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+The Project Gutenberg EBook of The Children's Book of Stars, by G.E. Mitton
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
+
+
+Title: The Children's Book of Stars
+
+Author: G.E. Mitton
+
+Release Date: May 17, 2009 [EBook #28853]
+
+Language: English
+
+Character set encoding: ISO-8859-1
+
+*** START OF THIS PROJECT GUTENBERG EBOOK THE CHILDREN'S BOOK OF STARS ***
+
+
+
+
+Produced by Siobhan Hillman, Brenda Lewis, Janet Blenkinship
+and the Online Distributed Proofreading Team at
+http://www.pgdp.net
+
+
+
+
+
+
+
+
+
+  THE CHILDREN'S BOOK OF STARS
+
+  THE CHILDREN'S BOOK OF STARS
+
+  MITTON
+
+  A.&C. BLACK
+
+  [Illustration: THE MOON-CHILD MUST KEEP ON RUNNING ROUND HER. P. 11.]
+
+
+  THE CHILDREN'S BOOK OF STARS
+
+  +-------------------------------------------------+
+  |  BY THE SAME AUTHOR                             |
+  |                                                 |
+  |  CHILDREN'S BOOK OF LONDON                      |
+  |                                                 |
+  |  CONTAINING 12 FULL-PAGE ILLUSTRATIONS          |
+  |  IN COLOUR BY JOHN WILLIAMSON                   |
+  |  PRICE =6s.=                                    |
+  |                                                 |
+  |  'The stories are told in a way that is bound   |
+  |  to rivet attention, and the historical sketches|
+  |  will leave a lasting impression on the minds   |
+  |  of young readers which will be very useful     |
+  |  when their studies in history become more      |
+  |  advanced.'--_Scotsman._                        |
+  |                                                 |
+  |                                                 |
+  |  ANIMAL AUTOBIOGRAPHIES                         |
+  |                                                 |
+  |  THE DOG                                        |
+  |                                                 |
+  |  WITH 12 FULL-PAGE ILLUSTRATIONS IN             |
+  |  COLOUR BY J. WILLIAMSON                        |
+  |                                                 |
+  |  PRICE =6s.=                                    |
+  |                                                 |
+  |  'A true life history, written "out of the      |
+  |  fulness of first-hand knowledge" by an author  |
+  |  who is thoroughly acquainted with all the      |
+  |  ways of "the friend of man."'--_Glasgow        |
+  |  Herald._                                       |
+  |                                                 |
+  |  'The story is admirably told in clear and      |
+  |  fascinating language.'--_Freeman's Journal._   |
+  |                                                 |
+  |  A. & C. BLACK. SOHO SQUARE. LONDON, W.         |
+  |                                                 |
+  +-------------------------------------------------+
+
+  AGENTS
+
+  AMERICA  THE MACMILLAN COMPANY
+           64 & 66 FIFTH AVENUE, NEW YORK
+
+  CANADA   THE MACMILLAN COMPANY OF CANADA, LTD.
+           27 RICHMOND STREET WEST, TORONTO
+
+  INDIA   MACMILLAN & COMPANY, LTD.
+          MACMILLAN BUILDING, BOMBAY
+          309 BOW BAZAAR STREET, CALCUTTA
+
+
+  THE
+  CHILDREN'S BOOK
+  OF
+  STARS
+
+  BY
+
+  G. E. MITTON
+
+  AUTHOR OF
+  'THE CHILDREN'S BOOK OF LONDON,' 'ANIMAL AUTOBIOGRAPHIES:
+  THE DOG,' ETC.
+
+  LONDON
+  ADAM AND CHARLES BLACK
+  1907
+
+ _Published September, 1907_
+
+
+
+
+PREFACE
+
+
+It was the intention of the late Agnes Clerke to write the preface to
+this 'Children's Book of Stars.' Miss Clerke took a warm and sympathetic
+interest in the authoress and her work, but her lamented death occurred
+before this kindly intention could be fulfilled.
+
+I cannot pretend to write adequately as her substitute, but I could not
+resist the appeal made to me by the author, in the name and for the sake
+of her dear friend and mine, to write a few words of introduction.
+
+I am in no way responsible either for the plan or for any portion of
+this work, but I can commend it as a book, written in a simple and
+pleasant style, calculated to awaken the interest of intelligent
+children, and to enable parents otherwise ignorant or astronomy to
+answer many of those puzzling questions which such children often put.
+
+  DAVID GILL.
+
+
+
+
+AUTHOR'S NOTE
+
+
+This little work is the outcome of many suggestions on the part of
+friends who were anxious to teach their small children something of the
+marvels of the heavens, but found it exceedingly difficult to get hold
+of a book wherein the intense fascination of the subject was not lost in
+conventional phraseology--a book in which the stupendous facts were
+stated in language simple enough to be read aloud to a child without
+paraphrase.
+
+Whatever merit there may be in the present work is due entirely to my
+friend Agnes Clerke, the well-known writer on astronomy; the faults are
+all my own. She gave me the impetus to begin by her warm encouragement,
+and she helped me to continue by hearing every chapter read as it was
+written, and by discussing its successor and making suggestions for it.
+Thus she heard the whole book in MS. A week after the last chapter had
+been read to her I started on a journey lasting many months, and while
+I was in the Far East the news reached me of her death, by which the
+world is the poorer. For her sake, as he has stated, her friend Sir
+David Gill, K.C.B., kindly undertook to supply the missing preface.
+
+  G. E. MITTON.
+
+
+
+
+CONTENTS
+
+
+  CHAPTER I                                                         PAGE
+
+  THE EARTH                                                            1
+
+  CHAPTER II
+
+  HANGING IN SPACE                                                    13
+
+  CHAPTER III
+
+  THE SHINING MOON                                                    21
+
+  CHAPTER IV
+
+  THE EARTH'S BROTHERS AND SISTER                                     32
+
+  CHAPTER V
+
+  FOUR SMALL WORLDS                                                   48
+
+  CHAPTER VI
+
+  FOUR LARGE WORLDS                                                   67
+
+  CHAPTER VII
+
+  THE SUN                                                             89
+
+  CHAPTER VIII
+
+  SHINING VISITORS                                                   103
+
+  CHAPTER IX
+
+  SHOOTING STARS AND FIERY BALLS                                     120
+
+  CHAPTER X
+
+  THE GLITTERING HEAVENS                                             135
+
+  CHAPTER XI
+
+  THE CONSTELLATIONS                                                 148
+
+  CHAPTER XII
+
+  WHAT THE STARS ARE MADE OF                                         159
+
+  CHAPTER XIII
+
+  RESTLESS STARS                                                     170
+
+  CHAPTER XIV
+
+  THE COLOURS OF THE STARS                                           176
+
+  CHAPTER XV
+
+  TEMPORARY AND VARIABLE STARS                                       188
+
+  CHAPTER XVI
+
+  STAR CLUSTERS AND NEBULĘ                                           197
+
+
+
+
+ILLUSTRATIONS
+
+PRINTED IN COLOUR
+
+  THE MOON-CHILD MUST KEEP ON  RUNNING ROUND HER           _Frontispiece_
+
+                                                             FACING PAGE
+
+  THE EARTH AND MOON HANGING IN SPACE                                 16
+
+  THE ENGLISH SUMMER AND WINTER                                       40
+
+  JUPITER AND ONE OF HIS MOONS                                        70
+
+  THE PLANET SATURN AND TWO OF HIS MOONS                              78
+
+  FLAMES FROM THE SUN                                                100
+
+  THE COMET IN THE BAYEUX TAPESTRY                                   104
+
+  A STICK THRUST INTO THE WATER APPEARS CROOKED                      114
+
+  CONSTELLATIONS NEAR THE POLE STAR                                  150
+
+  ORION AND HIS NEIGHBOURS                                           154
+
+  THE SPECTRUM OF THE SUN AND SIRIUS                                 168
+
+
+
+
+ILLUSTRATIONS
+
+IN BLACK AND WHITE
+
+
+                                                                    PAGE
+
+  THE MOON                                                   _facing_ 24
+
+  AN ECLIPSE OF THE MOON                                              28
+
+  AN ECLIPSE OF THE SUN                                               29
+
+  THE MOON RAISING THE TIDES                                          30
+
+  COMPARATIVE SIZES OF THE PLANETS                                    35
+
+  DIFFERENT PHASES OF VENUS                                           51
+
+  ORBITS OF MARS, THE EARTH, VENUS, AND MERCURY                       55
+
+  MAP OF MARS                                                _facing_ 56
+
+  ORBITS OF THE EARTH AND MARS                                        63
+
+  JUPITER AND HIS PRINCIPAL MOONS                                     72
+
+  SUN-SPOTS                                                  _facing_ 98
+
+  A GREAT COMET                                                 "    118
+
+  THE GREAT NEBULA IN ANDROMEDA                                 "    202
+
+
+THE CHILDREN'S BOOK OF STARS
+
+
+
+
+CHAPTER I
+
+THE EARTH
+
+
+It is a curious fact that when we are used to things, we often do not
+notice them, and things which we do every day cease to attract our
+attention. We find an instance of this in the curious change that comes
+over objects the further they are removed from us. They grow smaller and
+smaller, so that at a distance a grown-up person looks no larger than a
+doll; and a short stick planted in the ground only a few feet away
+appears as long as a much longer one at ten times the distance. This
+process is going on all round us every minute: houses, trees, buildings,
+animals, all seem larger or smaller in proportion to their distance from
+us. Sometimes I have seen a row of raindrops hanging on a bar by the
+window. When the sun catches one of them, it shines so brilliantly that
+it is as dazzling as a star; but my sense tells me it is a raindrop, and
+not a star at all. It is only because it is so near it seems as bright
+and important as a mighty star very, very far away.
+
+We are so much accustomed to this fact that we get into a habit of
+judging the distance of things by their size. If we see two lights
+shining on a dark night, and one is much larger than the other, we think
+that the bright one must be nearer to us; yet it need not necessarily be
+so, for the two lights might possibly be at the same distance from us,
+and one be large and the other small. There is no way in which we can
+tell the truth by just looking at them. Now, if we go out on any fine
+moonlight night and look up at the sky, we shall see one object there
+apparently much larger than any other, and that is the moon, so the
+question that occurs to us at once is, Is the moon really very much
+larger than any of the stars, or does it only seem so because it is very
+much nearer to us? As a matter of fact, the moon is one of the smallest
+objects in view, only, as it is our nearest neighbour, it appears very
+conspicuous. Having learned this, we shall probably look about to see
+what else there is to attract attention, and we may notice one star
+shining very brilliantly, almost like a little lamp, rather low down in
+the sky, in that part of it where the sun has lately set. It is so
+beautifully bright that it makes all the others look insignificant in
+comparison, yet it is not really large compared with the others, only,
+as it comes nearer to us than anything else in the sky except the moon,
+it looks larger than it has any right to do in comparison with the
+others.
+
+After this we might jump to the conclusion that all the bright large
+stars are really small and near to us, and all the faintly shining ones
+large and far away. But that would not be true at all, for some bright
+ones are very far away and some faint ones comparatively near, so that
+all we can do is to learn about them from the people who have studied
+them and found out about them, and then we shall know of our own
+knowledge which of them seem bright only because they are nearer than
+the others, and which are really very, very brilliant, and so still
+shine brightly, though set in space at an almost infinite distance from
+us.
+
+The sun, as we all know, appears to cross the sky every day; he gets up
+in the east and drops down in the west, and the moon does the same,
+only the moon is unlike the sun in this, that it changes its shape
+continually. We see a crescent moon growing every night larger and
+larger, until it becomes full and fat and round, and then it grows
+thinner and thinner, until it dies away; and after a little while it
+begins again, and goes through all the same changes once more. I will
+tell you why this is so further on, when we have a chapter all about the
+moon.
+
+If you watch the stars quietly for at least five minutes, you will see
+that they too are moving steadily on in the same way as the sun and
+moon. Watch one bright star coming out from behind a chimney-pot, and
+after about five minutes you will see that it has changed its place. Yet
+this is not true of all, for if we watch carefully we shall find that
+some, fairly high up in the sky, do not appear to move at all. The few
+which are moving so slowly that they seem to us to stand still are at a
+part of the sky close to the Pole Star, so called because it is always
+above the North Pole of the earth. I will explain to you how to find it
+in the sky for yourselves later on, but now you can ask anyone to point
+it out. Watch it. It appears to be fixed in one place, while the other
+stars are swinging round it in circles. In fact, it is as if we on the
+earth were inside a great hollow globe or ball, which continually turned
+round, with the Pole Star near the top of the globe; and you know that
+if you put your finger on the spot at the top of a spinning globe or
+ball, you can hold it there while all the rest of the ball runs round.
+Now, if you had to explain things to yourself, you would naturally
+think: 'Here is the great solid earth standing still, and the sun and
+moon go round it; the stars are all turning round it too, just as if
+they were fixed on to the inside of a hollow globe; we on the earth are
+in the middle looking up at them; and this great globe is slowly
+wheeling round us night by night.'
+
+In the childhood of the world men believed that this was really
+true--that the earth was the centre of the universe, that the sun and
+moon and all the hosts of heaven were there solely to light and benefit
+us; but as the world grew wiser the wonders of creation were fathomed
+little by little. Some men devoted their whole lives to watching the
+heavens, and the real state of things was gradually revealed to them.
+The first great discovery was that of the daily movement of the earth,
+its rotation on its own axis, which makes it appear as if all these
+shining things went round it. It is indeed a very difficult matter to
+judge which of two objects is moving unless we can compare them both
+with something outside. You must have noticed this when you are sitting
+in a train at a station, and there is another train on the other side of
+yours. For if one of the trains moves gently, either yours or the other,
+you cannot tell which one it is unless you look at the station platform;
+and if your position remains the same in regard to that, you know that
+your train is still standing, while the other one beside it has begun to
+move. And I am quite sure that there is no one of us who has not, at one
+time or another, stood on a bridge and watched the water running away
+underneath until we felt quite dizzy, and it seemed as if the water were
+standing still and the bridge, with ourselves on it, was flying swiftly
+away backwards. It is only when we turn to the banks and find them
+standing still, that we realize the bridge is not moving, and that it is
+the running water that makes it seem to do so. These everyday instances
+show us how difficult it is to judge whether we are moving or an outside
+object unless we have something else to compare with it. And the
+marvellous truth is that, instead of the sun and moon and stars rolling
+round the earth, it is the earth that is spinning round day by day,
+while the sun and the stars are comparatively still; and, though the
+moon does move, yet when we see her get up in the east and go down in
+the west that is due to our own movement and not to hers.
+
+The earth turns completely round once in a day and night. If you take an
+orange and stick a knitting-needle through it, and hold it so that the
+needle is not quite straight up but a little slanting, and then twirl it
+round, you will get quite a good idea of the earth, though of course
+there is no great pole like a gigantic needle stuck through it, that is
+only to make it easy for you to hold it by. In spinning the orange you
+are turning it as the earth turns day by day, or, as astronomers express
+it, as it rotates on its axis.
+
+There is a story of a cruel Eastern King who told a prisoner that he
+must die if he did not answer three questions correctly, and the
+questions were very difficult; this is one of them:
+
+'How long would it take a man to go round the earth if he never stopped
+to eat or drink on the way?'
+
+And the prisoner answered promptly: 'If he rose with the sun and kept
+pace with it all day, and never stopped for a moment to eat or drink, he
+would take just twenty-four hours, Your Royal Highness.' For in those
+days it was supposed that the sun went round the earth.
+
+Everyone is so remarkably clever nowadays that I am sure there will be
+someone clever enough to object that, if what I have said is true, there
+would be a great draught, for the air would be rushing past us. But, as
+a matter of fact, the air goes with us too. If you are inside a railway
+carriage with the windows shut you do not feel the rush of air, because
+the air in the carriage travels with you; and it is the same thing on
+the earth. The air which surrounds the earth clings to it and goes round
+with it, so there is no continuous breeze from this cause.
+
+But the spinning round on its own axis is not the earth's only movement,
+for all the time it is also moving on round the sun, and once in a whole
+year it completes its journey and comes back to the place from whence it
+started. Thus the turning round like a top or rotating on its axis makes
+the day and night, and the going in a great ring or revolving round the
+sun makes the years.
+
+Our time is divided into other sections besides days and years. We have,
+for instance, weeks and months. The weeks have nothing to do with the
+earth's movements; they are only made by man to break up the months;
+but the months are really decided by something over which we have no
+control. They are due to the moon, and, as I have said already, the moon
+must have a chapter to herself, so we won't say any more about the
+months here.
+
+If any friend of ours goes to India or New Zealand or America, we look
+upon him as a great traveller; yet every baby who has lived one year on
+the earth has travelled millions of miles without the slightest effort.
+Every day of our lives we are all flung through space without knowing it
+or thinking of it. It is as if we were all shut up in a comfortable
+travelling car, and were provided with so many books and pictures and
+companions that we never cared to look out of the windows, so that hour
+by hour as we were carried along over miles of space we never gave them
+a thought. Even the most wonderful car ever made by man rumbles and
+creaks and shakes, so that we cannot help knowing it is moving; but this
+beautiful travelling carriage of ours called the earth makes never a
+creak or groan as she spins in her age-long journey. It is always
+astonishing to me that so few people care to look out of the window as
+we fly along; most of them are far too much absorbed in their little
+petty daily concerns ever to lift their eyes from them. It is true that
+sometimes the blinds are down, for the sky is thickly covered with
+clouds, and we cannot see anything even if we want to. It is true also
+that we cannot see much of the scenery in the daytime, for the sun
+shining on the air makes a veil of blue glory, which hides the stars;
+but on clear nights we can see on every side numbers of stars quite as
+interesting and beautiful as any landscape; and yet millions of people
+never look up, never give a thought to the wonderful scenery through
+which their car is rushing.
+
+By reason of the onward rush of the earth in space we are carried over a
+distance of at least eighteen miles every second. Think of it: as we
+draw a breath we are eighteen miles away in space from the point we were
+at before, and this goes on unceasingly day and night. These astonishing
+facts make us feel how small and feeble we are, but we can take comfort
+in the thought that though our bodies are insignificant, the brain of
+man, which has discovered these startling facts, must in itself be
+regarded as one of the most marvellous of all the mysteries amid which
+we live.
+
+Well, we have arrived at some idea of our earth's position; we know
+that the earth is turning round day by day, and progressing round the
+sun year by year, and that all around lie the sentinel stars, scattered
+on a background of infinite space. If you take an older boy or girl and
+let him or her stand in the middle to represent the sun, then a smaller
+one would be the earth, and the smallest of all the moon; only in truth
+we could never get anyone large enough to represent the sun fairly, for
+the biggest giant that ever lived would be much too small in proportion.
+The one representing the sun must stand in the middle, and turn slowly
+round and round. Then let the earth-child turn too, and all the time she
+is spinning like a top she must be also hastening on in a big ring round
+the sun; but she must not go too fast, for the little moon-child must
+keep on running round her all the time. And the moon-child must keep her
+face turned always to the earth, so that the earth never sees her back.
+That is an odd thing, isn't it? We have never seen the other side of the
+moon, which goes round us, always presenting the same face to us.
+
+The earth is not the only world going round the sun; she has many
+brothers and a sister; some are nearer to the sun than she is, and some
+are further away, but all circle round the great central light-giver in
+rings lying one outside the other. These worlds are called planets, and
+the earth is one of them, and one of the smaller ones, too, nothing so
+great and important as we might have imagined.
+
+
+
+
+CHAPTER II
+
+HANGING IN SPACE
+
+
+If you are holding something in your hand and you let it go, what
+happens? It falls to the ground, of course. Now, why should it do so?
+You will say: 'How could it do anything else?' But that is only because
+you are hampered by custom. Try to shake yourself free, and think, Why
+should it go down instead of up or any other way? The first man who was
+clever enough to find some sort of an answer to this question was the
+great philosopher Sir Isaac Newton, though he was not quite the first to
+be puzzled by it. After years of study he discovered that every thing
+attracts every other thing in proportion to their masses (which is what
+you know as weight) and their distance from each other. In more
+scientific language, we should say every _body_ instead of every
+_thing_, for the word body does not only mean a living body, but every
+lump or mass of matter in the universe. The earth is a body in this
+sense, and so is the table or anything else you could name. Now as the
+earth is immeasurably heavier than anything that is on it, it pulls
+everything toward itself with such force that the little pulls of other
+things upon each other are not noticed. The earth draws us all toward
+it. It is holding us down to it every minute of the day. If we want to
+move we have to exert another force in order to overcome this attraction
+of the earth, so we exert our own muscles and lift first one foot and
+then the other away from the earth, and the effort we make in doing this
+tires us. All the while you are walking or running you are exercising
+force to lift your feet away from the ground. The pull of the earth is
+called gravitation. Just remember that, while we go on to something else
+which is almost as astonishing.
+
+We know that nothing here on earth continues to move for ever;
+everything has to be kept going. Anything left to itself has a tendency
+to stop. Why is this? This is because here in the world there is
+something that fights against the moving thing and tries to stop it,
+whether it be sent along the ground or thrown up in the air. You know
+what friction is, of course. If you rub your hands along any rough
+substance you will quickly feel it, but on a smooth substance you feel
+it less. That is why if you send a stone spinning along a carpet or a
+rough road it stops comparatively soon, whereas if you use the same
+amount of force and send it along a sheet of ice it goes on moving much
+longer. This kind of resistance, which we call friction, is one of the
+causes which is at work to bring things to a standstill; and another
+cause is the resistance of the air, which is friction in another form.
+It may be a perfectly still day, yet if you are bicycling you are
+breaking through the air all the time, just as you would be through
+water in swimming, only the resistance of the air is less than that of
+water. As the friction or the resistance of the air, or both combined,
+gradually lessens the pace of the stone you sent off with such force,
+the gravitation of the earth begins to be felt. When the stone first
+started the force you gave to it was enough to overcome the gravitation
+force, but as the stone moves more slowly the earth-pull asserts itself,
+and the stone drops down to the ground and lies still upon the surface.
+Now, if there were no friction, and therefore no resistance, there would
+be no reason why anything once set moving should not go on moving for
+ever. The force you give to any object you throw is enough to overcome
+gravitation; and it is only when the first force has been diminished by
+friction that the earth asserts its authority and pulls the moving
+object toward it. If it were possible to get outside the air and out of
+reach of the pull of the earth, we might fling a ball off into space,
+and it would go on in a straight line until something pulled it to
+itself by the force of gravity.
+
+Gravitation affects everything connected with the earth; even our air is
+held to the earth by gravitation. It grows thinner and thinner as we get
+further away from the earth. At the top of a high mountain the air is so
+thin that men have difficulty in breathing, and at a certain height they
+could not breathe at all. As they cannot breathe in very fine air, it is
+impossible for them to tell by personal experiment exactly where the air
+ends; but they have tried to find out in other ways, and though
+different men have come to different conclusions on the subject, it is
+safe to say that at about two hundred miles above the earth there is
+nothing that could be called air. Thus we can now picture our spinning
+earth clothed in a garment of air that clings closely about her, and
+grows thinner and thinner until it melts away altogether, for there is
+no air in space.
+
+[Illustration: THE EARTH AND MOON HANGING IN SPACE]
+
+Now in the beginning God made the world, and set it off by a first
+impulse. We know nothing about the details, though further on you shall
+hear what is generally supposed to have taken place; we only know that,
+at some remote age, this world, probably very different from what it is
+now, together with the other planets, was sent spinning off into space
+on its age-long journey. These planets were not sent off at random, but
+must have had some particular connection with each other and with the
+sun, for they all belong to one system or family, and act and react on
+each other. Now, if they had been at rest and not in movement, they
+would have fallen right into the sun, drawn by the force of gravitation;
+then they would have been burned up, and there would have been an end of
+them. But the first force had imparted to them the impulse to go on in a
+straight line, so when the sun pulled the result was a movement between
+the two: the planets did not continue to move in a straight line,
+neither did they fall on to the sun, but they went on a course between
+the two--that is, a circle--for the sun never let them get right away
+from him, but compelled them to move in circles round him. There is a
+very common instance of this kind of thing which we can see, or perhaps
+feel, every day. If you try to sit still on a bicycle you tumble off,
+because the earth pulls you down to itself; but if, by using the force
+of your own muscles, you give the bicycle a forward movement this
+resists the earth-pull, and the result is the bicycle runs along the
+ground. It does not get right away from the earth, not even two or three
+feet above ground; it is held to the earth, but still it goes forward
+and does not fall over, for the movement is made up of the earth-pull,
+which holds it to the ground, and the forward movement, which propels it
+along. Then again, as another instance, if you tie a ball to a string
+and whirl it round you, so long as you keep on whirling it will not fall
+to the ground, but the moment you stop down it drops, for there is
+nothing to fight against the pull of gravitation. Thus we can picture
+the earth and all the planets as if they were swinging round the sun,
+held by invisible strings. It is the combination of two forces that
+keeps them in their places--the first force and the sun's pull. It is
+very wonderful to think of. Here we are swinging in space on a ball that
+seems only large to us because we are so much smaller ourselves; there
+is nothing above or below it but space, yet it travels on day by day and
+year by year, held by invisible forces that the brain of man has
+discovered and measured.
+
+Of course, every planet gives a pull at every other planet too, but
+these pulls are so small compared with that of the sun that we need not
+at present notice them. Then we come to another point. We said that
+every body pulled every other body in proportion to their weights and
+their distance. Now, gravity acts much more strongly when things are
+near together than when they are far away from each other; so that if a
+smaller body is near to another somewhat larger than itself, it is
+pulled by it much more strongly than by a very much larger one at a
+considerably greater distance. We have an instance of this in the case
+of the earth and moon: as the earth responds to the pull of the sun, so
+the moon responds to the pull of the earth. The moon is so comparatively
+near to the earth that the earth-pull forces her to keep on going round
+and round, instead of leaving her free to circle round the sun by
+herself; and yet if you think of it the moon does go round the sun too.
+Recall that game we had when the sun was in the middle, and the two
+smaller girls, representing the earth and moon, went round it. The
+moon-child turned round the earth-child, but all the while the
+earth-child was going round the sun, so that in a year's time the moon
+had been all round the sun too, only not in a straight line. The moon is
+something like a dog who keeps on dancing round and round you when you
+go for a walk. He does go for the walk too, but he does much more than
+that in the same time. Thus we have further completed our idea of our
+world. We see it now hanging in space, with no visible support, held in
+its place by two mighty forces; spinning on year after year, attended by
+its satellite the moon, while we run, and walk, and cry, and laugh, and
+play about on its surface--little atoms who, except for the brain that
+God has given them, would never even have known that they are
+continually moving on through endless space.
+
+
+
+
+CHAPTER III
+
+THE SHINING MOON
+
+
+'Once upon a time,' long, long ago, the earth was not a compact, round,
+hard body such as she is now, but much larger and softer, and as she
+rotated a fragment broke off from her; it did not go right away from
+her, but still went on circling round with the motion it had inherited
+from her. As the ages passed on both the earth and this fragment, which
+had been very hot, cooled down, and in cooling became smaller, so that
+the distance between them was greater than it had been before they
+shrank. And there were other causes also that tended to thrust the two
+further from each other. Yet, compared with the other heavenly bodies,
+they are still near, and by looking up into the sky at night you can
+generally see this mighty fragment, which is a quarter the diameter of
+the earth--that is to say, a quarter the width of the earth measured
+from side to side through the middle. It is--as, of course, you have
+guessed--the moon. The moon is the nearest body to us in all space, and
+so vast is the distance that separates us from the stars that we speak
+as if she were not very far off, yet compared with the size of the earth
+the space lying between us and her is very great. If you went right
+round the world at the thickest part--that is to say, in the region of
+the Equator--and when you arrived at your starting-point went off once
+again, and so on until you had been round ten times, you would only then
+have travelled about as far as from the earth to the moon!
+
+The earth is not the only planet which has a moon, or as it is called, a
+satellite, in attendance. Some of the larger planets have several, but
+there is not one to compare with our moon. Which would you prefer if you
+had the choice, three or four small moons, some of them not much larger
+than a very big bright star, or an interesting large body like our own
+moon? I know which I should say.
+
+'You say that the moon broke off from the earth, so perhaps there may be
+some people living on her,' I hear someone exclaim.
+
+If there is one thing we have found out certainly about the moon, it is
+that no life, as we know it, could exist there, for there is neither air
+nor water. Whether she ever had any air or water, and if so, why they
+disappeared, are questions we cannot answer. We only know that now she
+is a dead world. Bright and beautiful as she is, shedding on us a pale,
+pure light, in vivid contrast with the fiery yellow rays of the sun, yet
+she is dead and lifeless and still. We can examine her surface with the
+telescope, and see it all very plainly. Even with a large opera-glass
+those markings which, to the naked eye, seem to be like a queer
+distorted face are changed, and show up as the shadows of great
+mountains. We can only see one side of the moon, because as I have said,
+she keeps always the same face turned to the earth; but as she sways
+slightly in her orbit, we catch a glimpse of sometimes a little more on
+one side and sometimes a little more on the other, and so we can judge
+that the unseen part is very much the same as that turned toward us.
+
+At first it is difficult to realize what it means to have no air.
+Besides supporting life in every breath that is drawn by living
+creatures, the air does numerous other kind offices for us--for
+instance, it carries sound. Supposing the most terrific volcano exploded
+in an airless world, it could not be heard. The air serves as a screen
+by day to keep off the burning heat of the sun's rays, and as a blanket
+by night to keep in the heat and not let it escape too quickly. If there
+were no air there could be no water, for all water would evaporate and
+vanish at once. Imagine the world deprived of air; then the sun's rays
+would fall with such fierceness that even the strongest tropical sun we
+know would be as nothing in comparison with it, and every green thing
+would shrivel up and die; this scorching sun would shine out of a black
+sky in which the stars would all be visible in the daytime, not hidden
+by the soft blue veil of air, as they are now. At night the instant the
+sun disappeared below the horizon black darkness would set in, for our
+lingering twilight is due to the reflection of the sun in the upper
+layers of air, and a bitterness of deathly cold would fall upon the
+earth--cold fiercer than that of the Arctic regions--and everything
+would be frozen solid. It would need but a short time to reduce the
+earth to the condition of the moon, where there is nothing to shrivel
+up, nothing to freeze. Her surface is made up of barren, arid rocks, and
+her scenery consists of icy black shadows and scorching white plains.
+
+[Illustration: _Paris Observatory._
+
+THE MOON.]
+
+The black shadows define the mountains, and tremendous mountains they
+are. Most of them have craters. A crater is like a cup, and generally
+has a little peak in the middle of it. This is the summit of a volcano,
+and when the volcano has burst up and vomited out floods of lava and
+débris, this has fallen down in a ring a little distance away from it,
+leaving a clear space next to the peak, so that, as the mountain ceases
+vomiting and the lava cools down, the ring hardens and forms a circular
+ridge. The craters on the moon are immense, not only in proportion to
+her size, but immense even according to our ideas on the earth. One of
+the largest craters in our own world is in Japan, and this measures
+seven miles across, while in the moon craters of fifty, sixty, and even
+a hundred miles are by no means uncommon, though there are also hundreds
+and thousands of smaller ones. We can see the surface of the moon very
+plainly with the magnificent telescopes that have now been made, and
+with the best of these anything the size of a large town would be
+plainly visible. Needless to say, no town ever has been or ever will be
+seen upon the moon!
+
+All these mountains and craters show that at one time the moon must have
+been convulsed with terrific disturbances, far worse than anything that
+we have any knowledge of on our earth; but this must have been ages
+ago, while the moon still probably had an atmosphere of its own. Now it
+has long been quiet. Nothing changes there; even the forces that are
+always at work on the earth--namely, damp and mould and water--altering
+the surface and breaking up the rocks, do not act there, where there is
+no moisture of any sort. So far as we can see, the purpose of the moon
+is to be the servant of the earth, to give her light by night and to
+raise the tides. Beautiful light it is, soft and mysterious--light that
+children do not often have a chance of seeing, for they are generally in
+bed before the moon rises when she is at the full.
+
+We know that the moon has no heat of her own--she parted with all that
+long ago; she cannot give us glowing light from brilliant flames, as the
+sun does; she shines only by the reflection of the sun on her surface,
+and this is the reason why she appears to change her shape so
+constantly. She does not really change; the whole round moon is always
+there, only part of it is in shadow. Sometimes you can see the dark part
+as well as the bright. When there is a crescent moon it looks as if it
+were encircling the rest; some people call it, 'seeing the old moon in
+the new moon's arms.' I don't know if you would guess why it is we can
+see the dark part then, or how it is lighted up. It is by reason of our
+own shining, for we give light to the moon, as she does to us. The sun's
+rays strike on the earth, and are reflected on to the moon, so that the
+moon is lighted by earthshine as we are lighted by moonshine, and it is
+these reflected earth-rays that light up the dark part of the moon and
+enable us to see it. What a journey these rays have had! They travel
+from the sun to the earth, and the earth to the moon, and then back to
+the earth again! From the moon the earth must appear a much bigger and
+more glorious spectacle than she does to us--four times wider across and
+probably brighter--for the sun's light strikes often on our clouds,
+which shine more brilliantly than her surface.
+
+Once again we must use an illustration to explain the subject. Set a
+lamp in the middle of a dark room, and let that be the sun, then take a
+small ball to represent the earth and a smaller one for the moon. Place
+the moon-ball between the lamp and the earth-ball. You will see that the
+side turned to the earth-ball is dark, but if you move the moon to one
+side of the earth, then from the earth half of it appears light and half
+dark; if you put it right away from the lamp, on the outer side of the
+earth, it is all gloriously lit up, unless it happens to be exactly
+behind the earth, when the earth's shadow will darken it. This is the
+full explanation of all the changes of the moon.
+
+[Illustration: AN ECLIPSE OF THE MOON.]
+
+Does it ever fall within the earth's shadow? Yes, it does; for as it
+passes round the earth it is not always at the same level, but sometimes
+a little higher and sometimes a little lower, and when it chances to
+pass exactly behind it enters the shadow and disappears. That is what we
+call an eclipse of the moon. It is nothing more than the earth's shadow
+thrown on to the moon, and as the shadow is round that is one of the
+proofs that the earth is round too. But there is another kind of
+eclipse--the eclipse of the sun; and this is caused by the moon herself.
+For when she is nearest to the sun, at new moon--that is to say, when
+her dark side is toward us, and she happens to get exactly between us
+and the sun--she shuts out the face of the sun from us; for though she
+is tiny compared with him, she is so much nearer to us that she appears
+almost the same size, and can blot him right out. Thus the eclipses of
+both sun and moon are not difficult to understand: that of the moon can
+only happen at full moon, when she is furthest from the sun, and it is
+caused by the earth's shadow falling upon the moon; and that of the sun
+at new moon, when she is nearest to him, and it is caused by the solid
+body of the moon coming between us and the sun.
+
+[Illustration: AN ECLIPSE OF THE SUN.]
+
+Besides giving us light by night, the moon serves other important
+purposes, and the most important of all is the raising of the tides.
+Without the rising of the sea twice in every day and night our coasts
+would become foul and unwholesome, for all the dead fish and rotting
+stuff lying on the beach would poison the air. The sea tides scour our
+coasts day by day with never-ceasing energy, and they send a great
+breath of freshness up our large rivers to delight many people far
+inland. The moon does most of this work, though she is a little helped
+by the sun. The reason of this is that the moon is so near to the earth
+that, though her pull is a comparatively small one, it is very strongly
+felt. She cannot displace the actual surface to any great extent, as it
+is so solid; but when it comes to the water she can and does displace
+that, so that the water rises up in answer to her pull, and as the earth
+turns round the raised-up water lags behind, reaching backward toward
+the moon, and is drawn up on the beach, and makes high tide. But it is
+stopped there, and meantime, by reason of the earth's movement, the moon
+is left far behind, and pulls the water to itself further on, when the
+first high tide relapses and falls down again. At length the moon gets
+round to quite the opposite side of the earth to that where she began,
+and there she makes a high tide too; but as she draws the water to
+herself she draws also the solid earth beneath the water to her in some
+degree, and so pulls it away from the place where the first high tide
+occurred, leaving the water there deeper than before, and so causing a
+secondary high tide.
+
+[Illustration: THE MOON RAISING THE TIDES.]
+
+The sun has some influence on the tides too, and when moon and sun are
+in the same line, as at full and new moon, then the tides are highest,
+and are called spring tides; but when they pull in different directions,
+as when it is half-moon, then the tides are lowest and are called neap
+tides.
+
+
+
+
+CHAPTER IV
+
+THE EARTH'S BROTHERS AND SISTER
+
+
+The earth is not the only world that, poised in space, swings around the
+sun. It is one of a family called the Solar System, which means the
+system controlled and governed by the sun. When we look up at the
+glorious sky, star-studded night by night, it might seem to us that the
+stars move only by reason of the earth's rotation; but when men first
+began to study the heavens attentively--and this is so long ago that the
+record of it is not to be found--they noticed that, while every shining
+object in the sky was apparently moving round us, there were a few which
+also had another movement, a proper motion of their own, like the moon.
+These curious stars, which appeared to wander about among the other
+stars, they called planets, or wanderers. And the reason, which was
+presently discovered, of our being able to see these movements was that
+these planets are very much nearer to us than any of the real stars,
+and in fact form part of our own solar system, while the stars are at
+immeasurable distances away. Of all the objects in the heavens the
+planets are the most intensely interesting to us; for though removed
+from us by millions of miles, the far-reaching telescope brings some of
+them within such range that we can see their surfaces and discover their
+movements in a way quite impossible with the stars. And here, if
+anywhere, might we expect to find traces of other living beings like
+ourselves; for, after all the earth is but a planet, not a very large
+nor a very small one, and in no very striking position compared with the
+other planets; and thus, arguing by what seems common-sense, we say, If
+this one planet has living beings on its surface, may not the other
+planets prove to be homes for living beings also? Counting our own
+earth, there are eight of these worlds in our solar system, and also a
+number of tiny planets, called asteroids; these likewise go round the
+sun, but are very much smaller than any of the first eight, and stand in
+a class by themselves, so that when the planets are mentioned it is
+generally the eight large well-known planets which are referred to.
+
+If we go back for a moment to the illustration of the large lamp
+representing our sun, we shall now be able to fill in the picture with
+much more detail. The orbits of the planets, as their paths round the
+sun are called, lie like great circles one outside another at various
+distances, and do not touch or cut each other. Where do you suppose our
+own place to be? Will it be the nearest to the sun or the furthest away
+from him? As a matter of fact, it is neither, we come third in order
+from the sun, for two smaller planets, one very small and the other
+nearly as large as the earth, circle round and round the sun in orbits
+lying inside ours. Now if we want to place objects around our lamp-sun
+which will represent these planets in size, and to put them in places
+corresponding to their real positions, we should find no room large
+enough to give us the space we ought to have. We must take the lamp out
+into a great open field, where we shall not be limited by walls. Then
+the smallest planet, named Mercury, which lies nearest of all to the
+sun, would have to be represented by a pea comparatively close to the
+sun; Venus, the next, would be a greengage plum, and would be about
+twice as far away; then would come the earth, a slightly larger plum,
+about half as far again as Venus. After this there would be a lesser
+planet, called Mars, like a marble. These are the first four, all
+comparatively small; beyond them there is a vast gap, in which we find
+the asteroids, and after this we come to four larger planets, mighty
+indeed as regards ourselves, for if our earth were a greengage plum,
+the first of these, Jupiter, would have to be the size of a football at
+least, and the next, Saturn, a smaller football, while Uranus and
+Neptune, the two furthest out, would be about the size of the toy
+balloons children play with. The outermost one, Neptune, would be thirty
+times as far from the sun as we are.
+
+[Illustration: COMPARATIVE SIZES OF THE PLANETS.]
+
+This is the solar system, and in it the only thing that shines by its
+own light is the sun; all the rest, the planets and their moons, shine
+only because the rays of light from the sun strike on their surfaces and
+are reflected off again. Our earth shines like that, and from the nearer
+planets must appear as a brilliant star. The little solar system is
+separated by distances beyond the realm of thought from the rest of the
+universe. Vast as are the intervals between ourselves and our planetary
+neighbours, they are as nothing to the space that separates us from the
+nearest of the steady shining fixed stars. Why, removed as far from us
+as the stars, the sun himself would have sunk to a point of light; and
+as for the planets, the largest of them, Jupiter, could not possibly be
+seen. Thus, when we look at those stars across the great gulf of space,
+we know that though we see them they cannot see us, and that to them our
+sun must seem only a star; consequently we argue that perhaps these
+stars themselves are suns with families of planets attached to them; and
+though there are reasons for thinking that this is not the case with
+all, it may be with some. Now if, after learning this, we look again at
+the sky, we do so with very different eyes, for we realize that some of
+these shining bodies are like ourselves in many things, and are shining
+only with a light borrowed from the sun, while others are mighty glowing
+suns themselves, shining by their own light, some greater and brighter,
+some less than our sun. The next thing to do is to learn which are stars
+and which are planets.
+
+Of the planets you will soon learn to pick out one or two, and will
+recognize them even if they do change their places--for instance, Venus
+is at times very conspicuous, shining as an evening star in the west
+soon after the sun goes down, or us a morning star before he gets up,
+though you are not so likely to see her then; anyway, she is never found
+very far from the sun. Jupiter is the only other planet that compares
+with her in brilliancy, and he shines most beautifully. He is, of
+course, much further away from us than Venus, but so much larger that he
+rivals her in brightness. Saturn can be quite easily seen as a
+conspicuous object, too, if you know where to look for him, and Mars is
+sometimes very bright with a reddish glow. The others you would not be
+able to distinguish.
+
+It is to our earth's family of these eight large planets going steadily
+round the same sun that we must give our attention first, before going
+on to the distant stars. Many of the planets are accompanied by
+satellites or moons, which circle round them. We may say that the sun is
+our parent--father, mother, what you will--and that the planets are the
+family of children, and that the moons are _their_ children. Our earth,
+you see, has only one child, but that a very fine one, of which she may
+well be proud.
+
+When I say that the planets go round the sun in circles I am only
+speaking generally; as a matter of fact, the orbits of the planets are
+not perfect circles, though some are more circular than others. Instead
+of this they are as a circle might look if it were pressed in from two
+sides, and this is called an ellipse. The path of our own earth round
+the sun is one of the most nearly circular of them all, and yet even in
+her orbit she is a good deal nearer to the sun at one time than another.
+Would you be surprised to hear that she is nearer in our winter and
+further away in our summer? Yet that is the case. And for the first
+moment it seems absurd; for what then makes the summer hotter than the
+winter? That is due to an altogether different cause; it depends on the
+position of the earth's axis. If that axis were quite straight up and
+down in reference to the earth's path round the sun we should have equal
+days and nights all the year round, but it is not; it leans over a
+little, so that at one time the North Pole points towards the sun and at
+another time away from it, while the South Pole is pointing first away
+from it and then toward it in exactly the reverse way. When the North
+Pole points to the sun we in the Northern Hemisphere have our summer. To
+understand this you must look at the picture, which will make it much
+clearer than any words of mine can do. The dark part is the night, and
+the light part the day. When we are having summer any particular spot on
+the Northern Hemisphere has quite a long way to travel in the light, and
+only a very short bit in the dark, and the further north you go the
+longer the day and shorter the night, until right up near the North
+Pole, within the Arctic Circle, it is daylight all the time. You have,
+perhaps, heard of the 'midnight sun' that people go to see in the North,
+and what the expression means is that at what should be midnight the
+sun is still there. He seems just to circle round the horizon, never
+very far above, but never dipping below it.
+
+When the sun is high overhead, his rays strike down with much more force
+than when he is low. It is, for instance, hotter at mid-day than in the
+evening. Now, when the North Pole is bowed toward the sun, the sun
+appears to us to be higher in the sky. In the British Isles he never
+climbs quite to the zenith, as we call the point straight above our
+heads; he always keeps on the southern side of that, so that our shadows
+are thrown northward at mid-day, but yet he gets nearer to it than he
+does in winter. Look at the picture of the earth as it is in winter.
+Then we have long nights and short days, and the sun never appears to
+climb very high, because we are turned away from him. During the short
+days we do not receive a great deal of heat, and during the long night
+the heat we have received has time to evaporate to a great extent. These
+two reasons--the greater or less height of the sun in the sky and the
+length of the days--are quite enough to account for the difference
+between our summer and winter. There is one rather interesting point to
+remember, and that is that in the Northern Hemisphere, whether it is
+winter or summer, the sun is south at mid-day, so that you can always
+find the north then, for your shadow will point northwards.
+
+[Illustration: THE ENGLISH SUMMER (LEFT) AND WINTER (RIGHT).]
+
+New Zealand and Australia and other countries placed in the Southern
+Hemisphere, as we are in the Northern, have their summer while we have
+winter, and winter while we have summer, and their summer is warmer than
+ours, because it comes when the earth in its journey is three million
+miles nearer to the sun than in our summer.
+
+All this seems to refer to the earth alone, and this chapter should be
+about the planets; but, after all, what applies to one planet applies to
+another in some degree, and we can turn to the others with much more
+interest now to see if their axes are bowed toward the sun as ours is.
+It is believed that in the case of Mercury, in regard to its path round
+the sun, the axis is straight up and down; if it is the changes of the
+seasons must depend on the nearness of Mercury to the sun and nothing
+else, and as he is a great deal nearer at one time than another, this
+might make a very considerable difference. Some of the planets are like
+the earth in regard to the position of their axes, but the two outermost
+ones, Uranus and Neptune, are very peculiar, for one pole is turned
+right toward the sun and the other right away from it, so that in one
+hemisphere there is continuous day all the summer, in the other there is
+continuous night, and then the process is reversed. But these little
+peculiarities we shall have to note more particularly in the account of
+the planets separately.
+
+There is a curious fact in regard to the distances of the planets from
+the sun. Each one, after the first, is, very roughly, about double the
+distance from the sun of the one inside it. This holds good for all the
+first four, then there is a great gap where we might expect to find
+another planet, after which follow the four large planets. Now, this gap
+puzzled astronomers greatly; for though there seemed to be no reason why
+the planets should be at regular distances one outside the other, yet
+there the fact was, and that the series should be broken by a missing
+planet was annoying. So very careful search was made, and a thrill of
+excitement went all through the scientific world when it was known that
+a tiny planet had been discovered in the right place. But this was not
+the end of it, for within a few years three or four more tiny planets
+were observed not far from the first one, and, as years rolled on, one
+after another was discovered until now the number amounts to over six
+hundred and others are perpetually being added to the list! Here was a
+new feature in the solar system, a band of tiny planets not one of which
+was to be compared in size with the least of those already known. The
+largest may be about as large as Europe, and others perhaps about the
+size of Wales, while there may be many that have only a few square miles
+of surface altogether, and are too small for us to see. To account for
+this strange discovery many theories were advanced.
+
+One was that there had been a planet--it might be about the size of
+Mars--which had burst up in a great explosion, and that these were the
+pieces--a very interesting and exciting idea, but one which proved to be
+impossible. The explanation now generally accepted is a little
+complicated, and to understand it we must go back for a bit.
+
+When we were talking of the earth and the moon we realized that once
+long ago the moon must have been a part of the earth, at a time when the
+earth was much larger and softer than she now is; to put it in the
+correct way, we should say when she was less dense. There is no need to
+explain the word 'dense,' for in its ordinary sense we use it every day,
+but in an astronomical sense it does not mean exactly the same thing.
+Everything is made up of minute particles or atoms, and when these
+atoms are not very close together the body they compose is loose in
+texture, while if they are closer together the body is firmer. For
+instance, air is less dense than water, and water than earth, and earth
+than steel. You see at once by this that the more density a thing has
+the heavier it is; for as a body is attracted to another body by every
+atom or particle in it, so if it has more particles it will be more
+strongly attracted. Thus on the earth the denser things are really
+heavier. But 'weight' is only a word we use in connection with the
+earth; it means the earth's pulling power toward any particular thing at
+the surface, and if we were right out in space away from the earth, the
+pulling power of the earth would be less, and so the weight would be
+less; and as it would be impossible always to state just how far away a
+thing was from the earth, astronomers talk about density, which means
+the number of particles a body contains in proportion to other bodies.
+Thus the planet Jupiter is very much larger than the earth, but his
+density is less. That does not mean to say that if Jupiter were in one
+scale and the earth in the other he would weigh less, because he is so
+very much bigger he would outweigh the earth still; his total _mass_
+would be greater than that of the earth, but it means that a piece of
+Jupiter the same size as a piece of the earth would weigh less under the
+same conditions.
+
+Now, before there were any planets at all or any sun, in the place of
+our solar system was a vast gaseous cloud called a nebula, which slowly
+rotated, and this rotation was the first impulse or force which God gave
+it. It was not at all dense, and as it rotated a part broke off, and
+inheriting the first impulse, went on rotating too. The impulse would
+have sent it off in a straight line, but the pull of gravity from the
+nebula held it in place, and it circled round; then the nebula, as it
+rotated, contracted a little, and occupied less space and grew denser,
+and presently a second piece was thrown off, to become in time another
+planet. The same process was repeated with Saturn, and then with the
+huge Jupiter. The nebula was always rotating and always contracting. And
+as it behaved, so did the planets in their turn; they spun round and
+cooled and contracted, and the moons were flung off from them, just as
+they--the planets--had been flung off from the parent nebula.
+
+Now, after the original nebula had parted with the mighty mass of
+Jupiter, it never again made an effort so great, and for a long time
+the fragments that were detached were so small as hardly to be worth
+calling planets; they were the asteroids, little lumps and fragments
+that the nebula left behind. But as it still contracted in time there
+came Mars; and having recovered a little, the nebula with more energy
+got rid of the earth, and next Venus, and lastly little Mercury, the
+smallest of the eight planets. Then it contracted further, and perhaps
+you can guess what the remainder of it is--the sun; and by spinning in a
+plastic state the sun, like the earth, has become a globe, round and
+comparatively smooth; and its density is now too great to allow of its
+losing any more fragments, so, as far as we can see, the solar system is
+complete.
+
+This theory of the origin of the planets is called the nebula theory. We
+cannot prove it, but there are so many facts that can only be explained
+by it, we have strong reason for believing that something of the kind
+must have happened. When we come to speak of the starry heavens we shall
+see that there are many masses of glowing gas which are nebulę of the
+same sort, and which form an object-lesson in our own history.
+
+We have spoken rather lightly of the nebula rotating and throwing off
+planets; but we must not think of all this as having happened in a
+short time. It is almost as impossible for the human mind to conceive
+the ages required for such slow changes as to grasp the great gulfs of
+space that separate us from the stars. We can only do it by comparison.
+You know what a second is, and how the seconds race past without ceasing
+day and night. It makes one giddy to picture the seconds there are in a
+year; yet if each one of those seconds was a year in itself, what then?
+That seems a stupendous time, but it is nothing compared with the time
+needed to form a nebula into a planetary system. If we had five thousand
+of such years, with every second in them a year, we should then only
+have counted one billion real years, and billions must have passed since
+the sun was a gaseous nebula filling the outermost bounds of our
+system!
+
+
+
+
+CHAPTER V
+
+FOUR SMALL WORLDS
+
+
+What must the sun appear to Mercury, who is so much nearer to him than
+we are? To understand that we should have to imagine our sun increased
+to eight or nine times his apparent size, and pouring out far greater
+heat and light than anything that we have here, even in the tropics. It
+was at first supposed that Mercury must have an extra thick covering of
+clouds to protect him from this tremendous glare; but recent
+observations tend to prove that, far from this, he is singularly free
+from cloud. As this is so, no life as we know it could possibly exist on
+Mercury.
+
+His year--the time he takes to go round the sun and come back to the
+same place again--is eighty-eight days, or about one-quarter of ours. As
+his orbit is much more like an ellipse than a circle, it follows that he
+is much nearer to the sun at one time than at another--in fact, when he
+is nearest, the size of the sun must seem three and a half times
+greater than when he is furthest away from it! Even at the best Mercury
+is very difficult to observe, and what we can learn about him is not
+much; but, as we have heard, his axis is supposed to be upright. If so
+his seasons cannot depend on the bend toward or away from the sun, but
+must be influenced solely by the changes in his distance from the sun,
+which are much greater than in our own ease. There is some reason to
+believe, too, that Mercury's day and year are the same length. This
+means that as the planet circles round the sun he turns once. If this is
+so the sun will shine on one half of the planet, producing an
+accumulated heat terrific to think of; while the other side is plunged
+in blackness. The side which faces the sun must be heated to a pitch
+inconceivable to us during the nearer half of the orbit--a pitch at
+which every substance must be at boiling-point, and which no life as we
+know it could possibly endure. Seen from our point of view, Mercury goes
+through all the phases of the moon, as he shines by the reflected light
+of the sun; but this point we shall consider more particularly in regard
+to Venus, as Venus is nearer to us and easier to study. For a long time
+astronomers had a fancy that there might be another planet even nearer
+to the sun than Mercury, perhaps hidden from us by the great glare of
+the sun. They even named this imaginary planet Vulcan, and some thought
+they had seen it, but it is tolerably certain that Vulcan existed only
+in imagination. Mercury is the nearest planet to the sun, and also the
+smallest, of course excepting the asteroids. It is about three thousand
+miles in diameter, and as our moon is two thousand miles, it is not so
+much bigger than that. So far as we are concerned, it is improbable we
+shall ever know very much more about this little planet.
+
+But next we come to Venus, our beautiful bright neighbour, who
+approaches nearer to us than any other heavenly body except the moon.
+Alas! when she is nearest, she like Mercury, turns her dark side toward
+us, coming in between us and the sun, so that we cannot observe her at
+all.
+
+Everyone must have noticed Venus, however carelessly they have looked at
+the sky; but it is likely that far more people have seen her as an
+evening than a morning star, for most people are in bed when the sun
+rises, and it is only before sunrise or after sunset we can see Venus
+well. She is at her best from our point of view when she seems to us to
+be furthest from the sun, for then we can study her best, and at these
+times she appears like a half or three-quarter moon, as we only see a
+part of the side from which the sunlight is reflected. She shines like a
+little silver lamp, excelling every other planet, even Jupiter, the
+largest of all. If we look at her even with the naked eye, we can see
+that she is elongated or drawn out, but her brilliance prevents us from
+seeing her shape exactly; to do this we must use a telescope.
+
+[Illustration: DIFFERENT PHASES OF VENUS.]
+
+It is a curious fact that some planets shine much more brightly than
+others, without regard to their size--that is to say, the surface on
+which the sun's rays strike is of greater reflecting power in some than
+in others. One of the brightest things in Nature that we can imagine is
+a bank of snow in sunlight; it is so dazzling that we have to look away
+or wink hard at the sight; and the reflective power of the surface of
+Venus is as dazzling as if she were made of snow. This is probably
+because the light strikes on the upper surface of the clouds which
+surround her. In great contrast to this is the surface of Mercury, which
+reflects as dully as a mass of lead. Our own moon has not a high
+reflecting power, as will be easily understood if we imagine what the
+world would be if condemned to perpetual moonlight only. It would,
+indeed, be a sad deprivation if the mournful cold light of the moon,
+welcome enough as a change from sunlight, were to take the place of
+sunlight in the daytime.
+
+For a very long time astronomers could not discover what time Venus took
+in rotating on her own axis--that is to say, what the length of her day
+was. She is difficult to observe, and in order to find out the rotation
+it is necessary to note some fixed object on the surface which turns
+round with the planet and comes back to the same place again, so that
+the time it takes in its journey can be measured. But the surface of
+Venus is always changing, so that it is impossible to judge at all
+certainly. Opinions differ greatly, some astronomers holding that
+Venus's day is not much longer than an earthly day, while others believe
+that the planet's day is equal to her year, just as in the case of
+Mercury. Venus's year is 225 days, or about seven and a half of our
+months, and if, indeed, her day and year are the same length, very
+peculiar effects would follow. For instance, terrible heat would be
+absorbed by the side of the planet facing the sun in the perpetual
+summer; and the cold which would be felt in the dreary winter's night
+would far exceed our bitterest Arctic climate. We cannot but fancy that
+any beings who might live on a planet of this kind must be different
+altogether from ourselves. Then, there is another point: even here on
+earth very strong winds are caused by the heating of the tropics; the
+hot air, being lighter than the cold air, rises, and the colder air from
+the poles rushes in to supply its place. This causes wind, but the winds
+which would be raised on Venus by the rush of air from the icy side of
+the planet to the hot one would be tornadoes such as we could but
+faintly dream of. It is, of course, useless to speculate when we know so
+little, but in a subject so intensely interesting we cannot help
+guessing a little.
+
+Venus is only slightly smaller than the earth, and her density is not
+very unlike ours; therefore the pull of gravity must be pretty much
+there what it is here--that is to say, things will weigh at her surface
+about the same as they do here. Her orbit is nearly a circle, so that
+her distance from the sun does not vary much, and the heat will not be
+much greater from this cause at one time of the year than another.
+
+As her orbit is tilted up a little she does not pass between us and the
+sun at each revolution, but occasionally she does so, and this passing
+is called a transit. Many important facts have been learned by watching
+these transits. Mercury also has transits across the sun, but as she is
+so much smaller than Venus they are not of such great importance. It was
+by the close observation of Venus during her transits that the distance
+from the earth to the sun was first measured. Not until the year 2004
+will another transit of Venus occur.
+
+It is not difficult to imagine that the earth must appear a splendid
+spectacle from Venus, whence she is seen to great advantage. When
+nearest to us she must see us like a little moon, with markings as the
+continents and seas rotate, and these will change as they are obscured
+by the clouds rolling over them. At the North and South Poles will be
+glittering ice-caps, growing larger and smaller as they turn toward or
+away from the sun. A brilliant spectacle!
+
+[Illustration: ORBITS OF MARS, THE EARTH, VENUS, AND MERCURY.]
+
+We might say with a sigh, 'If only we could see such a world!' Well, we
+can see a world--not indeed, so large as Venus, yet a world that comes
+almost as near to us as Venus does, and which, unlike her, is outside us
+in order from the sun, so that when it is nearest to us the full
+sunlight is on it. This is Mars, our neighbour on the other side, and of
+all the fascinating objects in the sky Mars is the most fascinating, for
+there, if anywhere, should we be likely to discover beings like
+ourselves!
+
+Mars takes rather more than half an hour longer to rotate than we do,
+and as he is so much smaller than the earth, this means that he moves
+round more slowly. His axis is bent at nearly the same angle as ours is.
+Mars is much smaller than the earth, his diameter is about twice that of
+the moon, and his density is about three-quarters that of the earth, so
+that altogether, with his smaller size and less density, anything
+weighing a hundred pounds here would only weigh some forty pounds on
+Mars; and if, by some miraculous agency, you were suddenly transported
+there, you would find yourself so light that you could jump enormous
+distances with little effort, and skip and hop as if you were on
+springs.
+
+[Illustration: _Memoirs of the British Astronomical Association._
+
+MAP OF MARS.]
+
+Look at the map of Mars, in which the surface appears to be cut up into
+land and water, continents and oceans. The men who first observed Mars
+with accuracy saw that some parts were of a reddish colour and others
+greenish, and arguing from our own world, they called the greenish parts
+seas and the reddish land. For a long while no one doubted that we
+actually looked on a world like our own, more especially as there was
+supposed to be a covering of atmosphere. The so-called land and water
+are much more cut up and mixed together than ours, it is true. Here and
+there is a large sea, like that marked 'Mare Australe,' but otherwise
+the water and the land are strangely intermingled. The red colour of the
+part they named land puzzled astronomers a good deal, for our land seen
+at the same distance would not appear so red, and they came at last to
+the conclusion that vegetation on Mars must be red instead of green! But
+after a while another disturbing fact turned up to upset their theories,
+and that was that they saw canals, or what they called canals, on Mars.
+These were long, straight, dark markings, such as you see on the map.
+It is true that some people never saw these markings at all, and
+disbelieved in their existence; but others saw them clearly, and watched
+them change--first go fainter and then darker again. And quite recently
+a photograph has been obtained which shows them plainly, so they must
+have an existence, and cannot be only in the eye of the observer, as the
+most sceptical people were wont to suggest. But further than this, one
+astronomer announced that some of these lines appeared to be double, yet
+when he looked at them again they had grown single. It was like a
+conjuring trick. Great excitement was aroused by this, for if the canals
+were altered so greatly it really did look as if there were intelligent
+beings on Mars capable of working at them. In any case, if these are
+really canals, to make them would be a stupendous feat, and if they are
+artificial--that is, made by beings and not natural--they show a very
+high power of engineering. Imagine anyone on earth making a canal many
+miles wide and two thousand miles long! It is inconceivable, but that is
+the feat attributed to the Martians. The supposed doubling of the
+canals, as I say, caused a great deal of talk, and very few people could
+see that they were double at all. Even now the fact is doubted, yet
+there seems every reason to believe it is true. They do not all appear
+to be double, and those that do are always the same ones, while others
+undoubtedly remain single all the time. But the canals do not exhaust
+the wonders of Mars. At each pole there is an ice-cap resembling those
+found at our own poles, and this tells us pretty plainly something about
+the climate of Mars, and that there is water there.
+
+This ice-cap melts when the pole which it surrounds is directed toward
+the sun, and sometimes in a hot summer it dwindles down almost to
+nothing, in a way that the ice-caps at the poles of the earth never do.
+A curious appearance has been noticed when it is melting: a dark shadow
+seems to grow underneath the edge of it and extends gradually, and as it
+extends the canals near it appear much darker and clearer than they did
+before, and then the canals further south undergo the same change. This
+looks as if the melting of the snow filled up the canals with water, and
+was a means of watering the planet by a system totally different from
+anything we know here, where our poles are surrounded by oceans, and the
+ice-caps do not in the least affect our water-supply. But, then, another
+strange fact had to be taken into consideration. These straight lines
+called canals ran out over the seas occasionally, and it was impossible
+to believe that if they were canals they could do that. Other things
+began to be discussed, such as the fact that the green parts of Mars did
+not always remain green. In what is the springtime of Mars they are so,
+but afterwards they become yellow, and still later in the season parts
+near the pole turn brown. Thus the idea that the greenish parts are seas
+had to be quite given up, though it appeared so attractive. The idea now
+generally believed is that the greenish parts are vegetation--trees and
+bushes and so on, and that the red parts are deserts of reddish sand,
+which require irrigation--that is to say, watering--before anything can
+be grown on them. The apparent doubling of the canals may be due to the
+green vegetation springing up along the banks. This might form two broad
+lines, while the canal itself would not be seen, and when the vegetation
+dies down, we should see only the trench of the canal, which would
+possibly appear faint and single. Therefore the arrangements on Mars
+appear to be a rich and a barren season on each hemisphere, the growth
+being caused by the melting of the polar ice-cap, which sends floods
+down even beyond the Equator. If we could imagine the same thing on
+earth we should have to think of pieces of land lying drear and dry and
+dead in winter between straight canal-like ditches of vast size. A
+little water might remain in these ditches possibly, but not enough to
+water the surrounding land. Then, as summer progressed, we should hear,
+'The floods are coming,' and each deep, huge canal would be filled up
+with a tide of water, penetrating further and further. The water drawn
+up into the air would fall in dew or rain. Vegetation would spring up,
+especially near the canal banks, and instead of dreary wastes rich
+growths would cover the land, gradually dying down again in the winter.
+So far Mars seems in some important respects very different from the
+earth. He is also less favourably placed than we are, for being so much
+further from the sun, he receives very much less heat and light. His
+years are 687 of our days, or one year and ten and a half months, and
+his atmosphere is not so dense as ours. With this greater distance from
+the sun and less air we might suppose the temperature would be very cold
+indeed, and that the surface would be frost-bound, not only at the
+poles, but far down towards the Equator. Instead of this being so, as we
+have seen, the polar caps melt more than those on the earth. We can
+only surmise there must be some compensation we do not know of that
+softens down the rigour of the seasons, and makes them milder than we
+should suppose possible.
+
+Of course, the one absorbing question is, Are there people on Mars? To
+this it is at present impossible to reply. We can only say the planet
+seems in every way fitted to support life, even if it is a little
+different from our earth. It is most certainly a living world, not a
+dead one like the moon, and as our knowledge increases we may some day
+be able to answer the question which so thrills us.
+
+Our opportunities for the observation of Mars vary very greatly, for as
+the earth's orbit lies inside that of Mars, we can best see him when we
+are between him and the sun. Of course, it must be remembered that the
+earth and the other planets are so infinitely small in regard to the
+space between them that there is no possibility of any one of them
+getting in such a position that it would throw a shadow on any other or
+eclipse it. The planets are like specks in space, and could not
+interfere with one another in this way. When Mars, therefore, is in a
+line with us and the sun we can see him best, but some of these times
+are better than others, for this reason--the earth's orbit is nearly a
+circle, and that of Mars more of an ellipse.
+
+[Illustration: ORBITS OF THE EARTH AND MARS.]
+
+Look at the illustration and remember that Mars' year is not quite two
+of ours--that is to say, every time we swing round our orbit we catch
+him up in a different place, for he will have progressed less than half
+his orbit while we go right round ours.
+
+Sometimes when we overtake him he may be at that part which is furthest
+away from us, or he may be at that part which is nearest to us, and if
+he is in the latter position we can see him best. Now at these, the most
+favourable times of all, he is still more than thirty-five millions of
+miles away--that is to say, one hundred and forty times as far as the
+moon, yet comparatively we can see him very well. He is coming nearer
+and nearer to us, and very soon will be nearer than he has been since
+1892, or fifteen years ago. Then many telescopes will be directed on
+him, and much may be learned about him.
+
+For a long time it was supposed that Mars had no moons, and when Dean
+Swift wrote 'Gulliver's Travels' he wanted to make the Laputans do
+something very clever, so he described their discovery of two moons
+attending Mars, and to make it quite absurd he said that when they
+observed these moons they found that one of them went round the planet
+in about ten hours. Now, as Mars takes more than twenty-four hours to
+rotate, this was considered ridiculous, for no moon known then took
+less time to go round its primary world than the primary world took to
+turn on its own axis. Our own moon, of course, takes thirty times as
+long--that is a month contains thirty days. Then one hundred and fifty
+years later this jest of Dean Swift's came true, for two moons were
+really discovered revolving round Mars, and one of them does actually
+take less time to complete its orbit than the planet does to
+rotate--namely, a little more than seven hours! So the absurdity in
+'Gulliver's Travels' was a kind of prophecy!
+
+These two moons are very small, the outer one perhaps five or six miles
+in diameter, and the inner one about seven; therefore from Mars the
+outer one, Deimos, cannot look much more than a brilliant star, and the
+inner one would be but a fifth part the apparent width of our own moon.
+So Mars is not very well off, after all. Still, there is great variety,
+for it must be odd to see the same moon appearing three times in the
+day, showing all the different phases as it goes from new to full, even
+though it is small!
+
+Such wonderful discoveries have already been made that it is not too
+much to say that perhaps some day we may be able to establish some sort
+of communication with Mars, and if it be inhabited by any intelligent
+beings, we may be able to signal to them; but it is almost impossible
+that any contrivance could bridge the gulf of airless space that
+separates us, and it is not likely that holiday trips to Mars will ever
+become fashionable!
+
+
+
+
+CHAPTER VI
+
+FOUR LARGE WORLDS
+
+
+I have told you about the four lesser worlds of which our earth is one,
+and you know that beyond Mars, the last of them, there lies a vast
+space, in which are found the asteroids, those strange small planets
+circling near to each other, like a swarm of bees. After this there
+comes Jupiter, who is so enormous, so superb in size compared with us,
+that he might well serve as the sun of a little system of his own. You
+remember that we represented him by a football, while the earth was only
+a greengage plum. But Jupiter himself is far less in comparison with the
+sun than we are in comparison with him. He differs from the planets we
+have heard about up to the present in that he seems to glow with some
+heat that he does not receive from the sun. The illumination which makes
+him appear as a star to us is, of course, merely reflected sunlight, and
+what we see is the external covering, his envelope of cloud.
+
+There is every reason to believe that the great bulk of Jupiter is
+still at a high temperature. We know that in the depths of the earth
+there is still plenty of heat, which every now and then makes its
+presence felt by bursting up through the vents we call volcanoes, the
+weak spots in the earth's crust; but our surface long ago cooled, for
+the outside of any body gets cool before the inside, as you may have
+found if ever you were trying to eat hot porridge, and circled round the
+edge of the plate with a spoon. A large body cools more slowly than a
+small one, and it is possible that Jupiter, being so much larger than we
+are, has taken longer to cool. One reason we have for thinking this is
+that he is so very light compared with his size--in other words, his
+density is so small that it is not possible he could be made of
+materials such as the earth is made of.
+
+As I said, when we study him through telescopes we see just the
+exterior, the outer envelope of cloud, and as we should expect, this
+changes continually, and appears as a series of belts, owing to the
+rotation of the planet. Jupiter's rotation is very rapid; though he is
+so much greater than the earth, he takes less than half the time the
+earth does to turn round--that is to say, only ten hours. His days and
+nights of five hours each seem short to us, accustomed to measure
+things by our own estimates. But we must remember that everything is
+relative; that is to say, there is really no such thing as fast or slow;
+it is all by comparison. A spider runs fast compared with a snail, but
+either is terribly slow compared with an express train; and the speed of
+an express train itself is nothing to the velocity of light.
+
+In the same way there is nothing absolutely great or small; it is all by
+comparison. We say how marvellous it is that a little insect has all the
+mechanism of life in its body when it is so tiny, but if we imagine that
+insect magnified by a powerful microscope until it appears quite large,
+the marvel ceases. Again, imagine a man walking on the surface of the
+earth as seen from a great distance through a telescope: he would seem
+less than an insect, and we might ask how could the mechanism of life be
+compressed into anything so small? Thus, when we say enormous or tiny we
+must always remember we are only speaking by the measurements of our own
+standards.
+
+There is nothing very striking about Jupiter's orbit. He takes between
+eleven and twelve of our years to get round the sun, so you see, though
+his day is shorter, his year is longer than ours. And this is not only
+because his path is much larger, but because by the law of gravity the
+more distant a planet is from the sun the more slowly it travels, so
+that while the earth speeds over eighteen miles Jupiter has only done
+eight. Of course, we must be careful to remember the difference between
+rotation and revolution. Jupiter rotates much quicker than the
+earth--that is to say, he turns round more quickly--but he actually gets
+over the ground more slowly. The sun appears much smaller to him than it
+does to us, and he receives considerably less light and heat. There are
+various spots on his surface, and one remarkable feature is a dark mark,
+which is called the 'great red spot.' If as we suppose what we see of
+the planet is merely the cloudy upper atmosphere, we should not expect
+to find anything permanent there, for the markings would change from day
+to day, and this they do with this exception--that this spot, dark red
+in colour, has been seen for many years, turning as the planet turned.
+It was first noticed in 1878, and was supposed to be some great mountain
+or excrescence peeping up through the clouds. It grew stronger and
+darker for several years, and then seemed to fade, and was not so easily
+seen, and though still remaining it is now pale. But, most startling
+to say, it has shifted its position a little--that is, it takes a few
+seconds longer to get round the planet than it did at first. A few
+seconds, you will say, but that is nothing! It does not seem much, but
+it shows how marvellously accurate astronomers are. Discoveries of vast
+importance have been made from observing a few seconds' discrepancy in
+the time the heavenly bodies take in their journeys, and the fact that
+this spot takes a little longer in its rotation than it did at first
+shows that it cannot be attached to the body of the planet. It is
+impossible for it to be the summit of a mountain or anything of that
+sort. What can it be? No one has yet answered that question.
+
+[Illustration: JUPITER AND ONE OF HIS MOONS]
+
+When we get to the chapter on the sun, we shall find curiosities
+respecting the spots there as well.
+
+Jupiter has seven moons, and four of these are comparatively large. They
+have the honour of having been the first heavenly bodies ever actually
+discovered, for the six large planets nearest the sun have been known so
+long that there is no record of their first discovery, and of course our
+own moon has always been known. Galileo, who invented the telescope,
+turned it on to the sky in 1610, when our King Charles I. was on the
+throne, and he saw these curious bodies which at first he could not
+believe to be moons. The four which he saw vary in size from two
+thousand one hundred miles in diameter to nearly three thousand six
+hundred. You remember our own moon is two thousand miles across, so even
+the smallest is larger than she. They go round at about the same level
+as the planet's Equator, and therefore they cross right in front of him,
+and go behind him once in every revolution. Since then the other three
+have been discovered in the band of Jupiter's satellites--one a small
+moon closer to him than any of the first set, and two others further
+out. It was by observation of the first four, however, that very
+interesting results were obtained. Mathematicians calculated the time
+that these satellites ought to disappear behind Jupiter and reappear
+again, but they found that this did not happen exactly at the time
+predicted; sometimes the moons disappeared sooner than they should have
+done, and sometimes later. Then this was discovered to have some
+relation to the distance of our earth from Jupiter. When he was at the
+far side of his immense orbit he was much more distant from us than when
+he was on the nearer side--in fact, the difference may amount to more
+than three hundred millions of miles. And it occurred to some clever man
+that the irregularities in time we noticed in the eclipses of the
+satellites corresponded with the distance of Jupiter from us. The
+further he drew away from us, the later were the eclipses, and as he
+came nearer they grew earlier. By a brilliant inspiration, this was
+attributed to the time light took to travel from them to us, and this
+was the first time anyone had been able to measure the velocity or speed
+of light. For all practical purposes, on the earth's surface we hold
+light to be instantaneous, and well we may, for light could travel more
+than eight times round the world in one second. It makes one's brain
+reel to think of such a thing. Then think how far Jupiter must be away
+from us at the furthest, when you hear that sometimes these eclipses
+were delayed seventeen minutes--minutes, not seconds--because it took
+that time for light to cross the gulf to us!
+
+[Illustration: JUPITER AND HIS PRINCIPAL MOONS.]
+
+Sound is very slow compared with light, and that is why, if you watch a
+man hammering at a distance, the stroke he gives the nail does not
+coincide with the bang that reaches you, for light gets to you
+practically at once, and the sound comes after it. No sound can travel
+without air, as we have heard, therefore no sound reaches us across
+space. If the moon were to blow up into a million pieces we should see
+the amazing spectacle, but should hear nothing of it. Light travels
+everywhere throughout the universe, and by the use of this universal
+carrier we have learnt all that we know about the stars and planets.
+When the time that light takes to travel had been ascertained by means
+of Jupiter's satellites, a still more important problem could be
+solved--that was our own distance from the sun, which before had only
+been known approximately, and this was calculated to be ninety-two
+millions seven hundred thousand miles, though sometimes we are a little
+nearer and sometimes a little further away.
+
+Jupiter is marvellous, but beyond him lies the most wonderful body in
+the whole solar system. We have found curiosities on our way out: we
+have studied the problem of the asteroids, of the little moon that goes
+round Mars in less time than Mars himself rotates; we have considered
+the 'great red spot' on Jupiter, which apparently moves independently
+of the planet; but nothing have we found as yet to compare with the
+rings of Saturn. May you see this amazing sight through a telescope one
+day!
+
+Look at the picture of this wonderful system, and think what it would be
+like if the earth were surrounded with similar rings! The first question
+which occurs to all of us is what must the sky look like from Saturn?
+What must it be to look up overhead and see several great hoops or
+arches extending from one horizon to another, reflecting light in
+different degrees of intensity? It would be as if we saw several immense
+rainbows, far larger than any earthly rainbow, and of pure light, not
+split into colours, extending permanently across the sky, and now and
+then broken by the black shadow of the planet itself as it came between
+them and the sun. However, we must begin at the beginning, and find out
+about Saturn himself before we puzzle ourselves over his rings. Saturn
+is not a very great deal less than Jupiter, though, so small are the
+other planets in comparison, that if Saturn and all the rest were rolled
+together, they would not make one mass so bulky as Jupiter! Saturn is
+so light--in other words, his density is so small--that he is actually
+lighter than water. He is the lightest, in comparison with his size, of
+any of the planets. Therefore he cannot be made largely of solid land,
+as our earth is, but must be to a great extent, composed of air and
+gaseous vapour, like his mighty neighbour. He approaches at times as
+near to Jupiter as Jupiter does to us, and on these occasions he must
+present a splendid spectacle to Jupiter. He takes no less than
+twenty-nine and a half of our years to complete his stately march around
+the sun, and his axis is a little more bent than ours; but, of course,
+at his great distance from the sun, this cannot have the same effect on
+the seasons that it does with us. Saturn turns fast on his axis, but not
+so fast as Jupiter, and in turning his face, or what we call his
+surface, presents much the same appearance to us that we might expect,
+for it changes very frequently and looks like cloud belts.
+
+The marvellous feature about Saturn is, of course, the rings. There are
+three of these, lying one within the other, and separated by a fine line
+from each other. The middle one is much the broadest, probably about ten
+thousand miles in width, and the inner one, which is the darkest, was
+not discovered until some time after the others. As the planet swings in
+his orbit the rings naturally appear very different to us at different
+times. Sometimes we can only see them edgewise, and then even in the
+largest telescope they are only like a streak of light, and this shows
+that they cannot be more than fifty or sixty miles in thickness. The one
+which is nearest to Saturn's surface does not approach him within ten
+thousand miles. Saturn has no less than ten satellites, in addition to
+the rings, so that his midnight sky must present a magnificent
+spectacle. The rings, which do not shine by their own light but by
+reflected sunlight, are solid enough to throw a shadow on the body of
+the planet, and themselves receive his shadow. Sometimes for days
+together a large part of Saturn must suffer eclipse beneath the
+encircling rings, but at other times, at night, when the rings are clear
+of the planet's body, so that the light is not cut off from them, they
+must appear as radiant arches of glory spanning the sky.
+
+The subject of these rings is so complicated by the variety of their
+changes that it is difficult for us even to think about it. It is one of
+the most marvellous of all the features of our planetary system. What
+are these rings? what are they made of? It has been positively proved
+that they cannot be made of continuous matter, either liquid or solid,
+for the force of gravity acting on them from the planet would tear them
+to pieces. What, then, can they be? It is now pretty generally believed
+that they are composed of multitudes of tiny bodies, each separate, and
+circling separately round the great planet, as the asteroids circle
+round the sun. As each one is detached from its neighbour and obeys its
+own impulses, there is none of the strain and wrench there would be were
+they all connected. According to the laws which govern planetary bodies,
+those which are nearest to the planet will travel more quickly than
+those which are further away. Of course, as we look at them from so
+great a distance, and as they are moving, they appear to us to be
+continuous. It is conjectured that the comparative darkness of the
+inside ring is caused by the fact that there are fewer of the bodies
+there to reflect the sunlight. Then, in addition to the rings, enough
+themselves to distinguish him from all other planets, there are the ten
+moons of richly-endowed Saturn to be considered. It is difficult to
+gather much about these moons, on account of our great distance from
+them. The largest is probably twice the diameter of our own moon. One of
+them seems to be much brighter--that is to say, of higher reflecting
+power--on one side than the other, and by distinguishing the sides
+and watching carefully, astronomers have come to the conclusion that it
+presents always the same face to Saturn in the same way as our own moon
+does to us; in fact, there is reason to think that all the moons of
+large planets do this.
+
+[Illustration: THE PLANET SATURN WITH TWO OF HIS MOONS.]
+
+All the moons lie outside the rings, and some at a very great distance
+from Saturn, so that they can only appear small as seen from him. Yet at
+the worst they must be brighter than ordinary stars, and add greatly to
+the variations in the sky scenery of this beautiful planet. In
+connection with Saturn's moons there is another of those astonishing
+facts that are continually cropping up to remind us that, however much
+we know, there is such a vast deal of which we are still ignorant. So
+far in dealing with all the planets and moons in the solar system we
+have made no remark on the way they rotate or revolve, because they all
+go in the same direction, and that direction is called
+counter-clockwise, which means that if you stand facing a clock and turn
+your hand slowly round the opposite direction to that in which the hands
+go, you will be turning it in the same way that the earth rotates on its
+axis and revolves in its orbit. It is, perhaps, just as well to give
+here a word of caution. Rotating of course means a planet's turning on
+its own axis, revolving means its course in its orbit round the sun.
+Mercury, Venus, Earth, Mars, Jupiter, and all their moons, as well as
+Saturn himself, rotate on their axes in this one
+direction--counter-clockwise--and revolve in the same direction as they
+rotate. Even the queer little moon of Mars, which runs round him quicker
+than he rotates, obeys this same rule. Nine of Saturn's moons follow
+this example, but one independent little one, which has been named
+Phoebe, and is far out from the planet, actually revolves in the
+opposite way. We cannot see how it rotates, but if, as we said just now,
+it turns the same face always to Saturn, then of course it rotates the
+wrong way too. A theory has been suggested to account for this curious
+fact, but it could not be made intelligible to anyone who has not
+studied rather high mathematics, so there we must just leave it, and put
+it in the cabinet of curiosities we have already collected on our way
+out to Saturn.
+
+For ages past men have known and watched the planets lying within the
+orbit of Saturn, and they had made up their minds that this was the
+limit of our system. But in 1781 a great astronomer named Herschel was
+watching the heavens through a telescope when he noticed one strange
+object that he was certain was no star. The vast distance of the stars
+prevents their having any definite outline, or what is called a disc.
+The rays dart out from them in all directions and there is no 'edge' to
+them, but in the case of the planets it is possible to see a disc with a
+telescope, and this object which attracted Herschel's attention had
+certainly a disc. He did not imagine he had discovered a new planet,
+because at that time the asteroids had not been found, and no one
+thought that there could be any more planets. Yet Herschel knew that
+this was not a star, so he called it a comet! He was actually the first
+who discovered it, for he knew it was not a fixed star, but it was after
+his announcement of this fact that some one else, observing it
+carefully, found it to be a real planet with an orbit lying outside that
+of Saturn, then the furthest boundary of the solar system. Herschel
+suggested calling it Georgius Sidus, in honour of George III., then
+King; but luckily this ponderous name was not adopted, and as the other
+planets had been called after the Olympian deities, and Uranus was the
+father of Saturn, it was called Uranus. It was subsequently found that
+this new planet had already been observed by other astronomers and
+catalogued as a star no less than seventeen times, but until Herschel's
+clear sight had detected the difference between it and the fixed stars
+no one had paid any attention to it. Uranus is very far away from the
+sun, and can only sometimes be seen as a small star by people who know
+exactly where to look for him. In fact, his distance from the sun is
+nineteen times that of the earth.
+
+Yet to show at all he must be of great size, and that size has actually
+been found out by the most delicate experiments. If we go back to our
+former comparison, we shall remember that if the earth were like a
+greengage plum, then Uranus would be in comparison about the size of one
+of those coloured balloons children play with; therefore he is much
+larger than the earth.
+
+In this far distant orbit the huge planet takes eighty-four of our years
+to complete one of his own. A man on the earth will have grown from
+babyhood to boyhood, from boyhood to the prime of life, and lived longer
+than most men, while Uranus has only once circled in his path.
+
+But in dealing with Uranus we come to another of those startling
+problems of which astronomy is full. So far we have dealt with planets
+which are more or less upright, which rotate with a rotation like that
+of a top. Now take a top and lay it on one side on the table, with one
+of its poles pointing toward the great lamp we used for the sun and the
+other pointing away. That is the way Uranus gets round his path, on his
+side! He rotates the wrong way round compared with the planets we have
+already spoken of, but he revolves the same way round the sun that all
+the others do. It seems wonderful that even so much can be found out
+about a body so far from us, but we know more: we have discovered that
+Uranus is made of lighter material than the earth; his density is less.
+How can that be known? Well, you remember every body attracts every
+other body in proportion to the atoms it contains. If, therefore, there
+were any bodies near to Uranus, it could be calculated by his influence
+on them what was his own mass, which, as you remember, is the word we
+use to express what would be weight were it at the earth's surface; and
+far away as Uranus is, the bodies from which such calculations may be
+made have been discovered, for he has no less than four satellites, or
+moons. Considering now the peculiar position of the planet, we might
+expect to find these moons revolving in a very different way from
+others, and this is indeed the case. They turn round the planet at
+about its Equator--that is to say, if you hold the top representing
+Uranus as was suggested just now, these moons would go above and below
+the planet in passing round it. Only we must remember there is really no
+such thing as above and below absolutely. We who are on one side of the
+world point up to the sky and down to the earth, while the people on the
+other side of the earth, say at New Zealand, also point up to the sky
+and down to the earth, but their pointings are directly the opposite of
+ours. So when we speak of moons going above and below that is only
+because, for the moment, we are representing Uranus as a top we hold in
+our hands, and so we speak of above and below as they are to us.
+
+It was Herschel who discovered these satellites, as well as the planet,
+and for these great achievements he occupies one of the grandest places
+in the rōle of names of which England is proud. But he did much more
+than this: his improvements in the construction of telescopes, and his
+devotion to astronomy in many other ways, would have caused him to be
+remembered without anything else.
+
+Of Uranus's satellites one, the nearest, goes round in about two and a
+half days, and the one that is furthest away takes about thirteen and a
+half days, so both have a shorter period than our moon.
+
+The discovery of Uranus filled the whole civilized world with wonder.
+The astronomers who had seen him, but missed finding out that he was a
+planet, must have felt bitterly mortified, and when he was discovered he
+was observed with the utmost accuracy and care. The calculations made to
+determine his path in the sky were the easier because he had been noted
+as a star in several catalogues previously, so that his position for
+some time past was known. Everybody who worked at astronomy began to
+observe him. From these facts mathematicians set to work, and, by
+abstruse calculations, worked out exactly the orbit in which he ought to
+move; then his movements were again watched, and behold he followed the
+path predicted for him; but there was a small difference here and there:
+he did not follow it exactly. Now, in the heavens there is a reason for
+everything, though we may not always be clever enough to find it out,
+and it was easily guessed that it was not by accident that Uranus did
+not precisely follow the path calculated for him. The planets all act
+and react on one another, as we know, according to their mass and their
+distance, and in the calculations the pull of Jupiter on Saturn and of
+Saturn on Uranus were known and allowed for. But Uranus was pulled by
+some unseen influence also.
+
+A young Englishman named Adams, by some abstruse and difficult
+mathematical work far beyond the power of ordinary brains, found out not
+only the fact that there must be another planet nearly as large as
+Uranus in an orbit outside his, but actually predicted where such a
+planet might be seen if anyone would look for it. He gave his results to
+a professor of astronomy at Cambridge. Now, it seems an easy thing to
+say to anyone, 'Look out for a planet in such and such a part of the
+sky,' but in reality, when the telescope is turned to that part of the
+sky, stars are seen in such numbers that, without very careful
+comparison with a star chart, it is impossible to say which are fixed
+stars and which, if any, is an intruder. There happened to be no star
+chart of this kind for the particular part of the sky wanted, and thus a
+long time elapsed and the planet was not identified. Meantime a young
+Frenchman named Leverrier had also taken up the same investigation, and,
+without knowing anything of Adams' work, had come to the same
+conclusion. He sent his results to the Berlin Observatory, where a star
+chart such as was wanted was actually just being made. By the use of
+this the Berlin astronomers at once identified this new member of our
+system, and announced to the astonished world that another large planet,
+making eight altogether, had been discovered. Then the English
+astronomers remembered that they too held in their hands the means for
+making this wonderful discovery, but, by having allowed so much time to
+elapse, they had let the honour go to France. However, the names of
+Adams and Leverrier will always be coupled together as the discoverers
+of the new planet, which was called Neptune. The marvel is that by pure
+reasoning the mind of man could have achieved such results.
+
+If the observation of Uranus is difficult, how much more that of
+Neptune, which is still further plunged in space! Yet by patience a few
+facts have been gleaned about him. He is not very different in size from
+Uranus. He also is of very slight density. His year includes one hundred
+and sixty-five of ours, so that since his discovery in 1846 he has only
+had time to get round less than a third of his path. His axis is even
+more tilted over than that of Uranus, so that if we compare Uranus to a
+top held horizontally, Neptune will be like a top with one end pointing
+downwards. He rotates in this extraordinary position, in the same manner
+as Uranus--namely, the other way over from all the other planets, but he
+revolves, as they all do, counter-clockwise.
+
+Seen from Neptune the sun can only appear about as large as Venus
+appears to us at her best, and the light and heat received are but one
+nine-hundreth part of what he sends us. Yet so brilliant is sunshine
+that even then the light that falls on Neptune must be very
+considerable, much more than that which we receive from Venus, for the
+sun itself glows, and from Venus the light is only reflected. The sun,
+small as it must appear, will shine with the radiance of a glowing
+electric light. To get some idea of the brilliance of sunlight, sit near
+a screen of leaves on some sunny day when the sun is high overhead, and
+note the intense radiance of even the tiny rays which shine through the
+small holes in the leaves. The scintillating light is more glorious than
+any diamond, shooting out coloured rays in all directions. A small sun
+the apparent size of Venus would, therefore, give enough light for
+practical purposes to such a world as Neptune, even though to us a world
+so illuminated would seem to be condemned to a perpetual twilight.
+
+
+
+
+CHAPTER VII
+
+THE SUN
+
+
+So far we have referred to the sun just so much as was necessary to show
+the planets rotating round him, and to acknowledge him as the source of
+all our light and heat; but we have not examined in detail this
+marvellous furnace that nourishes all the life on our planet and burns
+on with undiminished splendour from year to year, without thought or
+effort on our part. To sustain a fire on the earth much time and care
+and expense are necessary; fuel has to be constantly supplied, and men
+have to stoke the fire to keep it burning. Considering that the sun is
+not only vastly larger than all the fires on the earth put together, but
+also than the earth itself, the question very naturally occurs to us,
+Who supplies the fuel, and who does the stoking on the sun? Before we
+answer this we must try to get some idea of the size of this stupendous
+body. It is not the least use attempting to understand it by plain
+figures, for the figures would be too great to make any impression on
+us--they would be practically meaningless; we must turn to some other
+method. Suppose, for instance, that the sun were a hollow ball; then, if
+the earth were set at the centre, the moon could revolve round her at
+the same distance she is now, and there would be as great a distance
+between the moon and the shell of the sun as there is between the moon
+and the earth. This gives us a little idea of the size of the sun.
+Again, if we go back to that solar system in which we represented the
+planets by various objects from a pea to a football, and set a lamp in
+the centre to do duty for the sun, what size do you suppose that lamp
+would have to be really to represent the sun in proportion to the
+planets? Well, if our greengage plum which did duty for the earth were
+about three-quarters of an inch in diameter we should want a lamp with a
+flame as tall as the tallest man you know, and even then it would not
+give a correct idea unless you imagined that man extending his arms
+widely, and you drew round him a circle and filled in all the circle
+with flame! If this glorious flame burnt clear and fair and bright,
+radiating beams of light all around, the little greengage plum would not
+have to be too near, or it would be shrivelled up as in the blast of a
+furnace. To place it at anything resembling the distance it is from the
+sun in reality you would have to walk away from the flaming light for
+about three hundred steps, and set it down there; then, after having
+done all this, you would have some little idea of the relative sizes of
+the sun and the earth, and of the distance between them.
+
+Of course, all the other planets would have to be at corresponding
+distances. On this same scale, Neptune, the furthest out, would be three
+miles from our artificial sun! It seems preposterous to think that some
+specks so small as to be quite invisible, specks that crawl about on
+that plum, have dared to weigh and measure the gigantic sun; but yet
+they have done it, and they have even decided what he is made of. The
+result of the experiments is that we know the sun to be a ball of
+glowing gas at a temperature so high that nothing we have on earth could
+even compare with it. Of his radiating beams extending in all directions
+few indeed fall on our little plum, but those that do are the source of
+all life, whether animal or vegetable. If the sun's rays were cut off
+from us, we should die at once. Even the coal we use to keep us warm is
+but sun's heat stored up ages ago, when the luxuriant tropical
+vegetation sprang up in the warmth and then fell down and was buried in
+the earth. At night we are still enjoying the benefit of the sun's
+rays--that is, of those which are retained by our atmosphere; for if
+none remained even the very air itself would freeze, and by the next
+morning not one inhabitant would be left alive to tell the awful tale.
+Yet all this life and growth and heat we receive on the whole earth is
+but one part in two thousand two hundred millions of parts that go out
+in all directions into space. It has been calculated that the heat which
+falls on to all the planets together cannot be more than one part in one
+hundred millions and the other millions of parts seem to us to be simply
+wasted.
+
+For untold ages the sun has been pouring out this prodigal profusion of
+glory, and as we know that this cannot go on without some sort of
+compensation, we want to understand what keeps up the fires in the sun.
+It is true that the sun is so enormous that he might go on burning for a
+very long time without burning right away; but, then, even if he is
+huge, his expenditure is also huge. If he had been made of solid coal he
+would have been all used up in about six thousand years, burning at the
+pace he does. Now, we know that the ancient Egyptians kept careful note
+of the heavenly bodies, and if the sun were really burning away he must
+have been very much larger in their time; but we have no record of this;
+on the contrary, all records of the sun even to five thousand years ago
+show that he was much the same as at present. It is evident that we must
+search elsewhere for an explanation. It has been suggested that his
+furnace is supplied by the number of meteors that fall into him. Meteors
+are small bodies of the same materials as the planets, and may be
+likened to the dust of the solar system. It is not difficult to
+calculate the amount of matter he would require on this assumption to
+keep him going, and the amount required is so great as to make it
+practically impossible that this is the source of his supply. We have
+seen that all matter influences all other matter, and the quantity of
+meteoric stuff that would be required to support the sun's expenditure
+would be enough to have a serious effect on Mercury, an effect that
+would certainly have been noticed. There can, therefore, be no such mass
+of matter near the sun, and though there is no doubt a certain number of
+meteors do fall into his furnaces day by day, it is not nearly enough to
+account for his continuous radiation. It seems after this as if nothing
+else could be suggested; but yet an answer has been found, an answer so
+wonderful that it is more like a fairy tale than reality.
+
+To begin at the beginning, we must go back to the time when the sun was
+only a great gaseous nebula filling all the space included in the orbit
+of Neptune. This nebula was not in itself hot, but as it rotated it
+contracted. Now, heat is really only a form of energy, and energy and
+heat can be interchanged easily. This is a very startling thing when
+heard for the first time, but it is known as surely as we know anything
+and has been proved again and again. When a savage wants to make a fire
+he turns a piece of hard wood very very quickly between his
+palms--twiddles it, we should say expressively--into a hole in another
+piece of wood, until a spark bursts out. What is the spark? It is the
+energy of the savage's work turned to heat. When a horse strikes his
+iron-shod hoofs hard on the pavement you see sparks fly; that is caused
+by the energy of the horse's leg. When you pump hard at your bicycle you
+feel your pump getting quite hot, for part of the energy you are putting
+into your work is transformed into heat; and so on in numberless
+instances. No energetic action of any kind in this world takes place
+without some of the energy being turned into heat, though in many
+instances the amount is so small as to be unnoticeable. Nothing falls
+to the ground without some heat being generated. Now, when this great
+nebula first began its remarkable career, by the action of gravity all
+the particles in it were drawn toward the centre; little by little they
+fell in, and the nebula became smaller. We are not now concerned with
+the origin of the planets--we leave that aside; we are only
+contemplating the part of the nebula which remained to become the sun.
+Now these particles being drawn inward each generated some heat, so as
+the nebula contracted its temperature rose. Throughout the ages, over
+the space of millions and millions of miles, it contracted and grew
+hotter. It still remained gaseous, but at last it got to an immense
+temperature, and is the sun as we know it. What then keeps it shining?
+It is still contracting, but slowly, so slowly that it is quite
+imperceptible to our finest instruments. It has been calculated that if
+it contracts two hundred and fifty feet in diameter in a year, the
+energy thus gained and turned into heat is quite sufficient to account
+for its whole yearly output. This is indeed marvellous. In comparison
+with the sun's size two hundred and fifty feet is nothing. It would take
+nine thousand years at this rate before any diminution could be noticed
+by our finest instruments! Here is a source of heat which can continue
+for countless ages without exhaustion. Thus to all intents and purposes
+we may say the sun's shining is inexhaustible. Yet we must follow out
+the train of reasoning, and see what will happen in the end, in eras and
+eras of time, if nothing intervenes. Well, some gaseous bodies are far
+finer and more tenuous than others, and when a gaseous body contracts it
+is all the time getting denser; as it grows denser and denser it at last
+becomes liquid, and then solid, and then it ceases to contract, as of
+course the particles of a solid body cannot fall freely toward the
+centre, as those of a gaseous body can. Our earth has long ago reached
+this stage. When solid the action ceases, and the heat is no more kept
+up by this source of energy, therefore the body begins to cool--surface
+first, and lastly the interior; it cools more quickly the smaller it is.
+Our moon has parted with all her heat long ago, while the earth still
+retains some internally. In the sun, therefore, we have an object-lesson
+of the stages through which all the planets must have passed. They have
+all once been glowing hot, and some may be still hot even on the
+surface, as we have seen there is reason to believe is the case with
+Jupiter.
+
+By this marvellous arrangement for the continued heat of the sun we can
+see that the warmth of our planets is assured for untold ages. There is
+no need to fear that the sun will wear out by burning. His brightness
+will continue for ages beyond the thoughts of man.
+
+Besides this, a few other things have been discovered about him. He is,
+of course, exceptionally difficult to observe; for though he is so
+large, which should make it easy, he is so brilliant that anyone
+regarding him through a telescope without the precaution of prepared
+glasses to keep off a great part of the light would be blinded at once.
+One most remarkable fact about the sun is that his surface is flecked
+with spots, which appear sometimes in greater numbers and sometimes in
+less, and the reason and shape of these spots have greatly exercised
+men's minds. Sometimes they are large enough to be seen without a
+telescope at all, merely by looking through a piece of smoked or
+coloured glass, which cuts off the most overpowering rays. When they are
+visible like this they are enormous, large enough to swallow many earths
+in their depths. At other times they may be observed by the telescope,
+then they may be about five thousand miles across. Sometimes one spot
+can be followed by an astronomer as it passes all across the sun,
+disappears at the edge, and after a lapse of time comes back again round
+the other edge. This first showed men that the sun, like all the
+planets, rotated on his axis, and gave them the means of finding out how
+long he took in doing so. But the spots showed a most surprising result,
+for they took slightly different times in making their journey round the
+sun, times which differed according to their position. For instance, a
+spot near the equator of the sun took twenty-five days to make the
+circuit, while one higher up or lower down took twenty-six days, and one
+further out twenty-seven; so that if these spots are, as certainly
+believed, actually on the surface, the conclusion is that the sun does
+not rotate all in one piece, but that some parts go faster than others.
+No one can really explain how this could be, but it is certainly more
+easily understood in the case of a body of gas than of a solid body,
+when it would be simply impossible to conceive. The spots seem to keep
+principally a little north and a little south of the equator; there are
+very few actually at it, and none found near the poles, but no reason
+for this distribution has been discovered. It has been noted that about
+every eleven years the greatest number of spots appears, and that
+they become fewer again, mounting up in number to the next eleven years,
+and so on. All these curious facts show there is much yet to be solved
+about the sun. The spots were supposed for long to be eruptions bursting
+up above the surface, but now they are generally held to be deep
+depressions like saucers, probably caused by violent tempests, and it is
+thought that the inrush of cooler matter from above makes them look
+darker than the other parts of the sun's surface. But when we use the
+words 'cooler' and 'darker,' we mean only by comparison, for in reality
+the dark parts of the spots are brighter than electric light.
+
+[Illustration: _Royal Observatory, Greenwich._
+
+SUN-SPOTS.]
+
+The fact that the spots are in reality depressions or holes is shown by
+their change of appearance as they pass over the face of the sun toward
+the edge; for the change of shape is exactly that which would be caused
+by foreshortening.
+
+It sounds odd to say that the best time for observing the sun is during
+a total eclipse, for then the sun's body is hidden by the moon. But yet
+to a certain extent this is true, and the reason is that the sun's own
+brilliance is our greatest hindrance in observing him, his rays are so
+dazzling that they light up our own atmosphere, which prevents us seeing
+the edges. Now, during a total eclipse, when nearly all the rays are
+cut off, we can see marvellous things, which are invisible at other
+times. But total eclipses are few and far between, and so when one is
+approaching astronomers make great preparations beforehand.
+
+A total eclipse is not visible from all parts of the world, but only
+from that small part on which the shadow of the moon falls, and as the
+earth travels, this shadow, which is really a round spot, passes along,
+making a dark band. In this band astronomers choose the best
+observatories, and there they take up their stations. The dark body of
+the moon first appears to cut a little piece out of the side of the sun,
+and as it sails on, gradually blotting out more and more, eager
+telescopes follow it; at last it covers up the whole sun, and then a
+marvellous spectacle appears, for all round the edges of the black moon
+are seen glorious red streamers and arches and filaments of marvellous
+shapes, continually changing. These are thrown against a background of
+pale green light that surrounds the black moon and the hidden sun. In
+early days astronomers thought these wonderful coloured streamers
+belonged to the moon; but it was soon proved that they really are part
+of the sun, and are only invisible at ordinary times, because our
+atmosphere is too bright to allow them to be seen. An instrument has
+now been invented to cut off most of the light of the sun, and when this
+is attached to a telescope these prominences, as they are called, can be
+seen at any time, so that there is no need to wait for an eclipse.
+
+[Illustration: THE EARTH AS IT WOULD APPEAR IN COMPARISON WITH THE
+FLAMES SHOOTING OUT FROM THE SUN.]
+
+What are these marvellous streamers and filaments? They are what they
+seem, eruptions of fiery matter discharged from the ever-palpitating sun
+thousands of miles into surrounding space. They are for ever shooting
+out and bursting and falling back, fireworks on a scale too enormous for
+us to conceive. Some of these brilliant flames extend for three hundred
+thousand miles, so that in comparison with one of them the whole world
+would be but a tiny ball, and this is going on day and night without
+cessation. Look at the picture where the artist has made a little black
+ball to represent the earth as she would appear if she could be seen in
+the midst of the flames shooting out from the sun. Do not make a mistake
+and think the earth really could be in this position; she is only shown
+there so that you may see how tiny she is in comparison with the sun.
+All the time you have lived and your father, and grandfather, and right
+back to the beginnings of English history, and far, far further into the
+dim ages, this stupendous exhibition of energy and power has continued,
+and only of late years has anyone known anything about it; even now a
+mere handful of people do know, and the rest, who are warmed and fed and
+kept alive by the gracious beams of this great revolving glowing
+fireball, never give it a thought.
+
+I said just now a pale green halo surrounded the sun, extending far
+beyond the prominences; this is called the corona and can only be seen
+during an eclipse. It surrounds the sun in a kind of shell, and there is
+reason to believe that it too is made of luminous stuff ejected by the
+sun in its burning fury. It is composed of large streamers or filaments,
+which seem to shoot out in all directions; generally these are not much
+larger than the apparent width of the sun, but sometimes they extend
+much further. The puzzle is, this corona cannot be an atmosphere in any
+way resembling that of our earth; for the gravitational force of the
+sun, owing to its enormous size, is so great that it would make any such
+atmosphere cling to it much more densely near to the surface, while it
+would be thinner higher up, and the corona is not dense in any way, but
+thin and tenuous throughout. This makes it very difficult to explain; it
+is supposed that some kind of electrical force enters into the problem,
+but what it is exactly we are far from knowing yet.
+
+
+
+
+CHAPTER VIII
+
+SHINING VISITORS
+
+
+Our solar system is set by itself in the midst of a great space, and so
+far as we have learnt about it in this book everything in it seems
+orderly: the planets go round the sun and the satellites go round the
+planets, in orbits more or less regular; there seems no place for
+anything else. But when we have considered the planets and the
+satellites, we have not exhausted all the bodies which own allegiance to
+the sun. There is another class, made up of strange and weird members,
+which flash in and out of the system, coming and going in all directions
+and at all times--sometimes appearing without warning, sometimes
+returning with a certain regularity, sometimes retiring to infinite
+depths of space, where no human eye will ever see them more. These
+strange visitors are called comets, and are of all shapes and sizes and
+never twice alike. Even as we watch them they grow and change, and then
+diminish in splendour. Some are so vast that men see them as flaming
+signs in the sky, and regard them with awe and wonder; some cannot be
+seen at all without the help of the telescope. From the very earliest
+ages those that were large enough to be seen without glasses have been
+regarded with astonishment. Men used to think that they were signs from
+heaven foretelling great events in the world. Timid people predicted
+that the end of the world would come by collision with one of them.
+Others, again, fancifully likened them to fishes in that sea of space in
+which we swim--fishes gigantic and terrifying, endowed with sense and
+will.
+
+It is perhaps unnecessary to say that comets are no more alive than is
+our own earth, and as for causing the end of the world by collision,
+there is every reason to believe the earth has been more than once right
+through a comet's tail, and yet no one except scientific men even
+discovered it. These mysterious visitors from the outer regions of space
+were called comets from a Greek word signifying hair, for they often
+leave a long luminous trail behind, which resembles the filaments of a
+woman's hair. It is not often that one appears large and bright enough
+to be seen by the naked eye, and when it does it is not likely to be
+soon forgotten. In the year 1910 such a comet is expected, a comet
+which at its former appearance compelled universal attention by its
+brilliancy and strangeness. At the time of the Norman Conquest of
+England a comet believed to be the very same one was stretching its
+glorious tail half across the sky, and the Normans seeing it, took it as
+a good omen, fancying that it foretold their success. The history of the
+Norman Conquest was worked in tapestry--that is to say, in what we
+should call crewels on a strip of linen--and in this record the comet
+duly appears. Look at him in the picture as the Normans fancied him. He
+has a red head with blue flames starting from it, and several tails. The
+little group of men on the left are pointing and chattering about him.
+We can judge what an impression this comet must have made to be recorded
+in such an important piece of work.
+
+[Illustration: THE COMET IN THE BAYEUX TAPESTRY.]
+
+But we are getting on too fast. We have yet to learn how anyone can know
+that the comet which appeared at the time of the Norman Conquest is the
+same as that which has come back again at different times, and above
+all, how anyone can tell that it will come again in the year 1910. All
+this involves a long story.
+
+Before the invention of telescopes of course only those comets could be
+seen which were of great size and fine appearance. In those days men
+did not realize that our world was but one of a number and of no great
+importance except to ourselves, and they always took these blazing
+appearances in the heavens as a particular warning to the human race.
+But when astronomers, by the aid of the telescope, found that for one
+comet seen by the eye there were hundreds which no mortal eye unaided
+could see, this idea seemed, to say the least of it, unlikely. Yet even
+then comets were looked upon as capricious visitors from outer space;
+odd creatures drawn into our system by the attraction of the sun, who
+disappeared, never to return. It was Newton, the same genius who
+disclosed to us the laws of gravity, who first declared that comets
+moved in orbits, only that these orbits were far more erratic than any
+of those followed by the planets.
+
+So far we have supposed that the planets were all on what we should call
+a level--that is to say, we have regarded them as if they were floating
+in a sea of water around the sun; but this is only approximately
+correct, for the orbits of the planets are not all at one level. If you
+had a number of slender hoops or rings to represent the planetary
+orbits, you would have to tilt one a little this way and another a
+little that way, only never so far but that a line through the centre
+of the hoop from one side to another could pass through the sun. The way
+in which the planetary orbits are tilted is slight in comparison with
+that of the orbits of comets, for these are at all sorts of angles--some
+turned almost sideways, and others slanting, and all of them are
+ellipses long drawn out and much more irregular than the planetary
+orbits; but erratic as they are, in every case a line drawn through the
+sun and extended both ways would touch each side of the orbits.
+
+A great astronomer called Halley, who was born in the time of the
+Commonwealth, was lucky enough to see a very brilliant comet, and the
+sight interested him so much that he made all the calculations necessary
+to find out just in what direction it was travelling in the heavens. He
+found out that it followed an ellipse which brought it very near to the
+sun at one part of its journey, and carried it far beyond the orbit of
+the earth, right out to that of Neptune, at the other. Then he began to
+search the records for other comets which had been observed before his
+time. He found that two particularly bright ones had been carefully
+noted--one about seventy-five years before that which he had seen, and
+the other seventy-five years before that again. Both these comets had
+been watched so scientifically that the paths in which they had
+travelled could be computed. A brilliant inspiration came to Halley. He
+believed that instead of these three, his own and the other two, being
+different comets, they were the same one, which returned to the sun
+about every seventy-five years. This could be proved, for if this idea
+were correct, of course the comet would return again in another
+seventy-five years, unless something unforeseen occurred. But Halley was
+in the prime of life: he could not hope to live to see his forecast
+verified. The only thing he could do was to note down exact particulars,
+by means of which others who lived after him might recognize his comet.
+And so when the time came for its return, though Halley was in his
+grave, numbers of astronomers were watching eagerly to see the
+fulfilment of his prediction. The comet did indeed appear, and since
+then it has been seen once again, and now we expect it to come back in
+the year 1910, when you and I may see it for ourselves. When the
+identity of the comet was fully established men began to search further
+back still, to compare the records of other previous brilliant comets,
+and found that this one had been noticed many times before, and once as
+I said, at the time of the Norman Conquest. Halley's comet is peculiar
+in many ways. For instance, it is unusual that so large and interesting
+a comet should return within a comparatively limited time. It is the
+smaller comets, those that can only be seen telescopically, that usually
+run in small orbits. The smallest orbits take about three and a half
+years to traverse, and some of the largest orbits known require a period
+of one hundred and ten thousand years. Between these two limits lies
+every possible variety of period. One comet, seen about the time
+Napoleon was born, was calculated to take two thousand years to complete
+its journey, and another, a very brilliant one seen in 1882, must
+journey for eight hundred years before it again comes near to the sun.
+But we never know what might happen, for at any moment a comet which has
+traversed a long solitary pathway in outer darkness may flash suddenly
+into our ken, and be for the first time noted and recorded, before
+flying off at an angle which must take it for ever further and further
+from the sun.
+
+Everything connected with comets is mysterious and most fascinating.
+From out of the icy regions of space a body appears; what it is we know
+not, but it is seen at first as a hairy or softly-glowing star, and it
+was thus that Herschel mistook Uranus for a comet when he first
+discovered it. As it draws nearer the comet sends out some fan-like
+projections toward the sun, enclosing its nucleus in filmy wrappings
+like a cocoon of light, and it travels faster and faster. From its head
+shoots out a tail--it may be more than one--growing in splendour and
+width, and always pointing away from the sun. So enormous are some of
+these tails that when the comet's head is close to the sun the tail
+extends far beyond the orbit of the earth. Faster still and faster flies
+the comet, for as we have seen it is a consequence of the law of
+gravitation that the nearer planets are to the sun the faster they move
+in their orbits, and the same rule applies to comets too. As the comet
+dashes up to the sun his pace becomes something indescribable; it has
+been reckoned for some comets at three hundred miles a second! But
+behold, as the head flies round the sun the tail is always projected
+outwards. The nucleus or head may be so near to the sun that the heat it
+receives would be sufficient to reduce molten iron to vapour; but this
+does not seem to affect it: only the tail expands. Sometimes it becomes
+two or more tails, and as it sweeps round behind the head it has to
+cover a much greater space in the same time, and therefore it must
+travel even faster than the head. The pace is such that no calculations
+can account for it, if the tail is composed of matter in any sense as we
+know it. Then when the sun is passed the comet sinks away again, and as
+it goes the tail dies down and finally disappears. The comet itself
+dwindles to a hairy star once more and goes--whither? Into space so
+remote that we cannot even dream of it--far away into cold more
+appalling than anything we could measure, the cold of absolute space.
+More and more slowly it travels, always away and away, until the sun, a
+short time back a huge furnace covering all the sky, is now but a faint
+star. Thus on its lonely journey unseen and unknown the comet goes.
+
+This comet which we have taken as an illustration is a typical one, but
+all are not the same. Some have no tails at all, and never develop any;
+some change utterly even as they are watched. The same comet is so
+different at different times that the only possible way of identifying
+it is by knowing its path, and even this is not a certain method, for
+some comets appear to travel at intervals along the same path!
+
+Now we come to the question that must have been in the mind of everyone
+from the beginning of this chapter, What are comets? This question no
+one can answer definitely, for there are many things so puzzling about
+these strange appearances that it is difficult even to suggest an
+explanation. Yet a good deal is known. In the first place, we are
+certain that comets have very little density--that is to say, they are
+indescribably thin, thinner than the thinnest kind of gas; and air,
+which we always think so thin, would be almost like a blanket compared
+with the material of comets. This we judge because they exercise no sort
+of influence on any of the planetary bodies they draw near to, which
+they certainly would do if they were made of any kind of solid matter.
+They come sometimes very close to some of the planets. A comet was so
+near to Jupiter that it was actually in among his moons. The comet was
+violently agitated; he was pulled in fact right out of his old path, and
+has been going on a new one ever since; but he did not exercise the
+smallest effect on Jupiter, or even on the moons. And, as I said earlier
+in this chapter, we on the earth have been actually in the folds of a
+comet's tail. This astonishing fact happened in June, 1861. One evening
+after the sun had set a golden-yellow disc, surrounded with filmy
+wrappings, appeared in the sky. The sun's light, diffused throughout
+our atmosphere, had prevented its being seen sooner. This was apparently
+the comet's head. It is described as 'though a number of light, hazy
+clouds were floating around a miniature full moon.' From this a cone of
+light extended far up into the sky, and when the head disappeared below
+the horizon this tail was seen to reach to the zenith. But that was not
+all. Strange shafts of light seemed to hang right overhead, and could
+only be accounted for by supposing that they were caused by another tail
+hanging straight above us, so that we looked up at it foreshortened by
+perspective. The comet's head lay between the earth and the sun, and its
+tail, which extended over many millions of miles, stretched out behind
+in such a way that the earth must have gone right through it. The fact
+that the comet exercised no perceptible influence on the earth at all,
+and that there were not even any unaccountable magnetic storms or
+displays of electricity, may reassure us so that if ever we do again
+come in contact with one of these extremely fine, thin bodies, we need
+not be afraid.
+
+There is another way in which we can judge of the wonderful tenuity or
+thinness of comets--that is, that the smallest stars can be seen
+through their tails, even though those tails must be many thousands of
+miles in thickness. Now, if the tails were anything approaching the
+density of our own atmosphere, the stars when seen through them would
+appear to be moved out of their places. This sounds odd, and requires a
+word of explanation. The fact is that anything seen through any
+transparent medium like water or air is what is called refracted--that
+is to say, the rays coming from it look bent. Everyone is quite familiar
+with this in everyday life, though perhaps they may not have noticed it.
+You cannot thrust a stick into the water without seeing that it looks
+crooked. Air being less dense than water has not quite so strong a
+refracting power, but still it has some. We cannot prove it in just the
+same way, because we are all inside the atmosphere ourselves, and there
+is no possibility of thrusting a stick into it from the outside! The
+only way we know it is by looking at something which is 'outside'
+already, and we find plenty of objects in the sky. As a matter of fact,
+the stars are all a little pulled out of their places by being seen
+through the air, and though of course we do not notice this, astronomers
+know it and have to make allowance for it. The effect is most
+noticeable in the case of the sun when he is going down, for the
+atmosphere bends his rays up, and though we see him a great glowing red
+ball on the horizon, and watch him, as we think, drop gradually out of
+sight, we are really looking at him for the last moment or two when he
+has already gone, for the rays are bent up by the air and his image
+lingers when the real sun has disappeared.
+
+[Illustration: A STICK THRUST INTO THE WATER APPEARS CROOKED.]
+
+Therefore in looking through the luminous stuff that forms a comet's
+tail astronomers might well expect to see the stars displaced, but not a
+sign of this appears. It is difficult to imagine, therefore, what the
+tail can be made of. The idea is that the sun exercises a sort of
+repulsive effect on certain elements found in the comet's head--that is
+to say, it pushes them away, and that as the head approaches the sun,
+these elements are driven out of it away from the sun in vapour. This
+action may have something to do with electricity, which is yet little
+understood; anyway, the effect is that, instead of attracting the matter
+toward itself, in which case we should see the comet's tails stretching
+toward the sun, the sun drives it away! In the chapter on the sun we had
+to imagine something of the same kind to account for the corona, and the
+corona and the comet's tails may be really akin to each other, and
+could perhaps be explained in the same way. Now we come to a stranger
+fact still. Some comets go right through the sun's corona, and yet do
+not seem to be influenced by it in the smallest degree. This may not
+seem very wonderful at first perhaps, but if you remember that a dash
+through anything so dense as our atmosphere, at a pace much less than
+that at which a comet goes, is enough to heat iron to a white heat, and
+then make it fly off in vapour, we get a glimpse of the extreme fineness
+of the materials which make the corona.
+
+Here is Herschel's account of a comet that went very near the sun:
+
+'The comet's distance from the sun's centre was about the 160th part of
+our distance from it. All the heat we enjoy on this earth comes from the
+sun. Imagine the heat we should have to endure if the sun were to
+approach us, or we the sun, to one 160th part of its present distance.
+It would not be merely as if 160 suns were shining on us all at once,
+but, 160 times 160, according to a rule which is well known to all who
+are conversant with such matters. Now, that is 25,600. Only imagine a
+glare 25,600 times fiercer than that of the equatorial sunshine at noon
+day with the sun vertical. In such a heat there is no substance we know
+of which would not run like water, boil, and be converted into smoke or
+vapour. No wonder the comet gave evidence of violent excitement, coming
+from the cold region outside the planetary system torpid and ice-bound.
+Already when arrived even in our temperate region it began to show signs
+of internal activity; the head had begun to develop, and the tail to
+elongate, till the comet was for a time lost sight of--not for days
+afterwards was it seen; and its tail, whose direction was reversed, and
+which could not possibly be the same tail it had before, had already
+lengthened to an extent of about ninety millions of miles, so that it
+must have been shot out with immense force in a direction away from the
+sun.'
+
+We remember that comets have sometimes more than one tail, and a theory
+has been advanced to account for this too. It is supposed that perhaps
+different elements are thrust away by the sun at different angles, and
+one tail may be due to one element and another to another. But if the
+comet goes on tail-making to a large extent every time it returns to the
+sun, what happens eventually? Do the tails fall back again into the head
+when out of reach of the sun's action? Such an idea is inconceivable;
+but if not, then every time a comet approaches the sun he loses
+something, and that something is made up of the elements which were
+formerly in the head and have been violently ejected. If this be so we
+may well expect to see comets which have returned many times to the sun
+without tails at all, for all the tail-making stuff that was in the head
+will have been used up, and as this is exactly what we do see, the
+theory is probably true.
+
+Where do the comets come from? That also is a very large question. It
+used to be supposed they were merely wanderers in space who happened to
+have been attracted by our sun and drawn into his system, but there are
+facts which go very strongly against this, and astronomers now generally
+believe that comets really belong to the solar system, that their proper
+orbits are ellipses, and that in the case of those which fly off at such
+an angle that they can never return they must at some time have been
+pulled out of their original orbit by the influence of one of the
+planets.
+
+[Illustration: _Royal Observatory, Cape of Good Hope._
+
+A GREAT COMET.]
+
+To get a good idea of a really fine comet, until we have the opportunity
+of seeing one for ourselves, we cannot do better than look at this
+picture of a comet photographed in 1901 at the Cape of Good Hope. It is
+only comparatively recently that photography has been applied to comets.
+When Halley's comet appeared last time such a thing was not thought
+of, but when he comes again numbers of cameras, fitted up with all the
+latest scientific appliances, will be waiting to get good impressions of
+him.
+
+
+
+
+CHAPTER IX
+
+SHOOTING STARS AND FIERY BALLS
+
+
+All the substances which we are accustomed to see and handle in our
+daily lives belong to our world. There are vegetables which grow in the
+earth, minerals which are dug out of it, and elementary things, such as
+air and water, which have always made up a part of this planet since man
+knew it. These are obvious, but there are other things not quite so
+obvious which also help to form our world. Among these we may class all
+the elements known to chemists, many of which have difficult names, such
+as oxygen and hydrogen. These two are the elements which make up water,
+and oxygen is an important element in air, which has nitrogen in it too.
+There are numbers and numbers of other elements perfectly familiar to
+chemists, of which many people never even hear the names. We live in the
+midst of these things, and we take them for granted and pay little
+attention to them; but when we begin to learn about other worlds we at
+once want to know if these substances and elements which enter so
+largely into our daily lives are to be found elsewhere in the universe
+or are quite peculiar to our own world. This question might be answered
+in several ways, but one of the most practical tests would be if we
+could get hold of something which had not been always on the earth, but
+had fallen upon it from space. Then, if this body were made up of
+elements corresponding with those we find here, we might judge that
+these elements are very generally diffused throughout the bodies in the
+solar system.
+
+It sounds in the highest degree improbable that anything should come
+hurling through the air and alight on our little planet, which we know
+is a mere speck in a great ocean of space; but we must not forget that
+the power of gravity increases the chances greatly, for anything coming
+within a certain range of the earth, anything small enough, that is, and
+not travelling at too great a pace, is bound to fall on to it. And,
+however improbable it seems, it is undoubtedly true that masses of
+matter do crash down upon the earth from time to time, and these are
+called meteorites. When we think of the great expanse of the oceans, of
+the ice round the poles, and of the desert wastes, we know that for
+every one of such bodies seen to fall many more must have fallen unseen
+by any human being. Meteors large enough to reach the earth are not very
+frequent, which is perhaps as well, and as yet there is no record of
+anyone's having been killed by them. Most of them consist of masses of
+stone, and a few are of iron, while various substances resembling those
+that we know here have been found in them. Chemists in analyzing them
+have also come across certain elements so far unknown upon earth, though
+of course there is no saying that these may not exist at depths to which
+man has not penetrated.
+
+A really large meteor is a grand sight. If it is seen at night it
+appears as a red star, growing rapidly bigger and leaving a trail of
+luminous vapour behind as it passes across the sky. In the daytime this
+vapour looks like a cloud. As the meteor hurls itself along there may be
+a deep continuous roar, ending in one supreme explosion, or perhaps in
+several explosions, and finally the meteor may come to the earth in one
+mass, with a force so great that it buries itself some feet deep in the
+soil, or it may burst into numbers of tiny fragments, which are
+scattered over a large area. When a meteor is found soon after its fall
+it is very hot, and all its surface has 'run,' having been fused by
+heat. The heat is caused by the friction of our atmosphere. The meteor
+gets entangled in the atmosphere, and, being drawn by the attraction of
+the earth, dashes through it. Part of the energy of its motion is turned
+to heat, which grows greater and greater as the denser air nearer to the
+earth is encountered; so that in time all the surface of the meteor runs
+like liquid, and this liquid, rising to a still higher temperature, is
+blown off in vapour, leaving a new surface exposed. The vapour makes the
+trail of fire or cloud seen to follow the meteor. If the process went on
+for long the meteor would be all dissipated in vapour, and in any case
+it must reach the earth considerably reduced in size.
+
+Numbers and numbers of comparatively small ones disappear, and for every
+one that manages to come to earth there must be hundreds seen only as
+shooting stars, which vanish and 'leave not a wrack behind.' When a
+meteor is seen to fall it is traced, and, whenever possible, it is found
+and placed in a museum. Men have sometimes come across large masses of
+stone and iron with their surfaces fused with heat. These are in every
+way like the recognized meteorites, except that no eye has noted their
+advent. As there can be no reasonable doubt that they are of the same
+origin as the others, they too are collected and placed in museums, and
+in any large museum you would be able to see both kinds--those which
+have been seen to come to earth and those which have been found
+accidentally.
+
+The meteors which appear very brilliant in their course across the sky
+are sometimes called fire-balls, which is only another name for the same
+thing. Some of these are brighter than the full moon, so bright that
+they cause objects on earth to cast a shadow. In 1803 a fiery ball was
+noticed above a small town in Normandy; it burst and scattered stones
+far and wide, but luckily no one was hurt. The largest meteorites that
+have been found on the earth are a ton or more in weight; others are
+mere stones; and others again just dust that floats about in the
+atmosphere before gently settling. Of course, meteors of this last kind
+could not be seen to fall like the larger ones, yet they do fall in such
+numbers that calculations have been made showing that the earth must
+catch about a hundred millions of meteors daily, having altogether a
+total weight of about a hundred tons. This sounds enormous, but
+compared with the weight of the earth it is very small indeed.
+
+Now that we have arrived at the fact that strange bodies do come
+hurtling down upon us out of space, and that we can actually handle and
+examine them, the next question is, Where do they come from? At one time
+it was thought that they were fragments which had been flung off by the
+earth herself when she was subject to violent explosions, and that they
+had been thrown far enough to resist the impulse to drop down upon her
+again, and had been circling round the sun ever since, until the earth
+came in contact with them again and they had fallen back upon her. It is
+not difficult to imagine a force which would be powerful enough to
+achieve the feat of speeding something off at such a velocity that it
+passed beyond the earth's power to pull it back, but nothing that we
+have on earth would be nearly strong enough to achieve such a feat.
+Imaginative writers have pictured a projectile hurled from a cannon's
+mouth with such tremendous force that it not only passed beyond the
+range of the earth's power to pull it back, but so that it fell within
+the influence of the moon and was precipitated on to her surface! Such
+things must remain achievements in imagination only; it is not possible
+for them to be carried out. Other ideas as to the origin of meteors
+were that they had been expelled from the moon or from the sun. It would
+need a much less force to send a projectile away from the moon than from
+the earth on account of its smaller size and less density, but the
+distance from the earth to the moon is not very great, and any
+projectile hurled forth from the moon would cross it in a comparatively
+short time. Therefore if the meteorites come from the moon, the moon
+must be expelling them still, and we might expect to see some evidence
+of it; but we know that the moon is a dead world, so this explanation is
+not possible. The sun, for its part, is torn by such gigantic
+disturbances that, notwithstanding its vast size, there is no doubt
+sufficient force there to send meteors even so far as the earth, but the
+chances of their encountering the earth would be small. Both these
+theories are now discarded. It is believed that the meteors are merely
+lesser fragments of the same kind of materials as the planets, circling
+independently round the sun; and a proof of this is that far more
+meteorites fall on that part of the earth which is facing forward in its
+journey than on that behind, and this is what we should expect if the
+meteors were scattered independently through space and it was by reason
+of our movements that we came in contact with them. There is no need to
+explain this further. Everyone knows that in cycling or driving along a
+road where there is a good deal of traffic both ways the people we meet
+are more in number than those who overtake us, and the same result would
+follow with the meteors; that is to say, in travelling through space
+where they were fairly evenly distributed we should meet more than we
+should be overtaken by.
+
+You remember that it was suggested the sun's fuel might be obtained from
+meteors, and this was proved to be not possible, even though there are
+no doubt unknown millions of these strange bodies circling throughout
+the solar system.
+
+There are so many names for these flashing bodies that we may get a
+little confused: when they are seen in the sky they are meteors, or
+fire-balls; when they reach the earth they are called meteorites, and
+also aerolites. Then there is another class of the same bodies called
+shooting stars, and these are in reality only meteors on a smaller
+scale; but there ought to be no confusion in our thoughts, for all these
+objects are small bodies travelling round the sun, and caught by the
+earth's influence.
+
+When you watch the sky for some time on a clear night, you will seldom
+fail to see at least one star flash out suddenly in a path of thrilling
+light and disappear, and you cannot be certain whether that star had
+been shining in the sky a minute before, or if it had appeared suddenly
+only in order to go out. The last idea is right. We must get rid at once
+of the notion that it would be possible for any fixed star to behave in
+this manner. To begin with, the fixed stars are many of them actually
+travelling at a great velocity at present, yet so immeasurably distant
+are they that their movement makes no perceptible difference to us. For
+one of them to appear to dash across the heavens as a meteor does would
+mean a velocity entirely unknown to us, even comparing it with the speed
+of light. No, these shooting stars are not stars at all, though they
+were so named, long before the real motions of the fixed stars were even
+dimly guessed at. As we have seen, they belong to the same class as
+meteors.
+
+I remember being told by a clergyman, years ago, that one night in
+November he had gone up to bed very late, and as he pulled up his blind
+to look at the sky, to his amazement he saw a perfect hail of shooting
+stars, some appearing every minute, and all darting in vivid trails of
+light, longer or shorter, though all seemed to come from one point. So
+marvellous was the sight that he dashed across the village street,
+unlocked the church door, and himself pulled the bell with all his
+might. The people in that quiet country village had long been in bed,
+but they huddled on their clothes and ran out of their pretty thatched
+cottages, thinking there must be a great fire, and when they saw the
+wonder in the sky they were amazed and cried out that the world must be
+coming to an end. The clergyman knew better than that, and was able to
+reassure them, and tell them he had only taken the most effectual means
+of waking them so that they might not miss the display, for he was sure
+as long as they lived they would never see such another sight. A star
+shower of this kind is certainly well worth getting up to see, but
+though uncommon it is not unique. There are many records of such showers
+having occurred in times gone by, and when men put together and examined
+the records they found that the showers came at regular intervals. For
+instance, every year about the same time in November there is a star
+shower, not comparable, it is true, with the brilliant one the clergyman
+saw, but still noticeable, for more shooting stars are seen then than at
+other times, and once in every thirty-three years there is a specially
+fine one. It happened in fact to be one of these that the village people
+were wakened up to see.
+
+Not all at once, but gradually, the mystery of these shower displays was
+solved. It was realized that the meteors need not necessarily come from
+one fixed place in the sky because they seemed to us to do so, for that
+was only an effect of perspective. If you were looking down a long,
+perfectly straight avenue of tree-trunks, the avenue would seem to close
+in, to get narrower and narrower at the far end until it became a point;
+but it would not really do so, for you would know that the trees at the
+far end were just the same distance from each other as those between
+which you were standing. Now, two meteors starting from the same
+direction at a distance from each other, and keeping parallel, would
+seem to us to start from a point and to open out wider and wider as they
+approached, but they would not really do so; it would only be, as in the
+case of the avenue, an effect of perspective. If a great many meteors
+did the same thing, they would appear to us all to start from one point,
+whereas really they would be on parallel lines, only as they rushed to
+meet us or we rushed to meet them this effect would be produced.
+Therefore the first discovery was that these meteors were thousands and
+thousands of little bodies travelling in lines parallel to each other,
+like a swarm of little planets. To judge that their path was not a
+straight line but a circle or ellipse was the next step, and this was
+found to be the case. From taking exact measurements of their paths in
+the sky an astronomer computed they were really travelling round the sun
+in a lengthened orbit, an ellipse more like a comet's orbit than that of
+a planet. But next came the puzzling question, Why did the earth
+apparently hit them every year to some extent, and once in thirty-three
+years seem to run right into the middle of them? This also was answered.
+One has only to imagine a swarm of such meteors at first hastening
+busily along their orbit, a great cluster all together, then, by the
+near neighbourhood of some planet, or by some other disturbing causes,
+being drawn out, leaving stragglers lagging behind, until at last there
+might be some all round the path, but only thinly scattered, while the
+busy, important cluster that formed the nucleus was still much thicker
+than any other part. Now, if the orbit that the meteors followed cut the
+orbit or path of the earth at one point, then every time the earth came
+to what we may call the level crossing she must run into some of the
+stragglers, and if the chief part of the swarm took thirty-three years
+to get round, then once in about thirty-three years the earth must
+strike right into it. This would account for the wonderful display. So
+long drawn-out is the thickest part of the swarm that it takes a year to
+pass the points at the level crossing. If the earth strikes it near the
+front one year, she may come right round in time to strike into the rear
+part of the swarm next year, so that we may get fine displays two years
+running about every thirty-three years. The last time we passed through
+the swarm was in 1899, and then the show was very disappointing. Here in
+England thick clouds prevented our seeing much, and there will not be
+another chance for us to see it at its best until 1932.
+
+These November meteors are called Leonids, because they _seem_ to come
+from a group of stars named Leo, and though the most noticeable they are
+not the only ones. A shower of the same kind occurs in August too, but
+the August meteors, called Perseids, because they seem to come from
+Perseus, revolve in an orbit which takes a hundred and forty-two years
+to traverse! So that only every one hundred and forty-second year could
+we hope to see a good display. When all these facts had been gathered
+up, it seemed without doubt that certain groups of meteors travelled in
+company along an elliptical orbit. But there remained still something
+more--a bold and ingenious theory to be advanced. It was found that a
+comet, a small one, only to be seen with the telescope, revolved in
+exactly the same orbit as the November meteors, and another one, larger,
+in exactly the same orbit as the August ones; hence it could hardly be
+doubted that comets and meteors had some connection with each other,
+though what that connection is exactly no one knows. Anyway, we can have
+no shadow of doubt when we find the comet following a marked path, and
+the meteors pursuing the same path in his wake, that the two have some
+mysterious affinity. There are other smaller showers besides these of
+November and August, and a remarkable fact is known about one of them.
+This particular stream was found to be connected with a comet named
+Biela's Comet, that had been many times observed, and which returned
+about every seven years to the sun. After it had been seen several
+times, this astonishing comet split in two and appeared as two comets,
+both of which returned at the end of the next seven years. But on the
+next two occasions when they were expected they never came at all, and
+the third time there came instead a fine display of shooting stars, so
+it really seemed as if these meteors must be the fragments of the lost
+comet.
+
+It is very curious and interesting to notice that in these star showers
+there is no certain record of any large meteorite reaching the earth;
+they seem to be made up of such small bodies that they are all
+dissipated in vapour as they traverse our air.
+
+
+
+
+CHAPTER X
+
+THE GLITTERING HEAVENS
+
+
+On a clear moonless night the stars appear uncountable. You see them
+twinkling through the leafless trees, and covering all the sky from the
+zenith, the highest point above your head, down to the horizon. It seems
+as if someone had taken a gigantic pepper-pot and scattered them far and
+wide so that some had fallen in all directions. If you were asked to
+make a guess as to how many you can see at one time, no doubt you would
+answer 'Millions!' But you would be quite wrong, for the number of stars
+that can be seen at once without a telescope does not exceed two
+thousand, and this, after the large figures we have been dealing with,
+appears a mere trifle. With a telescope, even of small power, many more
+are revealed, and every increase in the size of the telescope shows more
+still; so that it might be supposed the universe is indeed illimitable,
+and that we are only prevented from seeing beyond a certain point by our
+limited resources. But in reality we know that this cannot be so. If
+the whole sky were one mass of stars, as it must be if the number of
+them were infinite, then, even though we could not distinguish the
+separate items, we should see it bright with a pervading and diffused
+light. As this is not so, we judge that the universe is not unending,
+though, with all our inventions, we may never be able to probe to the
+end of it. We need not, indeed, cry for infinity, for the distances of
+the fixed stars from us are so immeasurable that to atoms like ourselves
+they may well seem unlimited. Our solar system is set by itself, like a
+little island in space, and far, far away on all sides are other great
+light-giving suns resembling our own more or less, but dwindled to the
+size of tiny stars, by reason of the great void of space lying between
+us and them. Our sun is, indeed, just a star, and by no means large
+compared with the average of the stars either. But, then, he is our own;
+he is comparatively near to us, and so to us he appears magnificent and
+unique. Judging from the solar system, we might expect to find that
+these other great suns which we call stars have also planets circling
+round them, looking to them for light and heat as we do to our sun.
+There is no reason to doubt that in some instances the conjecture is
+right, and that there may be other suns with attendant planets. It is
+however a great mistake to suppose that because our particular family in
+the solar system is built on certain lines, all the other families must
+be made on the same pattern. Why, even in our own system we can see how
+very much the planets differ from each other: there are no two the same
+size; some have moons and some have not; Saturn's rings are quite
+peculiar to himself, and Uranus and Neptune indulge in strange vagaries.
+So why should we expect other systems to be less varied?
+
+As science has advanced, the idea that these faraway suns must have
+planetary attendants as our sun has been discarded. The more we know the
+more is disclosed to us the infinite variety of the universe. For
+instance, so much accustomed are we to a yellow sun that we never think
+of the possibility of there being one of another colour. What would you
+say then to a ruby sun, or a blue one; or to two suns of different
+colours, perhaps red and green, circling round each other; or to two
+such suns each going round a dark companion? For there are dark bodies
+as well as shining bodies in the sky. These are some of the marvels of
+the starry sky, marvels quite as absorbing as anything we have found in
+the solar system.
+
+It requires great care and patience and infinite labour before the very
+delicate observations which alone can reveal to us anything of the
+nature of the fixed stars can be accomplished. It is only since the
+improvement in large telescopes that this kind of work has become
+possible, and so it is but recently men have begun to study the stars
+intimately, and even now they are baffled by indescribable difficulties.
+One of these is our inability to tell the distance of a thing by merely
+looking at it unless we also know its size. On earth we are used to
+seeing things appear smaller the further they are from us, and by long
+habit can generally tell the real size; but when we turn to the stars,
+which appear so much alike, how are we to judge how far off they are?
+Two stars apparently the same size and close together in the sky may
+really be as far one from another as the earth is from the nearest; for
+if the further one were very much larger than the nearer, they would
+then appear the same size.
+
+At first it was natural enough to suppose that the big bright stars of
+what we call the first magnitude were the nearest to us, and the less
+bright the next nearest, and so on down to the tiny ones, only revealed
+by the telescope, which would be the furthest away of all; but research
+has shown that this is not correct. Some of the brightest stars may be
+comparatively near, and some of the smallest may be near also. The size
+is no test of distance. So far as we have been able to discover, the
+star which seems nearest _is_ a first magnitude one, but some of the
+others which outshine it must be among the infinitely distant ones. Thus
+we lie in the centre of a jewelled universe, and cannot tell even the
+size of the jewels which cover its radiant robe.
+
+I say 'lie,' but that is really not the correct word. So far as we have
+been able to find out, there is no such thing as absolute rest in the
+universe--in fact, it is impossible; for even supposing any body could
+be motionless at first, it would be drawn by the attraction of its
+nearest neighbours in space, and gradually gain a greater and greater
+velocity as it fell toward them. Even the stars we call 'fixed' are all
+hurrying along at a great pace, and though their distance prevents us
+from seeing any change in their positions, it can be measured by
+suitable instruments. Our sun is no exception to this universal rule.
+Like all his compeers, he is hurrying busily along somewhere in
+obedience to some impulse of which we do not know the nature; and as he
+goes he carries with him his whole cortčge of planets and their
+satellites, and even the comets. Yes, we are racing through space with
+another motion, too, besides those of rotation and revolution, for our
+earth keeps up with its master attractor, the sun. It is difficult, no
+doubt, to follow this, but if you think for a moment you will remember
+that when you are in a railway-carriage everything in that carriage is
+really travelling along with it, though it does not appear to move. And
+the whole solar system may be looked at as if it were one block in
+movement. As in a carriage, the different bodies in it continue their
+own movements all the time, while sharing in the common movement. You
+can get up and change your seat in the train, and when you sit down
+again you have not only moved that little way of which you are
+conscious, but a great way of which you are not conscious unless you
+look out of the window. Now in the case of the earth's own motion we
+found it necessary to look for something which does not share in that
+motion for purposes of comparison, and we found that something in the
+sun, who shows us very clearly we are turning on our axis. But in the
+case of the motion of the solar system the sun is moving himself, so we
+have to look beyond him again and turn to the stars for confirmation.
+Then we find that the stars have motions of their own, so that it is
+very difficult to judge by them at all. It is as if you were bicycling
+swiftly towards a number of people all walking about in different
+directions on a wide lawn. They have their movements, but they all also
+have an apparent movement, really caused by you as you advance toward
+them; and what astronomers had to do was to separate the true movements
+of the stars from the false apparent movement made by the advance of the
+sun. This great problem was attacked and overcome, and it is now known
+with tolerable certainty that the sun is sweeping onward at a pace of
+about twelve miles a second toward a fixed point. It really matters very
+little to us where he is going, for the distances are so vast that
+hundreds of years must elapse before his movement makes the slightest
+difference in regard to the stars. But there is one thing which we can
+judge, and that is that though his course appears to be in a straight
+line, it is most probably only a part of a great curve so huge that the
+little bit we know seems straight.
+
+When we speak of the stars, we ought to keep quite clearly in our minds
+the fact that they lie at such an incredible distance from us that it is
+probable we shall never learn a great deal about them. Why, men have not
+even yet been able to communicate with the planet Mars, at its nearest
+only some thirty-five million miles from us, and this is a mere nothing
+in measuring the space between us and the stars. To express the
+distances of the stars in figures is really a waste of time, so
+astronomers have invented another way. You know that light can go round
+the world eight times in a second; that is a speed quite beyond our
+comprehension, but we just accept it. Then think what a distance it
+could travel in an hour, in a day; and what about a year? The distance
+that light can travel in a year is taken as a convenient measure by
+astronomers for sounding the depths of space. Measured in this way light
+takes four years and four months to reach us from the nearest star we
+know of, and there are others so much more distant that hundreds--nay,
+thousands--of years would have to be used to convey it. Light which has
+been travelling along with a velocity quite beyond thought, silently,
+unresting, from the time when the Britons lived and ran half naked on
+this island of ours, has only reached us now, and there is no limit to
+the time we may go back in our imaginings. We see the stars, not as they
+are, but as they were. If some gigantic conflagration had happened a
+hundred years ago in one of them situated a hundred light-years away
+from us, only now would that messenger, swifter than any messenger we
+know, have brought the news of it to us. To put the matter in figures,
+we are sure that no star can lie nearer to us than twenty-five billions
+of miles. A billion is a million millions, and is represented by a
+figure with twelve noughts behind it, so--1,000,000,000,000; and
+twenty-five such billions is the least distance within which any star
+can lie. How much farther away stars may be we know not, but it is
+something to have found out even that. On the same scale as that we took
+in our first example, we might express it thus: If the earth were a
+greengage plum at a distance of about three hundred of your steps from
+the sun, and Neptune were, on the same scale, about three miles away,
+the nearest fixed star could not be nearer than the distance measured
+round the whole earth at the Equator!
+
+All this must provoke the question, How can anyone find out these
+things? Well, for a long time the problem of the distances of the stars
+was thought to be too difficult for anyone to attempt to solve it, but
+at last an ingenious method was devised, a method which shows once more
+the triumph of man's mind over difficulties. In practice this method is
+extremely difficult to carry out, for it is complicated by so many other
+things which must be made allowance for; but in theory, roughly
+explained, it is not too hard for anyone to grasp. The way of it is
+this: If you hold up your finger so as to cover exactly some object a
+few feet distant from you, and shut first one eye and then the other,
+you will find that the finger has apparently shifted very considerably
+against the background. The finger has not really moved, but as seen
+from one eye or the other, it is thrown on a different part of the
+background, and so appears to jump; then if you draw two imaginary
+lines, one from each eye to the finger, and another between the two
+eyes, you will have made a triangle. Now, all of you who have done a
+little Euclid know that if you can ascertain the length of one side of a
+triangle, and the angles at each end of it, you can form the rest of the
+triangle; that is to say, you can tell the length of the other two
+sides. In this instance the base line, as it is called--that is to say
+the line lying between the two eyes--can easily be measured, and the
+angles at each end can be found by an instrument called a sextant, so
+that by simple calculation anyone could find out what distance the
+finger was from the eye. Now, some ingenious man decided to apply this
+method to the stars. He knew that it is only objects quite near to us
+that will appear to shift with so small a base line as that between the
+eyes, and that the further away anything is the longer must the base
+line be before it makes any difference. But this clever man thought that
+if he could only get a base line long enough he could easily compute the
+distance of the stars from the amount that they appeared to shift
+against their background. He knew that the longest base line he could
+get on earth would be about eight thousand miles, as that is the
+diameter of the earth from one side to the other; so he carefully
+observed a star from one end of this immense base line and then from the
+other, quite confident that this plan would answer. But what happened?
+After careful observations he discovered that no star moved at all with
+this base line, and that it must be ever so much longer in order to make
+any impression. Then indeed the case seemed hopeless, for here we are
+tied to the earth and we cannot get away into space. But the astronomer
+was nothing daunted. He knew that in its journey round the sun the earth
+travels in an orbit which measures about one hundred and eighty-five
+millions of miles across, so he resolved to take observations of the
+stars when the earth was at one side of this great circle, and again,
+six months later, when she had travelled to the other side. Then indeed
+he would have a magnificent base line, one of one hundred and
+eighty-five millions of miles in length. What was the result? Even with
+this mighty line the stars are found to be so distant that many do not
+move at all, not even when measured with the finest instruments, and
+others move, it may be, the breadth of a hair at a distance of several
+feet! But even this delicate measure, a hair's-breadth, tells its own
+tale; it lays down a limit of twenty-five billion miles within which no
+star can lie!
+
+This system which I have explained to you is called finding the star's
+parallax, and perhaps it is easier to understand when we put it the
+other way round and say that the hair's-breadth is what the whole orbit
+of the earth would appear to have shrunk to if it were seen from the
+distance of these stars!
+
+Many, many stars have now been examined, and of them all our nearest
+neighbour seems to be a bright star seen in the Southern Hemisphere. It
+is in the constellation or star group called Centaurus, and is the
+brightest star in it. In order to designate the stars when it is
+necessary to refer to them, astronomers have invented a system. To only
+the very brightest are proper names attached; others are noted according
+to the degree of their brightness, and called after the letters of the
+Greek alphabet: alpha, beta, gamma, delta, etc. Our own word 'alphabet'
+comes, you know, from the first two letters of this Greek series. As
+this particular star is the brightest in the constellation Centaurus, it
+is called Alpha Centauri; and if ever you travel into the Southern
+Hemisphere and see it, you may greet it as our nearest neighbour in the
+starry universe, so far as we know at present.
+
+
+
+
+CHAPTER XI
+
+THE CONSTELLATIONS
+
+
+From the very earliest times men have watched the stars, felt their
+mysterious influence, tried to discover what they were, and noted their
+rising and setting. They classified them into groups, called
+constellations, and gave such groups the names of figures and animals,
+according to the positions of the stars composing them. Some of these
+imaginary figures seem to us so wildly ridiculous that we cannot
+conceive how anyone could have gone so far out of their way as to invent
+them. But they have been long sanctioned by custom, so now, though we
+find it difficult to recognize in scattered groups of stars any likeness
+to a fish or a ram or a bear; we still call the constellations by their
+old names for convenience in referring to them.
+
+Supposing the axis of the earth were quite upright, straight up and down
+in regard to the plane at which the earth goes round the sun, then we
+should always see the same set of stars from the Northern and the same
+set of stars from the Southern Hemispheres all the year round. But as
+the axis is tilted slightly, we can, during our nights in the winter in
+the Northern Hemisphere, see more of the sky to the south than we can in
+the summer; and in the Southern Hemisphere just the reverse is the case,
+far more stars to the north can be seen in the winter than in the
+summer. But always, whether it is winter or summer, there is one fixed
+point in each hemisphere round which all the other stars seem to swing,
+and this is the point immediately over the North or the South Poles.
+There is, luckily, a bright star almost at the point at which the North
+Pole would seem to strike the sky were it infinitely lengthened. This is
+not one of the brightest stars in the sky, but quite bright enough to
+serve the purpose, and if we stand with our faces towards it, we can be
+sure we are looking due north. How can we discover this star for
+ourselves in the sky? Go out on any starlight night when the sky is
+clear, and see if you can find a very conspicuous set of seven stars
+called the Great Bear. I shall not describe the Great Bear, because
+every child ought to know it already, and if they don't, they can ask
+the first grown-up person they meet, and they will certainly be told.
+(See map.)
+
+[Illustration: CONSTELLATIONS NEAR THE POLE STAR.]
+
+Having found the Great Bear, you have only to draw an imaginary line
+between the two last stars forming the square on the side away from the
+tail, and carry it on about three times as far as the distance between
+those two stars, and you will come straight to the Pole Star. The two
+stars in the Great Bear which help one to find it are called the
+Pointers, because they point to it.
+
+The Great Bear is one of the constellations known from the oldest times;
+it is also sometimes called Charles's Wain, the Dipper, or the Plough.
+It is always easily seen in England, and seems to swing round the Pole
+Star as if held by an invisible rope tied to the Pointers. Besides the
+Great Bear there is, not far from it, the Little Bear, which is really
+very like it, only smaller and harder to find. The Pole Star is the last
+star in its tail; from it two small stars lead away parallel to the
+Great Bear, and they bring the eye to a small pair which form one side
+of a square just like that in the Great Bear. But the whole of the
+Little Bear is turned the opposite way from the Great Bear, and the tail
+points in the opposite direction. And when you come to think of it,
+it is very ridiculous to have called these groups Bears at all, or to
+talk about tails, for bears have no tails! So it would have been better
+to have called them foxes or dogs, or almost any other animal rather
+than bears.
+
+Now, if you look at the sky on the opposite side of the Pole Star from
+the Great Bear, you will see a clearly marked capital W made up of five
+or six bright stars. This is called Cassiopeia, or the Lady's Chair.
+
+In looking at Cassiopeia you cannot help noticing that there is a zone
+or broad band of very many stars, some exceedingly small, which
+apparently runs right across the sky like a ragged hoop, and Cassiopeia
+seems to be set in or on it. This band is called the Milky Way, and
+crosses not only our northern sky, but the southern sky too, thus making
+a broad girdle round the whole universe. It is very wonderful, and no
+one has yet been able to explain it. The belt is not uniform and even,
+but it is here and there broken up into streamers and chips, having the
+same appearance as a piece of ribbon which has been snipped about by
+scissors in pure mischief; or it may be compared to a great river broken
+up into many channels by rocks and obstacles in its course.
+
+The Milky Way is mainly made up of thousands and thousands of small
+stars, and many more are revealed by the telescope; but, as we see in
+Cassiopeia, there are large bright stars in it too, though, of course,
+these may be infinitely nearer to us, and may only appear to us to be in
+the Milky Way because they are between us and it.
+
+Now, besides the few constellations that I have mentioned, there are
+numbers of others, some of which are difficult to discover, as they
+contain no bright stars. But there are certain constellations which
+every one should know, because in them may be found some of the
+brightest stars, those of the first magnitude. Magnitude means size, and
+it is really absurd for us to say a star is of the first magnitude
+simply because it appears to us to be large, for, as I have explained
+already, a small star comparatively near to us might appear larger than
+a greater one further away. But the word 'magnitude' was used when men
+really thought stars were large or small according to their appearance,
+and so it is used to this day. They called the biggest and brightest
+first magnitude stars. Of these there are not many, only some twenty, in
+all the sky. The next brightest--about the brightness of the Pole Star
+and the stars in the Great Bear--are of the second magnitude, and so
+on, each magnitude containing stars less and less bright. When we come
+to stars of the sixth magnitude we have reached the limit of our sight,
+for seventh magnitude stars can only be seen with a telescope. Now that
+we understand what is meant by the magnitude, we can go back to the
+constellations and try to find some more.
+
+If you draw an imaginary line across the two stars forming the backbone
+of the Bear, starting from the end nearest the tail, and continue it
+onward for a good distance, you will come to a very bright star called
+Capella, which you will know, because near it are three little ones in a
+triangle. Now, Capella means a goat, so the small ones are called the
+kids. In winter Capella gets high up into the sky, and then there is to
+be seen below her a little cluster called the Pleiades. There is nothing
+else like this in the whole sky. It is formed of six stars, as it
+appears to persons of ordinary sight, and these stars are of the sixth
+magnitude, the lowest that can be seen by the naked eye. But though
+small, they are set so close together, and appear so brilliant,
+twinkling like diamonds, that they are one of the most noticeable
+objects in the heavens. A legend tells that there were once seven stars
+in the Pleiades clearly visible, and that one has now disappeared. This
+is sometimes spoken of as 'the lost Pleiad,' but there does not seem to
+be any foundation for the story. In old days people attached particular
+holiness or luck to the number seven, and possibly, when they found that
+there were only six stars in this wonderful group, they invented the
+story about the seventh.
+
+As the Pleiades rise, a beautiful reddish star of the first magnitude
+rises beneath them. It is called Aldebaran, and it, as well as the
+Pleiades, forms a part of the constellation of Taurus the bull. In
+England we can see in winter below Aldebaran the whole of the
+constellation of Orion, one of the finest of all the constellations,
+both for the number of the bright stars it contains and for the extent
+of the sky it covers. Four bright stars at wide distances enclose an
+irregular four-sided space in which are set three others close together
+and slanting downwards. Below these, again, are another three which seem
+to fall from them, but are not so bright. The figure of Orion as drawn
+in the old representations of the constellations is a very magnificent
+one. The three bright stars form his belt, and the three smaller ones
+the hilt of his sword hanging from it.
+
+[Illustration: ORION AND HIS NEIGHBOURS.]
+
+If you draw an imaginary line through the stars forming the belt and
+prolong it downwards slantingly, you will see, in the very height of
+winter, the brightest star in all the sky, either in the Northern or
+Southern Hemisphere. This is Sirius, who stands in a class quite by
+himself, for he is many times brighter than any other first magnitude
+star. He never rises very high above the horizon here, but on crisp,
+frosty nights may be seen gleaming like a big diamond between the
+leafless twigs and boughs of the rime-encrusted trees. Sirius is the Dog
+Star, and it is perhaps fortunate that, as he is placed, he can be seen
+sometimes in the southern and sometimes in the northern skies, so that
+many more people have a chance of looking at his wonderful brilliancy,
+than if he had been placed near the Pole star. In speaking of the
+supreme brightness of Sirius among the stars, we must remember that
+Venus and Jupiter, which outrival him, are not stars, but planets, and
+that they are much nearer to us. Sirius is so distant that the measures
+for parallax make hardly any impression on him, but, by repeated
+experiments, it has now been proved that light takes more than eight
+years to travel from him to us. So that, if you are eight years old, you
+are looking at Sirius as he was when you were a baby!
+
+Not far from the Pleiades, to the left as you face them, are to be
+found two bright stars nearly the same size; these are the Heavenly
+Twins, or Gemini.
+
+Returning now to the Great Bear, we find, if we draw a line through the
+middle and last stars of his tail, and carry it on for a little
+distance, we come fairly near to a cluster of stars in the form of a
+horseshoe; there is only one fairly bright one in it, and some of the
+others are quite small, but yet the horseshoe is distinct and very
+beautiful to look at. This is the Northern Crown. The very bright star
+not far from it is another first-class star called Arcturus.
+
+To the left of the Northern Crown lies Hercules, which is only mentioned
+because near it is the point to which the sun with all his system
+appears at present to be speeding.
+
+For other fascinating constellations, such as Leo or the Lion, Andromeda
+and Perseus, and the three bright stars by which we recognize Aquila the
+Eagle, you must wait awhile, unless you can get some one to point them
+out.
+
+Those which you have noted already are enough to lead you on to search
+for more.
+
+Perhaps some of you who live in towns and can see only a little strip of
+sky from the nursery or schoolroom windows have already found this
+chapter dull, and if so you may skip the rest of it and go on to the
+next. For the others, however, there is one more thing to know before
+leaving the subject, and that is the names of the string of
+constellations forming what is called the Zodiac. You may have heard the
+rhyme:
+
+  'The Ram, the Bull, the Heavenly Twins,
+  And next the Crab, the Lion shines,
+  The Virgin and the Scales;
+  The Scorpion, Archer, and He-goat,
+  The Man that holds the watering-pot,
+  The Fish with glittering tails.'
+
+This puts in a form easy to remember the signs of the constellations
+which lie in the Zodiac, an imaginary belt across the whole heavens. It
+is very difficult to explain the Zodiac, but I must try. Imagine for a
+moment the earth moving round its orbit with the sun in the middle. Now,
+as the earth moves the sun will be seen continually against a different
+background--that is to say, he will appear to us to move not only across
+our sky in a day by reason of our rotation, but also along the sky,
+changing his position among the stars by reason of our revolution. You
+will say at once that we cannot see the stars when the sun is there, and
+no more we can. But the stars are there all the same, and every month
+the sun seems to have moved on into a new constellation, according to
+astronomers' reckoning. If you count up the names of the constellations
+in the rhyme, you will find that there are just twelve, one for each
+month, and at the end of the year the sun has come round to the first
+one again. The first one is Aries the Ram, and the sun is seen projected
+or thrown against that part of the sky where Aries is, in April, when we
+begin spring; this is the first month to astronomers, and not January,
+as you might suppose. Perhaps you will learn to recognize all the
+constellations in the Zodiac one day; a few of them, such as the Bull
+and the Heavenly Twins, you know already if you have followed this
+chapter.
+
+
+
+
+CHAPTER XII
+
+WHAT THE STARS ARE MADE OF
+
+
+How can we possibly tell what the stars are made of? If we think of the
+vast oceans of space lying between them and us, and realize that we can
+never cross those oceans, for in them there is no air, it would seem to
+be a hopeless task to find out anything about the stars at all. But even
+though we cannot traverse space ourselves, there is a messenger that
+can, a messenger that needs no air to sustain him, that moves more
+swiftly than our feeble minds can comprehend, and this messenger brings
+us tidings of the stars--his name is Light. Light tells us many
+marvellous things, and not the least marvellous is the news he gives us
+of the workings of another force, the force of gravitation. In some ways
+gravitation is perhaps more wonderful than light, for though light
+speeds across airless space, it is stopped at once by any opaque
+substance--that is to say, any substance not transparent, as you know
+very well by your own shadows, which are caused by your bodies stopping
+the light of the sun. Light striking on one side of the earth does not
+penetrate through to the other, whereas gravitation does. You remember,
+of course, what the force of gravitation is, for we read about that very
+early in this book. It is a mysterious attraction existing between all
+matter. Every atom pulls every other atom towards itself, more or less
+strongly according to distance. Now, solid matter itself makes no
+difference to the force of gravitation, which acts through it as though
+it were not there. The sun is pulling the earth toward itself, and it
+pulls the atoms on the far side of the earth just as strongly as it
+would if there were nothing lying between it and them. Therefore, unlike
+light, gravitation takes no heed of obstacles in the way, but acts in
+spite of them. The gravitation of the earth holds you down just the
+same, though you are on the upper floor of a house, with many layers of
+wood and plaster between you and it. It cannot pull you down, for the
+floor holds you up, but it is gravitation that keeps your feet on the
+ground all the same. A clever man made up a story about some one who
+invented a kind of stuff which stopped the force of gravitation going
+through it, just as a solid body stops light; when this stuff was made,
+of course, it went right away off into space, carrying with it anyone
+who stood on it, as there was nothing to hold it to the earth! That was
+only a story, and it is not likely anyone could invent such stuff, but
+it serves to make clear the working of gravitation. These two tireless
+forces, light and gravitation, run throughout the whole universe, and
+carry messages of tremendous importance for those who have minds to
+grasp them. Without light we could know nothing of these distant worlds,
+and without understanding the laws of gravity we should not be able to
+interpret much that light tells us.
+
+To begin with light, what can we learn from it? We turn at once to our
+own great light-giver, the sun, to whom we owe not only all life, but
+also all the colour and beauty on earth. It is well known to men of
+science that colour lies in the light itself, and not in any particular
+object. That brilliant blue cloak of yours is not blue of itself, but
+because of the light that falls on it. If you cannot believe this, go
+into a room lighted only by gas, and hey, presto! the colour is changed
+as if it were a conjuring trick. You cannot tell now by looking at the
+cloak whether it is blue or green! Therefore you must admit that as the
+colour changes with the change of light it must be due to light, and not
+to any quality belonging to the material of the cloak. But, you may
+protest, if the colour is solely due to light, and light falls on
+everything alike, why are there so many colours? That is a very fair
+question. If the light that comes from the sun were of only one
+colour--say blue or red--then everything would be blue or red all the
+world over. Some doors in houses are made with a strip of red or blue
+glass running down the sides. If you have one in your house like that,
+go and look through it, and you will see an astonishing world made up of
+different tones of the same colour. Everything is red or blue, according
+to the colour of the glass, and the only difference in the appearance of
+objects lies in the different shades, whether things are light or dark.
+This is a world as it might appear if the sun's rays were only blue or
+only red. But the sun's light is not of one colour only, fortunately for
+us; it is of all the colours mixed together, which, seen in a mass, make
+the effect of white light. Now, objects on earth are only either seen by
+the reflected light of the sun or by some artificial light. They have no
+light of their own. Put them in the dark and they do not shine at all;
+you cannot see them. It is the sun's light striking on them that makes
+them visible. But all objects do not reflect the light equally, and this
+is because they have the power of absorbing some of the rays that strike
+on them and not giving them back at all, and only those rays that are
+given back show to the eye. A white thing gives back all the rays, and
+so looks white, for we have the whole of the sun's light returned to us
+again. But how about a blue thing? It absorbs all the rays except the
+blue, so that the blue rays are the only ones that come back or rebound
+from it again to meet our eyes, and this makes us see the object blue;
+and this is the case with all the other colours. A red object retains
+all rays except the red, which it sends back to us; a yellow object
+gives back only the yellow rays, and so on. What an extraordinary and
+mysterious fact! Imagine a brilliant flower-garden in autumn. Here we
+have tall yellow sunflowers with velvety brown centres, clustering pink
+and crimson hollyhocks, deep red and bright yellow peonies, slender
+fairy-like Japanese anemones, great bunches of mauve Michaelmas daisies,
+and countless others, and mingled with all these are many shades of
+green. Yet it is the light of the sun alone that falling on all these
+varied objects, makes that glorious blaze of colour; it seems
+incredible. It may be difficult to believe, but it is true beyond all
+doubt. Each delicate velvety petal has some quality in it which causes
+it to absorb certain of the sun's rays and send back the others, and its
+colour is determined by those it sends back.
+
+Well then how infinitely varied must be the colours hidden in the sun's
+light, colours which, mixed all together, make white light! Yes, this is
+so, for all colours that we know are to be found there. In fact, the
+colours that make up sunlight are the colours to be seen in the rainbow,
+and they run in the same order. Have you ever looked carefully at a
+rainbow? If not, do so at the next chance. You will see it begins by
+being dark blue at one end, and passes through all colours until it gets
+to red at the other.
+
+We cannot see a rainbow every day just when we want to, but we can see
+miniature rainbows which contain just the same colours as the real ones
+in a number of things any time the sun shines. For instance, in the
+cut-glass edge of an inkstand or a decanter, or in one of those
+old-fashioned hanging pieces of cut-glass that dangle from the
+chandelier or candle-brackets. Of course you have often seen these
+colours reflected on the wall, and tried to get them to shine upon your
+face. Or you have caught sight of a brilliant patch of colour on the
+wall and looked around to see what caused it, finally tracing it to some
+thick edge of shining glass standing in the sunlight. Now, the cut-glass
+edge shows these colours to you because it breaks up the light that
+falls upon it into the colours it is made of, and lets each one come out
+separately, so that they form a band of bright colours instead of just
+one ray of white light.
+
+This is perhaps a little difficult to understand, but I will try to
+explain. When a ray of white light falls on such a piece of glass, which
+is known as a prism, it goes in as white light at one side, but the
+three-cornered shape of the glass breaks it up into the colours it is
+made of, and each colour comes out separately at the other side--namely,
+from blue to red--like a little rainbow, and instead of one ray of white
+light, we have a broad band of all the colours that light is made of.
+
+Who would ever have thought a pretty plaything like this could have told
+us what we so much wanted to know--namely, what the sun and the stars
+are made of? It seems too marvellous to be true, yet true it is that for
+ages and ages light has been carrying its silent messages to our eyes,
+and only recently men have learnt to interpret them. It is as if some
+telegraph operator had been going steadily on, click, click, click, for
+years and years, and no one had noticed him until someone learnt the
+code of dot and dash in which he worked, and then all at once what he
+was saying became clear. The chief instrument in translating the message
+that the light brings is simply a prism, a three-cornered wedge of
+glass, just the same as those hanging lustres belonging to the
+chandeliers. When a piece of glass like this is fixed in a telescope in
+such a way that the sun's rays fall on it, then there is thrown on to a
+piece of paper or any other suitable background a broad coloured band of
+lovely light like a little rainbow, and this is called the sun's
+spectrum, and the instrument by which it is seen is called a
+spectroscope. But this in itself could tell us little; the message it
+brings lies in the fact that when it has passed through the telescope,
+so that it is magnified, it is crossed by hundreds of minute black
+lines, not placed evenly at all, but scattered up and down. There may be
+two so close together that they look like one, and then three far apart,
+and then some more at different distances. When this remarkable
+appearance was examined carefully it was found that in sunlight the
+lines that appeared were always exactly the same, in the same places,
+and this seemed so curious that men began to seek for an explanation.
+
+Someone thought of an experiment which might teach us something about
+the matter, and instead of letting sunlight fall on the prism, he made
+an artificial light by burning some stuff called sodium, and then
+allowed the band of coloured light to pass through the telescope; when
+he examined the spectrum that resulted, he found that, though numbers of
+lines to be found in the sun's spectrum were missing, there were a few
+lines here exactly matching a few of the lines in the sun's spectrum;
+and this could not be the result of chance only, for the lines are so
+mathematically exact, and are in themselves so peculiarly distributed,
+that it could only mean that they were due to the same cause. What could
+this signify, then, but that away up there in the sun, among other
+things, stuff called sodium, very well known to chemists on earth, is
+burning? After this many other substances were heated white-hot so as to
+give out light, in order to discover if the lines to be seen in their
+spectra were also to be found in the sun's spectrum. One of these was
+iron, and, astonishing to say, all the many little thread-like lines
+that appeared in its spectrum were reproduced to a hair's-breadth, among
+others, in the sun's spectrum. So we have found out beyond all
+possibility of doubt some of the materials of which the sun is made. We
+know that iron, sodium, hydrogen, and numerous other substances and
+elements, are all burning away there in a terrific furnace, to which any
+furnace we have on earth is but as the flicker of a match.
+
+It was not, of course, much use applying this method to the planets, for
+we know that the light which comes from them to us is only reflected
+sunlight, and this, indeed, was proved by means of the spectroscope. But
+the stars shine by their own light, and this opened up a wide field for
+inquiry. The difficulty was, of course, to get the light of one star
+separated from all the rest, because the light of one star is very faint
+and feeble to cast a spectrum at all. Yet by infinite patience
+difficulties were overcome. One star alone was allowed to throw its
+light into the telescope; the light passed through a prism, and showed a
+faint band of many colours, with the expected little black lines cutting
+across it more or less thickly. Examinations have thus been made of
+hundreds of stars. In the course of them some substances as yet unknown
+to us on earth have been encountered, and in some stars one
+element--hydrogen--is much stronger than in others; but, on the whole,
+speaking broadly, it has been satisfactorily shown that the stars are
+made on the same principles as our own sun, so that the reasoning of
+astronomers which had argued them to be suns was proved.
+
+[Illustration: THE SPECTRUM OF THE SUN AND SIRIUS.]
+
+We have here in the picture the spectrum of the sun and the spectrum of
+Arcturus. You can see that the lines which appear in the band of light
+belonging to Sirius are also in the band of light belonging to the sun,
+together with many others. This means that the substances flaming out
+and sending us light from the far away star are also giving out light
+from our own sun, and that the sun and Sirius both contain the same
+elements in their compositions.
+
+This, indeed, seems enough for the spectroscope to have accomplished; it
+has interpreted for us the message light brings from the stars, so that
+we know beyond all possibility of mistake that these glowing, twinkling
+points of light are brilliant suns in a state of intense heat, and that
+in them are burning elements with which we ourselves are quite familiar.
+But when the spectroscope had done that, its work was not finished, for
+it has not only told us what the stars are made of, but another thing
+which we could never have known without it--namely, if they are moving
+toward us or going away from us.
+
+
+
+
+CHAPTER XIII
+
+RESTLESS STARS
+
+
+You remember we have already remarked upon the difficulty of telling how
+far one star lies behind another, as we do not know their sizes. It is,
+to take another similar case, easy enough to tell if a star moves to one
+side or the other, but very difficult by ordinary observation to tell if
+it is advancing toward us or running away from us, for the only means we
+have of judging is if it gets larger or smaller, and at that enormous
+distance the fact whether it advances or recedes makes no difference in
+its size. Now, the spectroscope has changed all this, and we can tell
+quite as certainly if a star is coming toward us as we can if it moves
+to one side. I will try to explain this. You know, perhaps, that sound
+is caused by vibration in the air. The noise, whatever it is, jars the
+air and the vibrations strike on our ears. It is rather the same thing
+as the result of throwing a stone into a pond: from the centre of the
+splash little wavelets run out in ever-widening circles; so through the
+air run ever-widening vibrations from every sound. The more vibrations
+there are in a second the shriller is the note they make. In a high note
+the air-vibrations follow one another fast, pouring into one's ear at a
+terrific speed, so that the apparatus in the ear which receives them
+itself vibrates fiercely and records a high note, while a lower note
+brings fewer and slower vibrations in a second, and the ear is not so
+much disturbed. Have you ever noticed that if a railway engine is
+sweeping-toward you and screaming all the time, its note seems to get
+shriller and shriller? That is because the engine, in advancing, sends
+the vibrations out nearer to you, so more of them come in a second, and
+thus they are crowded up closer together, and are higher and higher.
+
+Now, light is also caused by waves, but they are not the same as sound
+waves. Light travels without air, whereas sound we know cannot travel
+without air, and is ever so much slower, and altogether a grosser,
+clumsier thing than light. But yet the waves or rays which make light
+correspond in some ways to the vibrations of sound. What corresponds to
+the treble on the piano is the blue end of the spectrum in light, and
+the bass is the red end. Now, when we are looking at the spectrum of
+any body which is advancing swiftly toward us, something of the same
+effect is observed as in the case of the shrieking engine. Take any star
+and imagine that that star is hastening toward us at a pace of three
+hundred miles a second, which is not at all an unusual rate for a star;
+then, if we examine the band of light, the spectrum, of such a star, we
+shall observe an extraordinary fact--all these little lines we have
+spoken of are shoved up toward the treble or blue end of the spectrum.
+They still remain just the same distances from each other, and are in
+twos and threes or single, so that the whole set of lines is unaltered
+as a set, but everyone of them is shifted a tiny fraction up toward the
+blue end of the spectrum, just a little displaced. Now if, instead of
+advancing toward us, this same star had been rushing away from us at a
+similar pace, all these lines would have been moved a tiny bit toward
+the red or bass end of the spectrum. This is known to be certainly true,
+so that by means of the spectroscope we can tell that some of these
+great sun-stars are advancing toward us and some receding from us,
+according to whether the multitudes of little lines in the spectrum are
+shifted slightly to the blue or the red end.
+
+You remember that it has been surmised that the pace the sun moves with
+his system is about twelve miles a second. This seems fast enough to us,
+who think that one mile a minute is good time for an express train, but
+it is slow compared with the pace of many of the stars. As I have said,
+some are travelling at a rate of between two hundred and three hundred
+miles a second; and it is due to the spectroscope that we know not only
+whether a star is advancing toward us or receding from us, but also
+whether the pace is great or not; it even tells us what the pace is, up
+to about half a mile a second, which is very marvellous. It is a curious
+fact that many of the small stars show greater movement than the large
+ones, which mayor may not mean that they are nearer to us.
+
+It may be taken as established that there is no such thing as absolute
+rest in the universe: everything, stars and nebulę alike, are moving
+somewhere; in an infinite variety of directions, with an infinite
+variety of speed they hasten this way and that. It would be impossible
+for any to remain still, for even supposing it had been so 'in the
+beginning,' the vast forces at work in the universe would not let it
+remain so. Out of space would come the persistent call of gravitation:
+atoms would cry silently to atoms. There could be no perfect equality
+of pull on all sides; from one side or another the pull would be the
+stronger. Slowly the inert mass would obey and begin falling toward it;
+it might be an inch at a time, but with rapid increase, until at last it
+also was hastening some whither in this universe which appears to us to
+be infinite.
+
+It must be remembered that these stars, even when moving at an enormous
+pace, do not change their places in the sky when regarded by ordinary
+observers. It would take thousands of years for any of the
+constellations to appear at all different from what they are now, even
+though the stars that compose them are moving in different directions
+with a great velocity, for a space of many millions of miles, at the
+distance of most of the stars, would be but as the breadth of a fine
+hair as seen by us on earth. So thousands of years ago men looked up at
+the Great Bear, and saw it apparently the same as we see it now; yet for
+all that length of time the stars composing it have been rushing in this
+direction and that at an enormous speed, but do not appear to us on the
+earth to alter their positions in regard to each other. I know of
+nothing that gives one a more overwhelming sense of the mightiness of
+the universe and the smallness of ourselves than this fact. From age to
+age men look on changeless heavens, yet this apparently stable universe
+is fuller of flux and reflux than is the restless ocean itself, and the
+very wavelets on the sea are not more numerous nor more restless than
+the stars that bestrew the sky.
+
+
+
+
+CHAPTER XIV
+
+THE COLOURS OF THE STARS
+
+
+Has it ever occurred to you that the stars are not all of the same
+colour? It is true that, just glancing at them casually, you might say
+they are all white; but if you examine them more carefully you cannot
+help seeing that some shine with a steely blue light, while others are
+reddish or yellowish. These colours are not easy to distinguish with the
+naked eye, and might not attract any attention at all unless they were
+pointed out; yet when attention is drawn to the fact, it is impossible
+to deny the redness of some, such as Aldebaran. But though we may admit
+this, we might add that the colours are so very faint and inconspicuous,
+that they might be, after all, only the result of imagination.
+
+To prove that the colours are constant and real we must use a telescope,
+and then we need have no further doubt of their reality, for instead of
+disappearing, the colours of some stars stand out quite vividly beyond
+the possibility of mistake. Red stars are a bright red, and they are the
+most easily seen of all, though the other colours, blue and yellow and
+green, are seen very decidedly by some people. The red stars have been
+described by various observers as resembling 'a drop of blood on a black
+field,' 'most magnificent copper-red,' 'most intense blood-red,' and
+'glowing like a live coal out of the darkness of space.' Some people see
+them as a shining red, like that of a glowing cloud at sunset. Therefore
+there can be no doubt that the colours are genuine enough, and are
+telling us some message. This message we are able to read, for we have
+begun to understand the language the stars speak to us by their light
+since the invention of the spectroscope. The spectroscope tells us that
+these colours indicate different stages in the development of the stars,
+or differences of constitution--that is to say, in the elements of which
+they are made. Our own sun is a yellow star, and other yellow stars are
+akin to him; while red and blue and green stars contain different
+elements, or elements in different proportions.
+
+Stars do not always remain the same colours for an indefinite time; one
+star may change slowly from yellow to white, and another from red to
+yellow; and there are instances of notable changes, such as that of the
+brilliant white Sirius, who was stated in old times by many different
+observers to be a red star. All this makes us think, and year by year
+thought leads us on to knowledge, and knowledge about these distant suns
+increases. But though we know a good deal now, there are still many
+questions we should like to ask which we cannot expect to have answered
+for a long time yet, if ever.
+
+The star colours have some meanings which we cannot even guess; we can
+only notice the facts regarding them. For instance, blue stars are never
+known to be solitary--they always have a companion, but why this should
+be so passes our comprehension. What is it in the constitution of a blue
+star which holds or attracts another? Whatever it may be, it is
+established by repeated instances that blue stars do not stand alone. In
+the constellation of Cygnus there are two stars, a blue and a yellow
+one, which are near enough to each other to be seen in the same
+telescope at the same time, and yet in reality are separated by an
+almost incredible number of billions of miles. But as we know that a
+blue star is never seen alone, and that it has often as its companion a
+yellowish or reddish star, it is probable that these two, situated at an
+enormous distance from one another, are yet in some mysterious way
+dependent on each other, and are not merely seen together because they
+happen to fall in the same field of view.
+
+Many double stars show most beautifully contrasted colours: among them
+are pairs of yellow and rose-red, golden and azure, orange and purple,
+orange and lilac, copper-colour and blue, apple-green and cherry-red,
+and so on. In the Southern Hemisphere there is a cluster containing so
+many stars of brilliant colours that Sir John Herschel named it 'the
+Jewelled Cluster.'
+
+I expect most of you have seen an advertisement of Pear's Soap, in which
+you are asked to stare at some red letters, and then look away to some
+white surface, such as a ceiling, when you will see the same letters in
+green. This is because green is the complementary or contrasting colour
+to red, and the same thing is the case with blue and yellow. When any
+one colour of either of these pairs is seen, it tends to make the other
+appear by reaction, and if the eye gazed hard at blue instead of red, it
+would next see yellow, and not green. Now, many people to whom this
+curious fact is known argue that perhaps the colours of the double
+stars are not real, but the effect of contrast only; for instance, they
+say a red star near a companion white one would tend to make the
+companion appear green, and so, of course, it would. But this does not
+account for the star colours, which are really inherent in the stars
+themselves, as may be proved by cutting off the light of one star, and
+looking only at the other, when its colour still appears unchanged.
+Another argument equally strong against the contrast theory is that the
+colours of stars in pairs are by no means always those which would
+appear if the effect was only due to complementary colours. It is not
+always blue and yellow or red and green pairs that we see, though these
+are frequent, but many others of various kinds, such as copper and blue,
+and ruddy and blue.
+
+We have therefore come to the conclusion that there are in this
+astonishing universe numbers of gloriously coloured suns, some of which
+apparently lie close together. What follows? Why, we want to know, of
+course, if these stars are really pairs connected with each other, or if
+they only appear so by being in the same line of sight, though one is
+infinitely more distant than the other. And that question also has been
+answered. There are now known thousands of cases in which stars,
+hitherto regarded as single, have been separated into two, or even more,
+by the use of a telescope. Of these thousands, some hundreds have been
+carefully investigated, and the result is that, though there are
+undoubtedly some in which the connexion is merely accidental, yet in by
+far the greater number of cases the two stars thus seen together have
+really some connexion which binds them to one another; they are
+dependent on one another. This has been made known to us by the working
+of the wonderful law of gravitation, which is obeyed throughout the
+whole universe. We know that by the operation of this law two mighty
+suns will be drawn toward each other with a certain pull, just as surely
+as we know that a stone let loose from the hand will fall upon the
+earth; so by noting the effect of two mighty suns upon each other many
+facts about them may be found out. By the most minute and careful
+measurements, by the use of the spectroscope, and by every resource
+known to science, astronomers have, indeed, actually found out with a
+near approach to exactness how far some of these great suns lie from
+each other, and how large they are in comparison with one another.
+
+The very first double star ever discovered was one which you have
+already seen, the middle one in the tail of the Great Bear. If you look
+at it you will be delighted to find that you can see a wee star close to
+it, and you will think you are looking at an example of a double star
+with your very own eyes; but you will be wrong, for that wee star is
+separated by untold distances from the large one to which it seems so
+near. In fact, any stars which can be seen to be separate by the naked
+eye must lie immeasurably far apart, however tiny seems the space
+between them. Such stars may possibly have some connexion with each
+other, but, at any rate in this case, such a connexion has not been
+proved. No, the larger star itself is made up of two others, which can
+only be seen apart in a telescope. Since this discovery double stars
+have been plentifully found in every part of the sky. The average space
+between such double stars as seen from our earth is--what do you think?
+It is the width of a single hair held up thirty-six feet from our eyes!
+This could not, of course, be seen without the use of a telescope or
+opera-glasses. It serves to give some impression of star distances when
+we think that the millions and millions of miles lying between those
+stars have shrunk to that hair's-breadth seen from our point of view.
+
+Twin stars circle together round a common centre of gravity, and are
+bound by the laws of gravitation just as the planets are. Our sun is a
+solitary star, with no companion, and therefore such a state of things
+seems to us to be incredible. Fancy two gigantic suns, one topaz-yellow
+and the other azure-blue, circling around in endless movement! Where in
+such a system would there be room for the planets? How could planets
+exist under the pull of two suns in opposite directions? Still more
+wonders are unfolded as the inquiry proceeds. Certain irregularities in
+the motions of some of these twin systems led astronomers to infer that
+they were acted upon by another body, though this other body was not
+discernible. In fact, though they could not see it, they knew it must be
+there, just as Adams and Leverrier knew of the existence of Neptune,
+before ever they had seen him, by the irregularities in the movements of
+Uranus. As the results showed, it was there, and was comparable in size
+to the twin suns it influenced, and yet they could not see it. So they
+concluded this third body must be dark, not light-giving like its
+companions. We are thus led to the strange conclusion that some of these
+systems are very complicated, and are formed not only of shining suns,
+but of huge dark bodies which cannot be called suns. What are they,
+then? Can they be immense planets? Is it possible that life may there
+exist? No fairy tale could stir the imagination so powerfully as the
+thought of such systems including a planetary body as large or larger
+than its sun or suns. If indeed life exists there, what a varied scene
+must be presented day by day! At one time both suns mingling their
+flashing rays may be together in the sky; at another time only one
+appears, a yellow or blue sun, as the case may be. The surface of such
+planets must undergo weird transformations, the foliage showing one day
+green, the next yellow, and the next blue; shadows of azure and orange
+will alternate! But fascinating as such thoughts are, we can get no
+further along that path.
+
+To turn from fancy to facts, we find that telescope and spectroscope
+have supplied us with quite enough matter for wonder without calling
+upon imagination. We have discovered that many of the stars which seem
+to shine with a pure single light are double, and many more consist not
+only of two stars, but of several, some of which may be dark bodies. The
+Pole Star was long known to be double, and is now discovered to have a
+third member in its system. These multiple systems vary from one
+another in almost every case. Some are made up of a mighty star and a
+comparatively small one; others are composed of stars equal in
+light-giving power--twin suns. Some progress swiftly round their orbits,
+some go slowly; indeed, so slowly that during the century they have been
+under observation only the very faintest sign of movement has been
+detected; and in other systems, which we are bound to suppose double,
+the stars are so slow in their movements that no progress seems to have
+been made at all.
+
+The star we know as the nearest to us in the heavens, Alpha Centauri, is
+composed of two very bright partners, which take about eighty-seven
+years to traverse their orbit. They sometimes come as near to each other
+as Saturn is to the sun. In the case of Sirius astronomers found out
+that he had a companion by reason of his irregularities of movement
+before they discovered that companion, which is apparently a very small
+star, only to be discerned with good telescopes. But here, again, it
+would be unwise to judge only by what we see. Though the star appears
+small, we know by the influence it exercises on Sirius that it is very
+nearly the same size as he is. Thus we judge that it is poor in
+light-giving property; in fact, its shining power is much less than that
+of its companion, though its size is so nearly equal. This is not
+wonderful, for Sirius's marvellous light-giving power is one of the
+wonders of the universe; he shines as brilliantly as twenty-nine or
+thirty of our suns!
+
+In some cases the dark body which we cannot see may even be larger than
+the shining one, through which alone we can know anything of it. Here we
+have a new idea, a hint that in some of these systems there may be a
+mighty earth with a smaller sun going round it, as men imagined our sun
+went around the earth before the real truth was found out.
+
+So we see that, when we speak of the stars as suns comparable with our
+sun, we cannot think of them all as being exactly on the same model.
+There are endless varieties in the systems; there are solitary suns like
+ours which may have a number of small planets going round them, as in
+the solar system; but there are also double suns going round each other,
+suns with mighty dark bodies revolving round them which may be planets,
+and huge dark bodies with small suns too. Every increase of knowledge
+opens up new wonders, and the world in which we live is but one kind of
+world amid an infinite number.
+
+In this chapter we have learnt an altogether new fact--the fact that the
+hosts of heaven comprise not only those shining stars we are accustomed
+to see, but also dark bodies equally massive, and probably equally
+numerous, which we cannot see. In fact, the regions of space may be
+strewn with such dark bodies, and we could have no possible means of
+discovering them unless they were near enough to some shining body to
+exert an influence upon it. It is not with his eyes alone, or with his
+senses, man knows of the existence of these great worlds, but often
+solely by the use of the powers of his mind.
+
+
+
+
+CHAPTER XV
+
+TEMPORARY AND VARIABLE STARS
+
+
+It is a clear night, nearly all the world is asleep, when an astronomer
+crosses his lawn on his way to his observatory to spend the dark hours
+in making investigations into profound space. His brilliant mind,
+following the rays of light which shoot from the furthest star, will
+traverse immeasurable distances, while the body is forgotten. Just
+before entering the observatory he pauses and looks up; his eye catches
+sight of something that arrests him, and he stops involuntarily. Yet any
+stranger standing beside him, and gazing where he gazes, would see
+nothing unusual. There is no fiery comet with its tail stretching across
+from zenith to horizon, no flaming meteor dashing across the darkened
+sky. But that there is something unusual to be seen is evident, for the
+astronomer breathes quickly, and after another earnest scrutiny of the
+object which has attracted him, he rushes into the observatory, searches
+for a star-chart, and examines attentively that part of the sky at
+which he has been gazing. He runs his finger over the chart: here and
+there are the well-known stars that mark that constellation, but here?
+In that part there is no star marked, yet he knows, for his own eyes
+have told him but a few moments ago, that here there is actually blazing
+a star, not large, perhaps, but clear enough to be seen without a
+telescope--a star, maybe, which no eye but his has yet observed!
+
+He hurries to his telescope, and adjusts it so as to bring the stranger
+into the field of view. A new star! Whence has it come? What does it
+mean?
+
+By the next day at the latest the news has flown over the wires, and all
+the scientific world is aware that a new star has been detected where no
+star ever was seen before. Hundreds of telescopes are turned on to it;
+its spectrum is noted, and it stands revealed as being in a state of
+conflagration, having blazed up from obscurity to conspicuousness. Night
+after night its brilliance grows, until it ranks with the brightest
+stars in heaven, and then it dies down and grows dim, gradually
+sinking--sinking into the obscurity from whence it emerged so briefly,
+and its place in the sky knows it no more. It may be there still, but
+so infinitely faint and far away that no power at our command can reveal
+it to us. And the amazing part of it is that this huge disaster, this
+mighty conflagration, is not actually happening as it is seen, but has
+happened many hundreds of years ago, though the message brought by the
+light carrier has but reached us now.
+
+There have not been a great many such outbursts recorded, though many
+may have taken place unrecorded, for even in these days, when trained
+observers are ceaselessly watching the sky, 'new' stars are not always
+noticed at once. In 1892 a new star appeared, and shone for two months
+before anyone noticed it. This particular one never rose to any very
+brilliant size. I twas situated in the constellation of Auriga, and was
+noticed on February 1. It remained fairly bright until March 6, when it
+began to die down; but it has now sunk so low that it can only be seen
+in the very largest telescopes.
+
+Photography has been most useful in recording these stars, for when one
+is noticed it has sometimes been found that it has been recorded on a
+photographic plate taken some time previously, and this shows us how
+long it has been visible. More and more photography becomes the useful
+handmaid of astronomers, for the photographic prepared plate is more
+sensitive to rays of light than the human eye, and, what is more useful
+still, such plates retain the rays that fall upon them, and fix the
+impression. Also on a plate these rays are cumulative--that is to say,
+if a very faint star shines continuously on a plate, the longer the
+plate is exposed, within certain limits, the clearer will the image of
+that star become, for the light rays fall one on the top of the other,
+and tend to enforce each other, and so emphasize the impression, whereas
+with our eyes it is not the same thing at all, for if we do not see an
+object clearly because it is too faint, we do not see it any better,
+however much we may stare at the place where it ought to be. This is
+because each light ray that reaches our eye makes its own impression,
+and passes on; they do not become heaped on each other, as they do on a
+photographic plate.
+
+One variable star in Perseus, discovered in 1901, rose to such
+brilliancy that for one night it was queen of the Northern Hemisphere,
+outshining all the other first-class stars.
+
+It rose into prominence with wonderful quickness, and sank equally fast.
+At its height it outshone our sun eight thousand times! This star was
+so far from us that it was reckoned its light must take about three
+hundred years to reach us, consequently the great conflagration, or
+whatever caused the outburst, must have taken place in the reign of
+James I., though, as it was only seen here in 1901, it was called the
+new star of the new century.
+
+When these new stars die down they sometimes continue to shine faintly
+for a long time, so that they are visible with a telescope, but in other
+cases they may die out altogether. We know very little about them, and
+have but small opportunity for observing them, and so it is not safe to
+hazard any theories to account for their peculiarities. At first men
+supposed that the great flame was made by a violent collision between
+two bodies coming together with great velocity so that both flared up,
+but this speculation has been shown by the spectroscope to be
+improbable, and now it is supposed by some people that two stars
+journeying through space may pass through a nebulous region, and thus
+may flare up, and such a theory is backed up by the fact that a very
+great number of such stars do seem to be mixed up in some strange way
+with a nebulous haze.
+
+All these new stars that we have been discussing so far have only
+blazed up once and then died down, but there is another class of stars
+quite as peculiar, and even more difficult to explain, and these are
+called variable stars. They get brighter and brighter up to a certain
+point, and then die down, only to become bright once more, and these
+changes occur with the utmost regularity, so that they are known and can
+be predicted beforehand. This is even more unaccountable than a sudden
+and unrepeated outburst, for one can understand a great flare-up, but
+that a star should flare and die down with regularity is almost beyond
+comprehension. Clearly we must look further than before for an
+explanation. Let us first examine the facts we know. Variable stars
+differ greatly from each other. Some are generally of a low magnitude,
+and only become bright for a short time, while others are bright most of
+the time and die down only for a short time. Others become very bright,
+then sink a little bit, but not so low as at first; then they become
+bright again, and, lastly, go right down to the lowest point, and they
+keep on always through this regular cycle of changes. Some go through
+the whole of these changes in three days, and others take much longer.
+The periods, as the intervals between the complete round of changes are
+called, vary, in fact, between three days and six hundred! It may seem
+impossible that changes covering so long as six hundred days could be
+known and followed, but there is nothing that the patience of
+astronomers will not compass.
+
+One very well-known variable star you can see for yourselves, and as an
+ounce of observation is worth a pound of hearsay, you might take a
+little trouble to find it. Go out on any clear starlight night and look.
+Not very far from Cassiopeia (W.), to the left as you face it, are three
+bright stars running down in a great curve. These are in the
+constellation called Perseus, and a little to the right of the middle
+and lowest one is the only variable star we can see in the sky without a
+telescope.
+
+This is Algol. For the greater part of three days he is a bright star of
+about the second magnitude, then he begins to fade, and for four and a
+half hours grows steadily dimmer. At the dimmest he remains for about
+twenty minutes, and then rises again to his ordinary brightness in three
+and a half hours. How can we explain this? You may possibly be able to
+suggest a reason. What do you say to a dark body revolving round Algol,
+or, rather, revolving with him round a common centre of gravity? If
+such a thing were indeed true, and if such a body happened to pass
+between us and Algol at each revolution, the light of Algol would be cut
+off or eclipsed in proportion to the size of such a body. If the dark
+body were the full size of Algol and passed right between him and us, it
+would cut off all the light, but if it were not quite the same size, a
+little would still be seen. And this is really the explanation of the
+strange changes in the brightness of Algol, for such a dark body as we
+are imagining does in reality exist. It is a large dark body, very
+nearly as large as Algol himself, and if, as we may conjecture, it is a
+mighty planet, we have the extraordinary example of a planet and its sun
+being nearly the same size. We have seen that the eclipse happens every
+three days, and this means, of course, that the planetary body must go
+round its sun in that time, so as to return again to its position
+between us and him, but the thing is difficult to believe. Why, the
+nearest of all our planets to the sun, the wee Mercury, takes
+eighty-seven days to complete its orbit, and here is a mighty body
+hastening round its sun in three! To do this in the time the large dark
+planet must be very near to Algol; indeed, astronomers have calculated
+that the surfaces of the two bodies are not more than about two million
+miles apart, and this is a trifle when we consider that we ourselves are
+more than forty-six times as far as that from the sun. At this distance
+Algol, as observed from the planet, will fill half the sky, and the heat
+he gives out must be something stupendous. Also the effects of
+gravitation must be queer indeed, acting on two such huge bodies so
+close together. If any beings live in such a strange world, the pull
+which draws them to their mighty sun must be very nearly equal to the
+pull which holds them to their own globe; the two together may
+counteract each other, but the effect must be strange!
+
+From irregularities in the movements of Algol it has been judged that
+there may be also in the same system another dark body, but of it
+nothing has been definitely ascertained.
+
+But all variable stars need not necessarily be due to the light being
+intercepted by a dark body. There are cases where two bright stars in
+revolving round each other produce the same effect; for when seen side
+by side the two stars give twice as much light as when one is hidden
+behind the other, and as they are seen alternately side by side and in
+line, they seem to alter regularly in lustre.
+
+
+
+
+CHAPTER XVI
+
+STAR CLUSTERS AND NEBULĘ
+
+
+Could you point out any star cluster in the sky? You could if you would
+only think for a minute, for one has been mentioned already. This is the
+cluster known as the Pleiades, and it is so peculiar and so different
+from anything else, that many people recognize the group and know where
+to look for it even before they know the Great Bear, the favourite
+constellation in the northern sky, itself. The Pleiades is a real star
+cluster, and the chief stars in it are at such enormous distances from
+one another that they can be seen separately by the eye unaided, whereas
+in most clusters the stars appear to be so close together that without a
+telescope they make a mere blur of brightness. For a long time it was
+supposed that the stars composing the Pleiades could not really be
+connected because of the great distances between them; for, as you know,
+even a hair's-breadth apparently between stars signifies in reality many
+millions of miles.
+
+Light travelling from the Pleiades to us, at that incomprehensible pace
+of which you already know, takes a hundred and ninety years to reach us!
+At this incredibly remote distance lies the main part of the cluster
+from us; but it is more marvellous still that we have every reason to
+believe that the outlying stars of this cluster are as far from the
+central ones as the nearest star we know of, Alpha Centauri, is from us!
+Little wonder was it, then, that men hesitated to ascribe to the
+Pleiades any real connection with each other, and supposed them to be
+merely an assemblage of stars which seemed to us to lie together.
+
+With the unaided eye we see comparatively few stars in the Pleiades. Six
+is the usual number to be counted, though people with very good sight
+have made out fourteen. Viewed through the telescope, however, the scene
+changes: into this part of space stars are crowded in astonishing
+profusion; it is impossible to count them, and with every increase in
+the power of the telescope still more are revealed. Well over a thousand
+in this small space seems no exaggerated estimate. Now, it is impossible
+to say how many of these really belong to the group, and how many are
+seen there accidentally, but observations of the most prominent ones
+have shown that they are all moving in exactly the same direction at
+the same pace. It would be against probability to conceive that such a
+thing could be the result of mere chance, considering the infinite
+variety of star movements in general, and so we are bound to believe
+that this wonderful collection of stars is a real group, and not only an
+apparent one.
+
+So splendid are the great suns that illuminate this mighty system, that
+at least fifty or sixty of them far surpass our own sun in brilliancy.
+Therefore when we look at that tiny sparkling group we must in
+imagination picture it as a vast cluster of mighty stars, all controlled
+and swayed by some dominant impulse, though separated by spaces enough
+to make the brain reel in thinking of them. If these suns possess also
+attendant planets, what a galaxy of worlds, what a universe within a
+universe is here!
+
+Other star clusters there are, not so conspicuous as the Pleiades, and
+most of these can only be seen through a telescope, so we may be
+thankful that we have one example so splendid within our own vision.
+There are some clusters so far and faintly shining that they were at
+first thought to be nebulę, and not stars at all; but the telescope
+gradually revealed the fact that many of these are made up of stars,
+and so people began to think that all faint shining patches of nebulous
+light were really star clusters, which would be resolved into stars if
+only we had better telescopes. Since the invention of the spectroscope,
+however, fresh light has been thrown on the matter, for the spectrum
+which is shown by some of the nebulous patches is not the same as that
+shown by stars, and we know that many of these strange appearances are
+not made up of infinitely distant stars.
+
+We are talking here quite freely about nebulę because we have met one
+long ago when we discussed the gradual evolution of our own system, and
+we know quite well that a nebula is composed of luminous faintly-glowing
+gas of extreme fineness and thinness. We see in the sky at the present
+time what we may take to be object-lessons in our own history, for we
+see nebulę of all sorts and sizes, and in some stars are mixed up, and
+in others stars are but dimly seen, so that it does not require a great
+stretch of the imagination to picture these stars as being born,
+emerging from the swaddling bands of filmy webs that have enwrapped
+them; and other nebulę seem to be gas only, thin and glowing, with no
+stars at all to be found in it. We still know very little about these
+mysterious appearances, but the work of classifying and resolving them
+is going on apace. Nebulę are divided into several classes, but the
+easiest distinction to remember is that between white nebulę and green
+nebulę. This is not to say that we can see some coloured green, but that
+green appears in the spectrum of some of the nebulę, while the spectrum
+of a white nebula is more like that of a star.
+
+It is fortunate for us that in the sky we can see without a telescope
+one instance of each of the several objects of interest that we have
+referred to.
+
+We have been able to see one very vivid example of a variable star; we
+have seen one very beautiful example of a star cluster; and it remains
+to look for one very good example of a white nebula.
+
+Just as in finding Algol you were doing a little bit of practical work,
+proving something of which you had read, so by seeing this nebula you
+will remember more about nebulę in general than by reading many chapters
+on the subject. This particular nebula is in Andromeda, and is not far
+from Algol; and it is not difficult to find. It is the only one that can
+be well seen without a telescope, and was known to the ancients; it is
+believed to have been mentioned in a book of the tenth century!
+
+If you take an imaginary line down from the two left-hand stars of
+Cassiopeia, and follow it carefully, you will come before long to a
+rather faint star, and close to it is the nebula.
+
+When you catch sight of it you will, perhaps, at first be disappointed,
+for all you will see is a soft blur of white, as if someone had laid a
+dab of luminous paint on the sky with a finger; but as you gaze at it
+night after night and realize its unchangeableness, realize also that it
+is a mass of glowing gas, an island in space, infinitely distant,
+unsupported and inexplicable, something of the wonder of it will creep
+over you.
+
+[Illustration: _Dr. Max Wolf._
+
+THE GREAT NEBULA IN ANDROMEDA.]
+
+Thousands of telescopic nebulę are now known, and have been examined,
+and they are of all shapes. Roughly, they have been divided up into
+several classes--those that seem to us to be round and those that are
+long ovals, like this one in Andromeda; but these may, of course, be
+only round ones seen edgewise by us; others are very irregular, and
+spread over an enormous part of the sky. The most remarkable of these is
+that in Orion, and if you look very hard at the middle star in the
+sword-hilt of Orion, you may be able to make out a faint mistiness.
+This, when seen through a telescope, becomes a wonderful and
+far-spreading nebula, with brighter and darker parts like gulfs in
+it, and dark channels. It has been sometimes called the Fish-mouth
+Nebula, from a fanciful idea as to its shape. Indeed, so extraordinarily
+varied are these curious structures, that they have been compared with
+numbers of different objects. We have some like brushes, others
+resembling fans, rings, spindles, keyholes; others like animals--a fish,
+a crab, an owl, and so on; but these suggestions are imaginative, and
+have nothing to do with the real problem. In _The System of the Stars_
+Miss Clerke says: 'In regarding these singular structures we seem to see
+surges and spray-flakes of a nebulous ocean, bewitched into sudden
+immobility; or a rack of tempest-driven clouds hanging in the sky,
+momentarily awaiting the transforming violence of a fresh onset.
+Sometimes continents of pale light are separated by narrow straits of
+comparative darkness; elsewhere obscure spaces are hemmed in by luminous
+inlets and channels.'
+
+One curious point about the Orion Nebula is that the star which seems to
+be in the midst of it resolves itself under the telescope into not one
+but six, of various sizes.
+
+Nebulę are in most cases too enormously remote from the earth for us to
+have any possible means of computing the distance; but we may take it
+that light must journey at least a thousand years to reach us from them,
+and in many cases much more. Therefore, if at the time of the Norman
+Conquest a nebula had begun to grow dim and fade away, it would, for all
+intents and purposes, still be there for us, and for those that come
+after us for several generations, though all that existed of it in
+reality would be its pale image fleeting onward through space in all
+directions in ever-widening circles.
+
+That nebulę do sometimes change we have evidence: there are cases in
+which some have grown indisputably brighter during the years they have
+been under observation, and some nebulę that have been recorded by
+careful observers seem to have vanished. When we consider that these
+strange bodies fill many, many times the area of our whole solar system
+to the outermost bounds of Neptune's orbit, it is difficult to imagine
+what force it is that acts on them to revive or quench their light. That
+that light is not the direct result of heat has long been known; it is
+probably some form of electric excitement causing luminosity, very much
+as it is caused in the comets. Indeed, many people have been tempted to
+think of the nebulę as the comets of the universe, and in some points
+there are, no doubt, strong resemblances between the two. Both shine in
+the same way, both are so faint and thin that stars can be seen through
+them; but the spectroscope shows us that to carry the idea too far would
+be wrong, as there are many differences in constitution.
+
+We have seen that there are dark stars as well as light stars; if so,
+may there not be dark nebulę as well as light ones? It may very well be
+so. We have seen that there are reasons for supposing our own system to
+have been at first a cool dark nebula rotating slowly. The heavens may
+be full of such bodies, but we could not discern them. Their thinness
+would prevent their hiding any stars that happened to be behind them. No
+evidence of their existence could possibly be brought to us by any
+channel that we know.
+
+It is true that, besides the dark rifts in the bright nebulę, which may
+themselves be caused by a darker and non-luminous gas, there are also
+strange rifts in the Milky Way, which at one time were conjectured to be
+due to a dark body intervening between us and the starry background.
+This idea is now quite discarded; whatever may cause them, it is not
+that. One of the most startling of these rifts is that called the
+Coal-Sack, in the Southern Hemisphere, and it occurs in a part of the
+sky otherwise so bright that it is the more noticeable. No possible
+explanation has yet been suggested to account for it.
+
+Thus it may be seen that, though much has been discovered, much remains
+to be discovered. By the patient work of generations of astronomers we
+have gained a clear idea of our own position in the universe. Here are
+we on a small globe, swinging round a far mightier and a self-luminous
+globe, in company with seven other planets, many of which, including
+ourselves, are attended by satellites or moons. Between the orbits of
+these planets is a ring or zone of tiny bodies, also going round the
+sun. Into this system flash every now and then strange luminous
+bodies--some coming but once, never to return; others returning again
+and again.
+
+Far out in space lies this island of a system, and beyond the gulfs of
+space are other suns, with other systems: some may be akin to ours and
+some quite different. Strewn about at infinite distances are star
+clusters, nebulę, and other mysterious objects.
+
+The whole implies design, creation, and the working of a mighty
+intelligence; and yet there are small, weak creatures here on this
+little globe who refuse to believe in a God, or who, while acknowledging
+Him, would believe themselves to know better than He.
+
+THE END
+
+BILLING AND SONS, LTD., PRINTERS, GUILDFORD
+
+
+
+
+
+End of Project Gutenberg's The Children's Book of Stars, by G.E. Mitton
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+    <meta http-equiv="Content-Type" content="text/html;charset=iso-8859-1" />
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+    <title>
+      The Project Gutenberg eBook of The Children's Book Of Stars, by G. E. Mitton.
+    </title>
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+<pre>
+
+The Project Gutenberg EBook of The Children's Book of Stars, by G.E. Mitton
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
+
+
+Title: The Children's Book of Stars
+
+Author: G.E. Mitton
+
+Release Date: May 17, 2009 [EBook #28853]
+
+Language: English
+
+Character set encoding: ISO-8859-1
+
+*** START OF THIS PROJECT GUTENBERG EBOOK THE CHILDREN'S BOOK OF STARS ***
+
+
+
+
+Produced by Siobhan Hillman, Brenda Lewis, Janet Blenkinship
+and the Online Distributed Proofreading Team at
+http://www.pgdp.net
+
+
+
+
+
+
+</pre>
+
+
+<div class="figcenter" style="width: 447px;">
+<img src="images/i-cover1.jpg" width="447" height="600" alt="" title="book cover" />
+</div>
+
+
+<h1>THE<br />
+  CHILDREN'S BOOK<br />
+  OF<br />
+  STARS</h1>
+
+  <h4>BY</h4>
+
+  <h2>G. E. MITTON</h2>
+
+  <p class="center">AUTHOR OF<br />
+  'THE CHILDREN'S BOOK OF LONDON,' 'ANIMAL AUTOBIOGRAPHIES:<br />
+  THE DOG,' ETC.<br /><br />
+
+  LONDON<br />
+  ADAM AND CHARLES BLACK<br />
+  1907<br /><br /><i>Published September, 1907</i></p>
+
+<p><span class='pagenum'><a name="Page_i" id="Page_i">[Pg i]</a></span></p>
+<div class="figcenter" style="width: 386px;">
+<a name="frontis" id="frontis"></a>
+<img src="images/i-004.jpg" width="386" height="600" alt="THE MOON-CHILD MUST KEEP ON RUNNING ROUND HER. P. 11." title="" />
+<span class="caption">THE MOON-CHILD MUST KEEP ON RUNNING ROUND HER. P. <a href='#Page_11'><b>11</b></a>.</span>
+</div>
+
+
+<p><span class='pagenum'><a name="Page_ii" id="Page_ii">[Pg ii]</a></span></p>
+<div class="centerbox bbox">
+<div class="centerbox1 bbox">
+  <h4><span class="smcap">By the Same Author</span></h4>
+</div>
+<p>&nbsp;</p>
+<div class="centerbox1 bbox">
+  <h2>CHILDREN'S BOOK OF LONDON</h2>
+
+  <p>CONTAINING 12 FULL-PAGE ILLUSTRATIONS<br />
+  IN COLOUR BY JOHN WILLIAMSON<br />
+  PRICE <b>6s.</b></p>
+
+  <p>'The stories are told in a way that is bound
+  to rivet attention, and the historical sketches
+  will leave a lasting impression on the minds
+  of young readers which will be very useful
+  when their studies in history become more
+  advanced.'&mdash;<i>Scotsman.</i></p>
+
+
+  <h3>ANIMAL AUTOBIOGRAPHIES</h3>
+
+  <h2>THE DOG</h2>
+
+  <p>WITH 12 FULL-PAGE ILLUSTRATIONS IN<br />
+  COLOUR BY J. WILLIAMSON<br />
+
+  PRICE <b>6s.</b></p>
+
+  <p>'A true life history, written "out of the
+  fulness of first-hand knowledge" by an author
+  who is thoroughly acquainted with all the
+  ways of "the friend of man."'&mdash;<i>Glasgow</i>
+  <i>Herald.</i></p>
+
+  <p>'The story is admirably told in clear and
+  fascinating language.'&mdash;<i>Freeman's Journal.</i></p>
+</div>
+<p>&nbsp;</p>
+<div class="centerbox1 bbox">
+  <p class="center">A. &amp; C. Black. Soho Square. London, W.</p>
+</div></div>
+
+<p><br /><br />&nbsp;</p>
+
+<div class='centered'>
+<table border="0" cellpadding="4" width="55%" cellspacing="0" summary="AGENTS">
+
+
+<tr><td>AGENTS</td><td>&nbsp;</td></tr>
+<tr><td>AMERICA</td><td>THE MACMILLAN COMPANY
+           64 &amp; 66 FIFTH AVENUE, NEW YORK</td></tr>
+
+<tr><td>CANADA</td><td>THE MACMILLAN COMPANY OF CANADA, LTD.
+           27 RICHMOND STREET WEST, TORONTO</td></tr>
+
+<tr><td>INDIA</td><td>MACMILLAN &amp; COMPANY, LTD.
+          MACMILLAN BUILDING, BOMBAY
+          309 BOW BAZAAR STREET, CALCUTTA</td></tr>
+
+</table></div>
+
+
+<p><span class='pagenum'><a name="Page_iii" id="Page_iii">[Pg iii]</a></span></p>
+
+
+
+
+<p><span class='pagenum'><a name="Page_iv" id="Page_iv">[Pg iv]</a></span></p>
+
+<p><span class='pagenum'><a name="Page_v" id="Page_v">[Pg v]</a></span></p>
+
+
+
+<hr style="width: 65%;" />
+<h2><a name="PREFACE" id="PREFACE"></a>PREFACE</h2>
+
+
+<p>It was the intention of the late Agnes Clerke to write the preface to
+this 'Children's Book of Stars.' Miss Clerke took a warm and sympathetic
+interest in the authoress and her work, but her lamented death occurred
+before this kindly intention could be fulfilled.</p>
+
+<p>I cannot pretend to write adequately as her substitute, but I could not
+resist the appeal made to me by the author, in the name and for the sake
+of her dear friend and mine, to write a few words of introduction.</p>
+
+<p>I am in no way responsible either for the plan or for any portion of
+this work, but I can commend it as a book, written in a simple and
+pleasant style, calculated to awaken the interest of intelligent
+children, and to enable parents otherwise ignorant or astronomy to
+answer many of those puzzling questions which such children often put.</p>
+
+<p class="author">
+DAVID GILL.<br />
+<span class='pagenum'><a name="Page_vii" id="Page_vii">[Pg vii]</a></span><span class='pagenum'><a name="Page_vi" id="Page_vi">[Pg vi]</a></span></p>
+
+
+
+<hr style="width: 65%;" />
+<h2><a name="AUTHORS_NOTE" id="AUTHORS_NOTE"></a>AUTHOR'S NOTE</h2>
+
+
+<p>This little work is the outcome of many suggestions on the part of
+friends who were anxious to teach their small children something of the
+marvels of the heavens, but found it exceedingly difficult to get hold
+of a book wherein the intense fascination of the subject was not lost in
+conventional phraseology&mdash;a book in which the stupendous facts were
+stated in language simple enough to be read aloud to a child without
+paraphrase.</p>
+
+<p>Whatever merit there may be in the present work is due entirely to my
+friend Agnes Clerke, the well-known writer on astronomy; the faults are
+all my own. She gave me the impetus to begin by her warm encouragement,
+and she helped me to continue by hearing every chapter read as it was
+written, and by discussing its successor and making suggestions for it.
+Thus she heard the whole book in MS. A week after the last chapter had
+been read to her I started on a journey lasting<span class='pagenum'><a name="Page_viii" id="Page_viii">[Pg viii]</a></span> many months, and while
+I was in the Far East the news reached me of her death, by which the
+world is the poorer. For her sake, as he has stated, her friend Sir
+David Gill, K.C.B., kindly undertook to supply the missing preface.</p>
+
+<p class="author">
+G. E. MITTON.<br />
+<span class='pagenum'><a name="Page_ix" id="Page_ix">[Pg ix]</a></span></p>
+
+
+
+<hr style="width: 65%;" />
+<h2><a name="CONTENTS" id="CONTENTS"></a>CONTENTS</h2>
+
+
+
+
+<div class='centered'>
+<table border="0" cellpadding="4" width="75%" cellspacing="0" summary="CONTENTS">
+<tr><td rowspan="33"><img src="images/i-spine1.jpg" width="143" height="600" alt="book spine" title="" /></td><td align='left'>CHAPTER I</td><td align='right'>PAGE</td></tr>
+<tr><td align='left'><span style="margin-left: 2em;">THE EARTH</span></td><td align='right'><a href='#Page_1'><b>1</b></a></td></tr>
+<tr><td align='left'>CHAPTER II</td></tr>
+<tr><td align='left'><span style="margin-left: 2em;">HANGING IN SPACE</span></td><td align='right'><a href='#Page_13'><b>13</b></a></td></tr>
+<tr><td align='left'>CHAPTER III</td></tr>
+<tr><td align='left'><span style="margin-left: 2em;">THE SHINING MOON</span></td><td align='right'><a href='#Page_21'><b>21</b></a></td></tr>
+<tr><td align='left'>CHAPTER IV</td></tr>
+<tr><td align='left'><span style="margin-left: 2em;">THE EARTH'S BROTHERS AND SISTER</span></td><td align='right'><a href='#Page_32'><b>32</b></a></td></tr>
+<tr><td align='left'>CHAPTER V</td></tr>
+<tr><td align='left'><span style="margin-left: 2em;">FOUR SMALL WORLDS</span></td><td align='right'><a href='#Page_48'><b>48</b></a></td></tr>
+<tr><td align='left'>CHAPTER VI</td></tr>
+<tr><td align='left'><span style="margin-left: 2em;">FOUR LARGE WORLDS</span></td><td align='right'><a href='#Page_67'><b>67</b></a></td></tr>
+<tr><td align='left'>CHAPTER VII</td></tr>
+<tr><td align='left'><span style="margin-left: 2em;">THE SUN</span></td><td align='right'><a href='#Page_89'><b>89</b></a></td></tr>
+<tr><td align='left'>CHAPTER VIII</td></tr>
+<tr><td align='left'><span style="margin-left: 2em;">SHINING VISITORS</span></td><td align='right'><a href='#Page_103'><b>103</b></a></td></tr>
+<tr><td align='left'>CHAPTER IX<span class='pagenum'><a name="Page_x" id="Page_x">[Pg x]</a></span></td></tr>
+<tr><td align='left'><span style="margin-left: 2em;">SHOOTING STARS AND FIERY BALLS</span></td><td align='right'><a href='#Page_120'><b>120</b></a></td></tr>
+<tr><td align='left'>CHAPTER X</td></tr>
+<tr><td align='left'><span style="margin-left: 2em;">THE GLITTERING HEAVENS</span></td><td align='right'><a href='#Page_135'><b>135</b></a></td></tr>
+<tr><td align='left'>CHAPTER XI</td></tr>
+<tr><td align='left'><span style="margin-left: 2em;">THE CONSTELLATIONS</span></td><td align='right'><a href='#Page_148'><b>148</b></a></td></tr>
+<tr><td align='left'>CHAPTER XII</td></tr>
+<tr><td align='left'><span style="margin-left: 2em;">WHAT THE STARS ARE MADE OF</span></td><td align='right'><a href='#Page_159'><b>159</b></a></td></tr>
+<tr><td align='left'>CHAPTER XIII</td></tr>
+<tr><td align='left'><span style="margin-left: 2em;">RESTLESS STARS</span></td><td align='right'><a href='#Page_170'><b>170</b></a></td></tr>
+<tr><td align='left'>CHAPTER XIV</td></tr>
+<tr><td align='left'><span style="margin-left: 2em;">THE COLOURS OF THE STARS</span></td><td align='right'><a href='#Page_176'><b>176</b></a></td></tr>
+<tr><td align='left'>CHAPTER XV</td></tr>
+<tr><td align='left'><span style="margin-left: 2em;">TEMPORARY AND VARIABLE STARS</span></td><td align='right'><a href='#Page_188'><b>188</b></a></td></tr>
+<tr><td align='left'>CHAPTER XVI</td></tr>
+<tr><td align='left'><span style="margin-left: 2em;">STAR CLUSTERS AND NEBUL&AElig;</span></td><td align='right'><a href='#Page_197'><b>197</b></a></td></tr>
+</table></div>
+<p><span class='pagenum'><a name="Page_xi" id="Page_xi">[Pg xi]</a></span></p>
+
+
+
+<hr style="width: 65%;" />
+<h2><a name="ILLUSTRATIONS" id="ILLUSTRATIONS"></a>ILLUSTRATIONS</h2>
+
+<h2>PRINTED IN COLOUR</h2>
+
+
+
+<div class='center'>
+<table border="0" cellpadding="2" width="65%" cellspacing="0" summary="PRINTED IN COLOUR">
+<tr><td align='left'>THE MOON-CHILD MUST KEEP ON RUNNING ROUND HER</td><td align='right'><a href='#frontis'><b><i>Frontispiece</i></b></a></td></tr>
+<tr><td>&nbsp;</td><td align="right">PAGE</td></tr>
+<tr><td align='left'>THE EARTH AND MOON HANGING IN SPACE</td><td align='right'><a href='#Page_16'><b>16</b></a></td></tr>
+<tr><td align='left'>THE ENGLISH SUMMER AND WINTER</td><td align='right'><a href='#Page_40'><b>40</b></a></td></tr>
+<tr><td align='left'>JUPITER AND ONE OF HIS MOONS</td><td align='right'><a href='#Page_70'><b>70</b></a></td></tr>
+<tr><td align='left'>THE PLANET SATURN AND TWO OF HIS MOONS</td><td align='right'><a href='#Page_78'><b>78</b></a></td></tr>
+<tr><td align='left'>FLAMES FROM THE SUN</td><td align='right'><a href='#Page_100'><b>100</b></a></td></tr>
+<tr><td align='left'>THE COMET IN THE BAYEUX TAPESTRY</td><td align='right'><a href='#Page_104'><b>104</b></a></td></tr>
+<tr><td align='left'>A STICK THRUST INTO THE WATER APPEARS CROOKED</td><td align='right'><a href='#Page_114'><b>114</b></a></td></tr>
+<tr><td align='left'>CONSTELLATIONS NEAR THE POLE STAR</td><td align='right'><a href='#Page_150'><b>150</b></a></td></tr>
+<tr><td align='left'>ORION AND HIS NEIGHBOURS</td><td align='right'><a href='#Page_154'><b>154</b></a></td></tr>
+<tr><td align='left'>THE SPECTRUM OF THE SUN AND SIRIUS</td><td align='right'><a href='#Page_168'><b>168</b></a></td></tr>
+</table></div>
+
+<p><span class='pagenum'><a name="Page_xii" id="Page_xii">[Pg xii]</a></span></p>
+
+
+<hr style="width: 65%;" />
+<h2>ILLUSTRATIONS</h2>
+
+<h2>IN BLACK AND WHITE</h2>
+
+
+
+
+
+
+<div class='centered'>
+<table border="0" cellpadding="2" width="65%" cellspacing="0" summary="IN BLACK AND WHITE">
+<tr><td>&nbsp;</td><td align="right">PAGE</td></tr>
+<tr><td align='left'>THE MOON</td><td align='right'><a href='#Page_24'><b>24</b></a></td></tr>
+<tr><td align='left'>AN ECLIPSE OF THE MOON</td><td align='right'><a href='#Page_28'><b>28</b></a></td></tr>
+<tr><td align='left'>AN ECLIPSE OF THE SUN</td><td align='right'><a href='#Page_29'><b>29</b></a></td></tr>
+<tr><td align='left'>THE MOON RAISING THE TIDES</td><td align='right'><a href='#Page_30'><b>30</b></a></td></tr>
+<tr><td align='left'>COMPARATIVE SIZES OF THE PLANETS</td><td align='right'><a href='#Page_35'><b>35</b></a></td></tr>
+<tr><td align='left'>DIFFERENT PHASES OF VENUS</td><td align='right'><a href='#Page_51'><b>51</b></a></td></tr>
+<tr><td align='left'>ORBITS OF MARS, THE EARTH, VENUS, AND MERCURY</td><td align='right'><a href='#Page_55'><b>55</b></a></td></tr>
+<tr><td align='left'>MAP OF MARS</td><td align='right'><a href='#Page_56'><b>56</b></a></td></tr>
+<tr><td align='left'>ORBITS OF THE EARTH AND MARS</td><td align='right'><a href='#Page_63'><b>63</b></a></td></tr>
+<tr><td align='left'>JUPITER AND HIS PRINCIPAL MOONS</td><td align='right'><a href='#Page_72'><b>72</b></a></td></tr>
+<tr><td align='left'>SUN-SPOTS</td><td align='right'><a href='#i128'><b>98</b></a></td></tr>
+<tr><td align='left'>A GREAT COMET</td><td align='right'><a href='#Page_118'><b>118</b></a></td></tr>
+<tr><td align='left'>THE GREAT NEBULA IN ANDROMEDA</td><td align='right'><a href='#Page_202'><b>202</b></a></td></tr>
+</table></div>
+<p><span class='pagenum'><a name="Page_1" id="Page_1">[Pg 1]</a></span></p>
+
+<hr style="width: 65%;" />
+<h1><br /><br />THE CHILDREN'S BOOK OF STARS<br /><br /></h1>
+
+
+
+<hr style="width: 65%;" />
+<h2><a name="CHAPTER_I" id="CHAPTER_I"></a>CHAPTER I</h2>
+
+<h3>THE EARTH</h3>
+
+
+<p>It is a curious fact that when we are used to things, we often do not
+notice them, and things which we do every day cease to attract our
+attention. We find an instance of this in the curious change that comes
+over objects the further they are removed from us. They grow smaller and
+smaller, so that at a distance a grown-up person looks no larger than a
+doll; and a short stick planted in the ground only a few feet away
+appears as long as a much longer one at ten times the distance. This
+process is going on all round us every minute: houses, trees, buildings,
+animals, all seem larger or smaller in proportion to their distance from
+us. Sometimes I have seen a row of rain<span class='pagenum'><a name="Page_2" id="Page_2">[Pg 2]</a></span>drops hanging on a bar by the
+window. When the sun catches one of them, it shines so brilliantly that
+it is as dazzling as a star; but my sense tells me it is a raindrop, and
+not a star at all. It is only because it is so near it seems as bright
+and important as a mighty star very, very far away.</p>
+
+<p>We are so much accustomed to this fact that we get into a habit of
+judging the distance of things by their size. If we see two lights
+shining on a dark night, and one is much larger than the other, we think
+that the bright one must be nearer to us; yet it need not necessarily be
+so, for the two lights might possibly be at the same distance from us,
+and one be large and the other small. There is no way in which we can
+tell the truth by just looking at them. Now, if we go out on any fine
+moonlight night and look up at the sky, we shall see one object there
+apparently much larger than any other, and that is the moon, so the
+question that occurs to us at once is, Is the moon really very much
+larger than any of the stars, or does it only seem so because it is very
+much nearer to us? As a matter of fact, the moon is one of the smallest
+objects in view, only, as it is our nearest neighbour, it appears very
+conspicuous. Having learned this, we shall probably look about to see
+what else there<span class='pagenum'><a name="Page_3" id="Page_3">[Pg 3]</a></span> is to attract attention, and we may notice one star
+shining very brilliantly, almost like a little lamp, rather low down in
+the sky, in that part of it where the sun has lately set. It is so
+beautifully bright that it makes all the others look insignificant in
+comparison, yet it is not really large compared with the others, only,
+as it comes nearer to us than anything else in the sky except the moon,
+it looks larger than it has any right to do in comparison with the
+others.</p>
+
+<p>After this we might jump to the conclusion that all the bright large
+stars are really small and near to us, and all the faintly shining ones
+large and far away. But that would not be true at all, for some bright
+ones are very far away and some faint ones comparatively near, so that
+all we can do is to learn about them from the people who have studied
+them and found out about them, and then we shall know of our own
+knowledge which of them seem bright only because they are nearer than
+the others, and which are really very, very brilliant, and so still
+shine brightly, though set in space at an almost infinite distance from
+us.</p>
+
+<p>The sun, as we all know, appears to cross the sky every day; he gets up
+in the east and drops down in the west, and the moon does the same,<span class='pagenum'><a name="Page_4" id="Page_4">[Pg 4]</a></span>
+only the moon is unlike the sun in this, that it changes its shape
+continually. We see a crescent moon growing every night larger and
+larger, until it becomes full and fat and round, and then it grows
+thinner and thinner, until it dies away; and after a little while it
+begins again, and goes through all the same changes once more. I will
+tell you why this is so further on, when we have a chapter all about the
+moon.</p>
+
+<p>If you watch the stars quietly for at least five minutes, you will see
+that they too are moving steadily on in the same way as the sun and
+moon. Watch one bright star coming out from behind a chimney-pot, and
+after about five minutes you will see that it has changed its place. Yet
+this is not true of all, for if we watch carefully we shall find that
+some, fairly high up in the sky, do not appear to move at all. The few
+which are moving so slowly that they seem to us to stand still are at a
+part of the sky close to the Pole Star, so called because it is always
+above the North Pole of the earth. I will explain to you how to find it
+in the sky for yourselves later on, but now you can ask anyone to point
+it out. Watch it. It appears to be fixed in one place, while the other
+stars are swinging round it in circles. In fact, it is as if we<span class='pagenum'><a name="Page_5" id="Page_5">[Pg 5]</a></span> on the
+earth were inside a great hollow globe or ball, which continually turned
+round, with the Pole Star near the top of the globe; and you know that
+if you put your finger on the spot at the top of a spinning globe or
+ball, you can hold it there while all the rest of the ball runs round.
+Now, if you had to explain things to yourself, you would naturally
+think: 'Here is the great solid earth standing still, and the sun and
+moon go round it; the stars are all turning round it too, just as if
+they were fixed on to the inside of a hollow globe; we on the earth are
+in the middle looking up at them; and this great globe is slowly
+wheeling round us night by night.'</p>
+
+<p>In the childhood of the world men believed that this was really
+true&mdash;that the earth was the centre of the universe, that the sun and
+moon and all the hosts of heaven were there solely to light and benefit
+us; but as the world grew wiser the wonders of creation were fathomed
+little by little. Some men devoted their whole lives to watching the
+heavens, and the real state of things was gradually revealed to them.
+The first great discovery was that of the daily movement of the earth,
+its rotation on its own axis, which makes it appear as if all these
+shining things went round it. It is indeed a<span class='pagenum'><a name="Page_6" id="Page_6">[Pg 6]</a></span> very difficult matter to
+judge which of two objects is moving unless we can compare them both
+with something outside. You must have noticed this when you are sitting
+in a train at a station, and there is another train on the other side of
+yours. For if one of the trains moves gently, either yours or the other,
+you cannot tell which one it is unless you look at the station platform;
+and if your position remains the same in regard to that, you know that
+your train is still standing, while the other one beside it has begun to
+move. And I am quite sure that there is no one of us who has not, at one
+time or another, stood on a bridge and watched the water running away
+underneath until we felt quite dizzy, and it seemed as if the water were
+standing still and the bridge, with ourselves on it, was flying swiftly
+away backwards. It is only when we turn to the banks and find them
+standing still, that we realize the bridge is not moving, and that it is
+the running water that makes it seem to do so. These everyday instances
+show us how difficult it is to judge whether we are moving or an outside
+object unless we have something else to compare with it. And the
+marvellous truth is that, instead of the sun and moon and stars rolling
+round the earth, it is the earth that is spinning round day by day,
+while<span class='pagenum'><a name="Page_7" id="Page_7">[Pg 7]</a></span> the sun and the stars are comparatively still; and, though the
+moon does move, yet when we see her get up in the east and go down in
+the west that is due to our own movement and not to hers.</p>
+
+<p>The earth turns completely round once in a day and night. If you take an
+orange and stick a knitting-needle through it, and hold it so that the
+needle is not quite straight up but a little slanting, and then twirl it
+round, you will get quite a good idea of the earth, though of course
+there is no great pole like a gigantic needle stuck through it, that is
+only to make it easy for you to hold it by. In spinning the orange you
+are turning it as the earth turns day by day, or, as astronomers express
+it, as it rotates on its axis.</p>
+
+<p>There is a story of a cruel Eastern King who told a prisoner that he
+must die if he did not answer three questions correctly, and the
+questions were very difficult; this is one of them:</p>
+
+<p>'How long would it take a man to go round the earth if he never stopped
+to eat or drink on the way?'</p>
+
+<p>And the prisoner answered promptly: 'If he rose with the sun and kept
+pace with it all day, and never stopped for a moment to eat or drink, he
+would take just twenty-four hours, Your Royal<span class='pagenum'><a name="Page_8" id="Page_8">[Pg 8]</a></span> Highness.' For in those
+days it was supposed that the sun went round the earth.</p>
+
+<p>Everyone is so remarkably clever nowadays that I am sure there will be
+someone clever enough to object that, if what I have said is true, there
+would be a great draught, for the air would be rushing past us. But, as
+a matter of fact, the air goes with us too. If you are inside a railway
+carriage with the windows shut you do not feel the rush of air, because
+the air in the carriage travels with you; and it is the same thing on
+the earth. The air which surrounds the earth clings to it and goes round
+with it, so there is no continuous breeze from this cause.</p>
+
+<p>But the spinning round on its own axis is not the earth's only movement,
+for all the time it is also moving on round the sun, and once in a whole
+year it completes its journey and comes back to the place from whence it
+started. Thus the turning round like a top or rotating on its axis makes
+the day and night, and the going in a great ring or revolving round the
+sun makes the years.</p>
+
+<p>Our time is divided into other sections besides days and years. We have,
+for instance, weeks and months. The weeks have nothing to do with the
+earth's movements; they are only made by man<span class='pagenum'><a name="Page_9" id="Page_9">[Pg 9]</a></span> to break up the months;
+but the months are really decided by something over which we have no
+control. They are due to the moon, and, as I have said already, the moon
+must have a chapter to herself, so we won't say any more about the
+months here.</p>
+
+<p>If any friend of ours goes to India or New Zealand or America, we look
+upon him as a great traveller; yet every baby who has lived one year on
+the earth has travelled millions of miles without the slightest effort.
+Every day of our lives we are all flung through space without knowing it
+or thinking of it. It is as if we were all shut up in a comfortable
+travelling car, and were provided with so many books and pictures and
+companions that we never cared to look out of the windows, so that hour
+by hour as we were carried along over miles of space we never gave them
+a thought. Even the most wonderful car ever made by man rumbles and
+creaks and shakes, so that we cannot help knowing it is moving; but this
+beautiful travelling carriage of ours called the earth makes never a
+creak or groan as she spins in her age-long journey. It is always
+astonishing to me that so few people care to look out of the window as
+we fly along; most of them are far too much absorbed in their<span class='pagenum'><a name="Page_10" id="Page_10">[Pg 10]</a></span> little
+petty daily concerns ever to lift their eyes from them. It is true that
+sometimes the blinds are down, for the sky is thickly covered with
+clouds, and we cannot see anything even if we want to. It is true also
+that we cannot see much of the scenery in the daytime, for the sun
+shining on the air makes a veil of blue glory, which hides the stars;
+but on clear nights we can see on every side numbers of stars quite as
+interesting and beautiful as any landscape; and yet millions of people
+never look up, never give a thought to the wonderful scenery through
+which their car is rushing.</p>
+
+<p>By reason of the onward rush of the earth in space we are carried over a
+distance of at least eighteen miles every second. Think of it: as we
+draw a breath we are eighteen miles away in space from the point we were
+at before, and this goes on unceasingly day and night. These astonishing
+facts make us feel how small and feeble we are, but we can take comfort
+in the thought that though our bodies are insignificant, the brain of
+man, which has discovered these startling facts, must in itself be
+regarded as one of the most marvellous of all the mysteries amid which
+we live.</p>
+
+<p>Well, we have arrived at some idea of our earth's<span class='pagenum'><a name="Page_11" id="Page_11">[Pg 11]</a></span> position; we know
+that the earth is turning round day by day, and progressing round the
+sun year by year, and that all around lie the sentinel stars, scattered
+on a background of infinite space. If you take an older boy or girl and
+let him or her stand in the middle to represent the sun, then a smaller
+one would be the earth, and the smallest of all the moon; only in truth
+we could never get anyone large enough to represent the sun fairly, for
+the biggest giant that ever lived would be much too small in proportion.
+The one representing the sun must stand in the middle, and turn slowly
+round and round. Then let the earth-child turn too, and all the time she
+is spinning like a top she must be also hastening on in a big ring round
+the sun; but she must not go too fast, for the little moon-child must
+keep on running round her all the time. And the moon-child must keep her
+face turned always to the earth, so that the earth never sees her back.
+That is an odd thing, isn't it? We have never seen the other side of the
+moon, which goes round us, always presenting the same face to us.</p>
+
+<p>The earth is not the only world going round the sun; she has many
+brothers and a sister; some are nearer to the sun than she is, and some
+are further<span class='pagenum'><a name="Page_12" id="Page_12">[Pg 12]</a></span> away, but all circle round the great central light-giver in
+rings lying one outside the other. These worlds are called planets, and
+the earth is one of them, and one of the smaller ones, too, nothing so
+great and important as we might have imagined.<span class='pagenum'><a name="Page_13" id="Page_13">[Pg 13]</a></span></p>
+
+
+
+<hr style="width: 65%;" />
+<h2><a name="CHAPTER_II" id="CHAPTER_II"></a>CHAPTER II</h2>
+
+<h3>HANGING IN SPACE</h3>
+
+
+<p>If you are holding something in your hand and you let it go, what
+happens? It falls to the ground, of course. Now, why should it do so?
+You will say: 'How could it do anything else?' But that is only because
+you are hampered by custom. Try to shake yourself free, and think, Why
+should it go down instead of up or any other way? The first man who was
+clever enough to find some sort of an answer to this question was the
+great philosopher Sir Isaac Newton, though he was not quite the first to
+be puzzled by it. After years of study he discovered that every thing
+attracts every other thing in proportion to their masses (which is what
+you know as weight) and their distance from each other. In more
+scientific language, we should say every <i>body</i> instead of every
+<i>thing</i>, for the word body does not only mean a living body, but every
+lump or mass of matter in the universe. The earth is a body in this<span class='pagenum'><a name="Page_14" id="Page_14">[Pg 14]</a></span>
+sense, and so is the table or anything else you could name. Now as the
+earth is immeasurably heavier than anything that is on it, it pulls
+everything toward itself with such force that the little pulls of other
+things upon each other are not noticed. The earth draws us all toward
+it. It is holding us down to it every minute of the day. If we want to
+move we have to exert another force in order to overcome this attraction
+of the earth, so we exert our own muscles and lift first one foot and
+then the other away from the earth, and the effort we make in doing this
+tires us. All the while you are walking or running you are exercising
+force to lift your feet away from the ground. The pull of the earth is
+called gravitation. Just remember that, while we go on to something else
+which is almost as astonishing.</p>
+
+<p>We know that nothing here on earth continues to move for ever;
+everything has to be kept going. Anything left to itself has a tendency
+to stop. Why is this? This is because here in the world there is
+something that fights against the moving thing and tries to stop it,
+whether it be sent along the ground or thrown up in the air. You know
+what friction is, of course. If you rub your hands along any rough
+substance you will quickly feel it,<span class='pagenum'><a name="Page_15" id="Page_15">[Pg 15]</a></span> but on a smooth substance you feel
+it less. That is why if you send a stone spinning along a carpet or a
+rough road it stops comparatively soon, whereas if you use the same
+amount of force and send it along a sheet of ice it goes on moving much
+longer. This kind of resistance, which we call friction, is one of the
+causes which is at work to bring things to a standstill; and another
+cause is the resistance of the air, which is friction in another form.
+It may be a perfectly still day, yet if you are bicycling you are
+breaking through the air all the time, just as you would be through
+water in swimming, only the resistance of the air is less than that of
+water. As the friction or the resistance of the air, or both combined,
+gradually lessens the pace of the stone you sent off with such force,
+the gravitation of the earth begins to be felt. When the stone first
+started the force you gave to it was enough to overcome the gravitation
+force, but as the stone moves more slowly the earth-pull asserts itself,
+and the stone drops down to the ground and lies still upon the surface.
+Now, if there were no friction, and therefore no resistance, there would
+be no reason why anything once set moving should not go on moving for
+ever. The force you give to any object you throw is enough to over<span class='pagenum'><a name="Page_16" id="Page_16">[Pg 16]</a></span>come
+gravitation; and it is only when the first force has been diminished by
+friction that the earth asserts its authority and pulls the moving
+object toward it. If it were possible to get outside the air and out of
+reach of the pull of the earth, we might fling a ball off into space,
+and it would go on in a straight line until something pulled it to
+itself by the force of gravity.</p>
+
+<div class="figcenter" style="width: 443px;">
+<img src="images/i-034.jpg" width="443" height="650" alt="THE EARTH AND MOON HANGING IN SPACE" title="" />
+<span class="caption">THE EARTH AND MOON HANGING IN SPACE</span>
+</div>
+
+<p>Gravitation affects everything connected with the earth; even our air is
+held to the earth by gravitation. It grows thinner and thinner as we get
+further away from the earth. At the top of a high mountain the air is so
+thin that men have difficulty in breathing, and at a certain height they
+could not breathe at all. As they cannot breathe in very fine air, it is
+impossible for them to tell by personal experiment exactly where the air
+ends; but they have tried to find out in other ways, and though
+different men have come to different conclusions on the subject, it is
+safe to say that at about two hundred miles above the earth there is
+nothing that could be called air. Thus we can now picture our spinning
+earth clothed in a garment of air that clings closely about her, and
+grows thinner and thinner until it melts away altogether, for there is
+no air in space.</p>
+
+<p><span class='pagenum'><a name="Page_17" id="Page_17">[Pg 17]</a></span></p>
+
+<p>Now in the beginning God made the world, and set it off by a first
+impulse. We know nothing about the details, though further on you shall
+hear what is generally supposed to have taken place; we only know that,
+at some remote age, this world, probably very different from what it is
+now, together with the other planets, was sent spinning off into space
+on its age-long journey. These planets were not sent off at random, but
+must have had some particular connection with each other and with the
+sun, for they all belong to one system or family, and act and react on
+each other. Now, if they had been at rest and not in movement, they
+would have fallen right into the sun, drawn by the force of gravitation;
+then they would have been burned up, and there would have been an end of
+them. But the first force had imparted to them the impulse to go on in a
+straight line, so when the sun pulled the result was a movement between
+the two: the planets did not continue to move in a straight line,
+neither did they fall on to the sun, but they went on a course between
+the two&mdash;that is, a circle&mdash;for the sun never let them get right away
+from him, but compelled them to move in circles round him. There is a
+very common instance of this kind of thing which we can<span class='pagenum'><a name="Page_18" id="Page_18">[Pg 18]</a></span> see, or perhaps
+feel, every day. If you try to sit still on a bicycle you tumble off,
+because the earth pulls you down to itself; but if, by using the force
+of your own muscles, you give the bicycle a forward movement this
+resists the earth-pull, and the result is the bicycle runs along the
+ground. It does not get right away from the earth, not even two or three
+feet above ground; it is held to the earth, but still it goes forward
+and does not fall over, for the movement is made up of the earth-pull,
+which holds it to the ground, and the forward movement, which propels it
+along. Then again, as another instance, if you tie a ball to a string
+and whirl it round you, so long as you keep on whirling it will not fall
+to the ground, but the moment you stop down it drops, for there is
+nothing to fight against the pull of gravitation. Thus we can picture
+the earth and all the planets as if they were swinging round the sun,
+held by invisible strings. It is the combination of two forces that
+keeps them in their places&mdash;the first force and the sun's pull. It is
+very wonderful to think of. Here we are swinging in space on a ball that
+seems only large to us because we are so much smaller ourselves; there
+is nothing above or below it but space, yet it travels on day by day and
+year by year, held by invisible<span class='pagenum'><a name="Page_19" id="Page_19">[Pg 19]</a></span> forces that the brain of man has
+discovered and measured.</p>
+
+<p>Of course, every planet gives a pull at every other planet too, but
+these pulls are so small compared with that of the sun that we need not
+at present notice them. Then we come to another point. We said that
+every body pulled every other body in proportion to their weights and
+their distance. Now, gravity acts much more strongly when things are
+near together than when they are far away from each other; so that if a
+smaller body is near to another somewhat larger than itself, it is
+pulled by it much more strongly than by a very much larger one at a
+considerably greater distance. We have an instance of this in the case
+of the earth and moon: as the earth responds to the pull of the sun, so
+the moon responds to the pull of the earth. The moon is so comparatively
+near to the earth that the earth-pull forces her to keep on going round
+and round, instead of leaving her free to circle round the sun by
+herself; and yet if you think of it the moon does go round the sun too.
+Recall that game we had when the sun was in the middle, and the two
+smaller girls, representing the earth and moon, went round it. The
+moon-child turned round the earth-child, but all the while the
+earth-child was going round the sun, so<span class='pagenum'><a name="Page_20" id="Page_20">[Pg 20]</a></span> that in a year's time the moon
+had been all round the sun too, only not in a straight line. The moon is
+something like a dog who keeps on dancing round and round you when you
+go for a walk. He does go for the walk too, but he does much more than
+that in the same time. Thus we have further completed our idea of our
+world. We see it now hanging in space, with no visible support, held in
+its place by two mighty forces; spinning on year after year, attended by
+its satellite the moon, while we run, and walk, and cry, and laugh, and
+play about on its surface&mdash;little atoms who, except for the brain that
+God has given them, would never even have known that they are
+continually moving on through endless space.<span class='pagenum'><a name="Page_21" id="Page_21">[Pg 21]</a></span></p>
+
+
+
+<hr style="width: 65%;" />
+<h2><a name="CHAPTER_III" id="CHAPTER_III"></a>CHAPTER III</h2>
+
+<h3>THE SHINING MOON</h3>
+
+
+<p>'Once upon a time,' long, long ago, the earth was not a compact, round,
+hard body such as she is now, but much larger and softer, and as she
+rotated a fragment broke off from her; it did not go right away from
+her, but still went on circling round with the motion it had inherited
+from her. As the ages passed on both the earth and this fragment, which
+had been very hot, cooled down, and in cooling became smaller, so that
+the distance between them was greater than it had been before they
+shrank. And there were other causes also that tended to thrust the two
+further from each other. Yet, compared with the other heavenly bodies,
+they are still near, and by looking up into the sky at night you can
+generally see this mighty fragment, which is a quarter the diameter of
+the earth&mdash;that is to say, a quarter the width of the earth measured
+from side to side through the middle. It is&mdash;as, of course, you have
+guessed<span class='pagenum'><a name="Page_22" id="Page_22">[Pg 22]</a></span>&mdash;the moon. The moon is the nearest body to us in all space, and
+so vast is the distance that separates us from the stars that we speak
+as if she were not very far off, yet compared with the size of the earth
+the space lying between us and her is very great. If you went right
+round the world at the thickest part&mdash;that is to say, in the region of
+the Equator&mdash;and when you arrived at your starting-point went off once
+again, and so on until you had been round ten times, you would only then
+have travelled about as far as from the earth to the moon!</p>
+
+<p>The earth is not the only planet which has a moon, or as it is called, a
+satellite, in attendance. Some of the larger planets have several, but
+there is not one to compare with our moon. Which would you prefer if you
+had the choice, three or four small moons, some of them not much larger
+than a very big bright star, or an interesting large body like our own
+moon? I know which I should say.</p>
+
+<p>'You say that the moon broke off from the earth, so perhaps there may be
+some people living on her,' I hear someone exclaim.</p>
+
+<p>If there is one thing we have found out certainly about the moon, it is
+that no life, as we know it, could exist there, for there is neither air
+nor water. Whether she ever had any air or water, and if<span class='pagenum'><a name="Page_23" id="Page_23">[Pg 23]</a></span> so, why they
+disappeared, are questions we cannot answer. We only know that now she
+is a dead world. Bright and beautiful as she is, shedding on us a pale,
+pure light, in vivid contrast with the fiery yellow rays of the sun, yet
+she is dead and lifeless and still. We can examine her surface with the
+telescope, and see it all very plainly. Even with a large opera-glass
+those markings which, to the naked eye, seem to be like a queer
+distorted face are changed, and show up as the shadows of great
+mountains. We can only see one side of the moon, because as I have said,
+she keeps always the same face turned to the earth; but as she sways
+slightly in her orbit, we catch a glimpse of sometimes a little more on
+one side and sometimes a little more on the other, and so we can judge
+that the unseen part is very much the same as that turned toward us.</p>
+
+<p>At first it is difficult to realize what it means to have no air.
+Besides supporting life in every breath that is drawn by living
+creatures, the air does numerous other kind offices for us&mdash;for
+instance, it carries sound. Supposing the most terrific volcano exploded
+in an airless world, it could not be heard. The air serves as a screen
+by day to keep off the burning heat of the sun's<span class='pagenum'><a name="Page_24" id="Page_24">[Pg 24]</a></span> rays, and as a blanket
+by night to keep in the heat and not let it escape too quickly. If there
+were no air there could be no water, for all water would evaporate and
+vanish at once. Imagine the world deprived of air; then the sun's rays
+would fall with such fierceness that even the strongest tropical sun we
+know would be as nothing in comparison with it, and every green thing
+would shrivel up and die; this scorching sun would shine out of a black
+sky in which the stars would all be visible in the daytime, not hidden
+by the soft blue veil of air, as they are now. At night the instant the
+sun disappeared below the horizon black darkness would set in, for our
+lingering twilight is due to the reflection of the sun in the upper
+layers of air, and a bitterness of deathly cold would fall upon the
+earth&mdash;cold fiercer than that of the Arctic regions&mdash;and everything
+would be frozen solid. It would need but a short time to reduce the
+earth to the condition of the moon, where there is nothing to shrivel
+up, nothing to freeze. Her surface is made up of barren, arid rocks, and
+her scenery consists of icy black shadows and scorching white plains.</p>
+
+<div class="figcenter" style="width: 456px;">
+<img src="images/i-044.jpg" width="456" height="600" alt="Paris Observatory.
+
+THE MOON." title="" />
+<span class="caption">Paris Observatory.
+
+THE MOON.</span>
+</div>
+
+<p>The black shadows define the mountains, and tremendous mountains they
+are. Most of them<span class='pagenum'><a name="Page_25" id="Page_25">[Pg 25]</a></span> have craters. A crater is like a cup, and generally
+has a little peak in the middle of it. This is the summit of a volcano,
+and when the volcano has burst up and vomited out floods of lava and
+d&eacute;bris, this has fallen down in a ring a little distance away from it,
+leaving a clear space next to the peak, so that, as the mountain ceases
+vomiting and the lava cools down, the ring hardens and forms a circular
+ridge. The craters on the moon are immense, not only in proportion to
+her size, but immense even according to our ideas on the earth. One of
+the largest craters in our own world is in Japan, and this measures
+seven miles across, while in the moon craters of fifty, sixty, and even
+a hundred miles are by no means uncommon, though there are also hundreds
+and thousands of smaller ones. We can see the surface of the moon very
+plainly with the magnificent telescopes that have now been made, and
+with the best of these anything the size of a large town would be
+plainly visible. Needless to say, no town ever has been or ever will be
+seen upon the moon!</p>
+
+<p>All these mountains and craters show that at one time the moon must have
+been convulsed with terrific disturbances, far worse than anything that
+we have any knowledge of on our earth; but this<span class='pagenum'><a name="Page_26" id="Page_26">[Pg 26]</a></span> must have been ages
+ago, while the moon still probably had an atmosphere of its own. Now it
+has long been quiet. Nothing changes there; even the forces that are
+always at work on the earth&mdash;namely, damp and mould and water&mdash;altering
+the surface and breaking up the rocks, do not act there, where there is
+no moisture of any sort. So far as we can see, the purpose of the moon
+is to be the servant of the earth, to give her light by night and to
+raise the tides. Beautiful light it is, soft and mysterious&mdash;light that
+children do not often have a chance of seeing, for they are generally in
+bed before the moon rises when she is at the full.</p>
+
+<p>We know that the moon has no heat of her own&mdash;she parted with all that
+long ago; she cannot give us glowing light from brilliant flames, as the
+sun does; she shines only by the reflection of the sun on her surface,
+and this is the reason why she appears to change her shape so
+constantly. She does not really change; the whole round moon is always
+there, only part of it is in shadow. Sometimes you can see the dark part
+as well as the bright. When there is a crescent moon it looks as if it
+were encircling the rest; some people call it, 'seeing the old moon in
+the new moon's arms.' I<span class='pagenum'><a name="Page_27" id="Page_27">[Pg 27]</a></span> don't know if you would guess why it is we can
+see the dark part then, or how it is lighted up. It is by reason of our
+own shining, for we give light to the moon, as she does to us. The sun's
+rays strike on the earth, and are reflected on to the moon, so that the
+moon is lighted by earthshine as we are lighted by moonshine, and it is
+these reflected earth-rays that light up the dark part of the moon and
+enable us to see it. What a journey these rays have had! They travel
+from the sun to the earth, and the earth to the moon, and then back to
+the earth again! From the moon the earth must appear a much bigger and
+more glorious spectacle than she does to us&mdash;four times wider across and
+probably brighter&mdash;for the sun's light strikes often on our clouds,
+which shine more brilliantly than her surface.</p>
+
+<p>Once again we must use an illustration to explain the subject. Set a
+lamp in the middle of a dark room, and let that be the sun, then take a
+small ball to represent the earth and a smaller one for the moon. Place
+the moon-ball between the lamp and the earth-ball. You will see that the
+side turned to the earth-ball is dark, but if you move the moon to one
+side of the earth, then from the earth half of it appears light and half
+dark; if you put it right<span class='pagenum'><a name="Page_28" id="Page_28">[Pg 28]</a></span> away from the lamp, on the outer side of the
+earth, it is all gloriously lit up, unless it happens to be exactly
+behind the earth, when the earth's shadow will darken it. This is the
+full explanation of all the changes of the moon.</p>
+
+<div class="figcenter" style="width: 650px;">
+<img src="images/i-049.jpg" width="650" height="209" alt="AN ECLIPSE OF THE MOON." title="" />
+<span class="caption">AN ECLIPSE OF THE MOON.</span>
+</div>
+
+<p>Does it ever fall within the earth's shadow? Yes, it does; for as it
+passes round the earth it is not always at the same level, but sometimes
+a little higher and sometimes a little lower, and when it chances to
+pass exactly behind it enters the shadow and disappears. That is what we
+call an eclipse of the moon. It is nothing more than the earth's shadow
+thrown on to the moon, and as the shadow is round that is one of the
+proofs that the earth is round too. But there is another kind of
+eclipse&mdash;the eclipse of the sun; and this is caused by the moon herself.
+For when she is nearest to the sun, at new moon&mdash;that is to say, when
+her dark side is<span class='pagenum'><a name="Page_29" id="Page_29">[Pg 29]</a></span> toward us, and she happens to get exactly between us
+and the sun&mdash;she shuts out the face of the sun from us; for though she
+is tiny compared with him, she is so much nearer to us that she appears
+almost the same size, and can blot him right out. Thus the eclipses of
+both sun and moon are not difficult to understand: that of the moon can
+only happen at full moon, when she is furthest from the sun, and it is
+caused by the earth's shadow falling upon the moon; and that of the sun
+at new moon, when she is nearest to him, and it is caused by the solid
+body of the moon coming between us and the sun.</p>
+
+<div class="figcenter" style="width: 650px;">
+<img src="images/i-050.jpg" width="650" height="210" alt="AN ECLIPSE OF THE SUN." title="" />
+<span class="caption">AN ECLIPSE OF THE SUN.</span>
+</div>
+
+<p>Besides giving us light by night, the moon serves other important
+purposes, and the most important of all is the raising of the tides.
+Without the rising of the sea twice in every day and night our coasts
+would become foul and unwholesome, for all the dead fish and rotting
+stuff lying on the beach would<span class='pagenum'><a name="Page_30" id="Page_30">[Pg 30]</a></span> poison the air. The sea tides scour our
+coasts day by day with never-ceasing energy, and they send a great
+breath of freshness up our large rivers to delight many people far
+inland. The moon does most of this work, though she is a little helped
+by the sun. The reason of this is that the moon is so near to the earth
+that, though her pull is a comparatively small one, it is very strongly
+felt. She cannot displace the actual surface to any great extent, as it
+is so solid; but when it comes to the water she can and does displace
+that, so that the water rises up in answer to her pull, and as the earth
+turns round the raised-up water lags behind, reaching backward toward
+the moon, and is drawn up<span class='pagenum'><a name="Page_31" id="Page_31">[Pg 31]</a></span> on the beach, and makes high tide. But it is
+stopped there, and meantime, by reason of the earth's movement, the moon
+is left far behind, and pulls the water to itself further on, when the
+first high tide relapses and falls down again. At length the moon gets
+round to quite the opposite side of the earth to that where she began,
+and there she makes a high tide too; but as she draws the water to
+herself she draws also the solid earth beneath the water to her in some
+degree, and so pulls it away from the place where the first high tide
+occurred, leaving the water there deeper than before, and so causing a
+secondary high tide.</p>
+
+<div class="figcenter" style="width: 650px;">
+<a name="i30" id="i30"></a>
+<img src="images/i-051.jpg" width="650" height="368" alt="THE MOON RAISING THE TIDES." title="" />
+<span class="caption">THE MOON RAISING THE TIDES.</span>
+</div>
+
+
+<p>The sun has some influence on the tides too, and when moon and sun are
+in the same line, as at full and new moon, then the tides are highest,
+and are called spring tides; but when they pull in different directions,
+as when it is half-moon, then the tides are lowest and are called neap
+tides.<span class='pagenum'><a name="Page_32" id="Page_32">[Pg 32]</a></span></p>
+
+
+
+<hr style="width: 65%;" />
+<h2><a name="CHAPTER_IV" id="CHAPTER_IV"></a>CHAPTER IV</h2>
+
+<h3>THE EARTH'S BROTHERS AND SISTER</h3>
+
+
+<p>The earth is not the only world that, poised in space, swings around the
+sun. It is one of a family called the Solar System, which means the
+system controlled and governed by the sun. When we look up at the
+glorious sky, star-studded night by night, it might seem to us that the
+stars move only by reason of the earth's rotation; but when men first
+began to study the heavens attentively&mdash;and this is so long ago that the
+record of it is not to be found&mdash;they noticed that, while every shining
+object in the sky was apparently moving round us, there were a few which
+also had another movement, a proper motion of their own, like the moon.
+These curious stars, which appeared to wander about among the other
+stars, they called planets, or wanderers. And the reason, which was
+presently discovered, of our being able to see these movements was that
+these planets are very much nearer to us than any of the real<span class='pagenum'><a name="Page_33" id="Page_33">[Pg 33]</a></span> stars,
+and in fact form part of our own solar system, while the stars are at
+immeasurable distances away. Of all the objects in the heavens the
+planets are the most intensely interesting to us; for though removed
+from us by millions of miles, the far-reaching telescope brings some of
+them within such range that we can see their surfaces and discover their
+movements in a way quite impossible with the stars. And here, if
+anywhere, might we expect to find traces of other living beings like
+ourselves; for, after all the earth is but a planet, not a very large
+nor a very small one, and in no very striking position compared with the
+other planets; and thus, arguing by what seems common-sense, we say, If
+this one planet has living beings on its surface, may not the other
+planets prove to be homes for living beings also? Counting our own
+earth, there are eight of these worlds in our solar system, and also a
+number of tiny planets, called asteroids; these likewise go round the
+sun, but are very much smaller than any of the first eight, and stand in
+a class by themselves, so that when the planets are mentioned it is
+generally the eight large well-known planets which are referred to.</p>
+
+<p>If we go back for a moment to the illustration of the large lamp
+representing our sun, we shall<span class='pagenum'><a name="Page_34" id="Page_34">[Pg 34]</a></span> now be able to fill in the picture with
+much more detail. The orbits of the planets, as their paths round the
+sun are called, lie like great circles one outside another at various
+distances, and do not touch or cut each other. Where do you suppose our
+own place to be? Will it be the nearest to the sun or the furthest away
+from him? As a matter of fact, it is neither, we come third in order
+from the sun, for two smaller planets, one very small and the other
+nearly as large as the earth, circle round and round the sun in orbits
+lying inside ours. Now if we want to place objects around our lamp-sun
+which will represent these planets in size, and to put them in places
+corresponding to their real positions, we should find no room large
+enough to give us the space we ought to have. We must take the lamp out
+into a great open field, where we shall not be limited by walls. Then
+the smallest planet, named Mercury, which lies nearest of all to the
+sun, would have to be represented by a pea comparatively close to the
+sun; Venus, the next, would be a greengage plum, and would be about
+twice as far away; then would come the earth, a slightly larger plum,
+about half as far again as Venus. After this there would be a lesser
+planet, called Mars, like a marble. These are the first<span class='pagenum'><a name="Page_35" id="Page_35">[Pg 35]</a></span> four, all
+comparatively small; beyond them there is a vast gap, in which we find
+the asteroids, and after this we come to four larger planets, mighty
+indeed as regards ourselves, for if our earth were<span class='pagenum'><a name="Page_36" id="Page_36">[Pg 36]</a></span> a greengage plum,
+the first of these, Jupiter, would have to be the size of a football at
+least, and the next, Saturn, a smaller football, while Uranus and
+Neptune, the two furthest out, would be about the size of the toy
+balloons children play with. The outermost one, Neptune, would be thirty
+times as far from the sun as we are.</p>
+
+<div class="figcenter" style="width: 584px;">
+<a name="i35" id="i35"></a>
+<img src="images/i-056.jpg" width="584" height="700" alt="COMPARATIVE SIZES OF THE PLANETS." title="" />
+<span class="caption">COMPARATIVE SIZES OF THE PLANETS.</span>
+</div>
+
+<p>This is the solar system, and in it the only thing that shines by its
+own light is the sun; all the rest, the planets and their moons, shine
+only because the rays of light from the sun strike on their surfaces and
+are reflected off again. Our earth shines like that, and from the nearer
+planets must appear as a brilliant star. The little solar system is
+separated by distances beyond the realm of thought from the rest of the
+universe. Vast as are the intervals between ourselves and our planetary
+neighbours, they are as nothing to the space that separates us from the
+nearest of the steady shining fixed stars. Why, removed as far from us
+as the stars, the sun himself would have sunk to a point of light; and
+as for the planets, the largest of them, Jupiter, could not possibly be
+seen. Thus, when we look at those stars across the great gulf of space,
+we know that though we see them they cannot see us, and that to them our
+sun must seem only a star; con<span class='pagenum'><a name="Page_37" id="Page_37">[Pg 37]</a></span>sequently we argue that perhaps these
+stars themselves are suns with families of planets attached to them; and
+though there are reasons for thinking that this is not the case with
+all, it may be with some. Now if, after learning this, we look again at
+the sky, we do so with very different eyes, for we realize that some of
+these shining bodies are like ourselves in many things, and are shining
+only with a light borrowed from the sun, while others are mighty glowing
+suns themselves, shining by their own light, some greater and brighter,
+some less than our sun. The next thing to do is to learn which are stars
+and which are planets.</p>
+
+<p>Of the planets you will soon learn to pick out one or two, and will
+recognize them even if they do change their places&mdash;for instance, Venus
+is at times very conspicuous, shining as an evening star in the west
+soon after the sun goes down, or us a morning star before he gets up,
+though you are not so likely to see her then; anyway, she is never found
+very far from the sun. Jupiter is the only other planet that compares
+with her in brilliancy, and he shines most beautifully. He is, of
+course, much further away from us than Venus, but so much larger that he
+rivals her in brightness. Saturn can be quite easily seen as a
+conspicuous<span class='pagenum'><a name="Page_38" id="Page_38">[Pg 38]</a></span> object, too, if you know where to look for him, and Mars is
+sometimes very bright with a reddish glow. The others you would not be
+able to distinguish.</p>
+
+<p>It is to our earth's family of these eight large planets going steadily
+round the same sun that we must give our attention first, before going
+on to the distant stars. Many of the planets are accompanied by
+satellites or moons, which circle round them. We may say that the sun is
+our parent&mdash;father, mother, what you will&mdash;and that the planets are the
+family of children, and that the moons are <i>their</i> children. Our earth,
+you see, has only one child, but that a very fine one, of which she may
+well be proud.</p>
+
+<p>When I say that the planets go round the sun in circles I am only
+speaking generally; as a matter of fact, the orbits of the planets are
+not perfect circles, though some are more circular than others. Instead
+of this they are as a circle might look if it were pressed in from two
+sides, and this is called an ellipse. The path of our own earth round
+the sun is one of the most nearly circular of them all, and yet even in
+her orbit she is a good deal nearer to the sun at one time than another.
+Would you be surprised to hear that she is nearer in our winter and
+further away in our summer? Yet that is the<span class='pagenum'><a name="Page_39" id="Page_39">[Pg 39]</a></span> case. And for the first
+moment it seems absurd; for what then makes the summer hotter than the
+winter? That is due to an altogether different cause; it depends on the
+position of the earth's axis. If that axis were quite straight up and
+down in reference to the earth's path round the sun we should have equal
+days and nights all the year round, but it is not; it leans over a
+little, so that at one time the North Pole points towards the sun and at
+another time away from it, while the South Pole is pointing first away
+from it and then toward it in exactly the reverse way. When the North
+Pole points to the sun we in the Northern Hemisphere have our summer. To
+understand this you must look at the picture, which will make it much
+clearer than any words of mine can do. The dark part is the night, and
+the light part the day. When we are having summer any particular spot on
+the Northern Hemisphere has quite a long way to travel in the light, and
+only a very short bit in the dark, and the further north you go the
+longer the day and shorter the night, until right up near the North
+Pole, within the Arctic Circle, it is daylight all the time. You have,
+perhaps, heard of the 'midnight sun' that people go to see in the North,
+and what the expression means is that at what should be midnight the<span class='pagenum'><a name="Page_40" id="Page_40">[Pg 40]</a></span>
+sun is still there. He seems just to circle round the horizon, never
+very far above, but never dipping below it.</p>
+
+<div class="figcenter" style="width: 700px;">
+<img src="images/i-062.jpg" width="700" height="481" alt="THE ENGLISH SUMMER (LEFT) AND WINTER (RIGHT)." title="" />
+<span class="caption">THE ENGLISH SUMMER (LEFT) AND WINTER (RIGHT).</span>
+</div>
+
+<p>When the sun is high overhead, his rays strike down with much more force
+than when he is low. It is, for instance, hotter at mid-day than in the
+evening. Now, when the North Pole is bowed toward the sun, the sun
+appears to us to be higher in the sky. In the British Isles he never
+climbs quite to the zenith, as we call the point straight above our
+heads; he always keeps on the southern side of that, so that our shadows
+are thrown northward at mid-day, but yet he gets nearer to it than he
+does in winter. Look at the picture of the earth as it is in winter.
+Then we have long nights and short days, and the sun never appears to
+climb very high, because we are turned away from him. During the short
+days we do not receive a great deal of heat, and during the long night
+the heat we have received has time to evaporate to a great extent. These
+two reasons&mdash;the greater or less height of the sun in the sky and the
+length of the days&mdash;are quite enough to account for the difference
+between our summer and winter. There is one rather interesting point to
+remember, and that is that in the Northern Hemisphere, whether it is
+winter or summer, the sun is south at mid-day,<span class='pagenum'><a name="Page_41" id="Page_41">[Pg 41]</a></span> so that you can always
+find the north then, for your shadow will point northwards.</p>
+
+
+
+<p>New Zealand and Australia and other countries placed in the Southern
+Hemisphere, as we are in the Northern, have their summer while we have
+winter, and winter while we have summer, and their summer is warmer than
+ours, because it comes when the earth in its journey is three million
+miles nearer to the sun than in our summer.</p>
+
+<p>All this seems to refer to the earth alone, and this chapter should be
+about the planets; but, after all, what applies to one planet applies to
+another in some degree, and we can turn to the others with much more
+interest now to see if their axes are bowed toward the sun as ours is.
+It is believed that in the case of Mercury, in regard to its path round
+the sun, the axis is straight up and down; if it is the changes of the
+seasons must depend on the nearness of Mercury to the sun and nothing
+else, and as he is a great deal nearer at one time than another, this
+might make a very considerable difference. Some of the planets are like
+the earth in regard to the position of their axes, but the two outermost
+ones, Uranus and Neptune, are very peculiar, for one pole is turned
+right toward the sun and the other right away from it, so that in<span class='pagenum'><a name="Page_42" id="Page_42">[Pg 42]</a></span> one
+hemisphere there is continuous day all the summer, in the other there is
+continuous night, and then the process is reversed. But these little
+peculiarities we shall have to note more particularly in the account of
+the planets separately.</p>
+
+<p>There is a curious fact in regard to the distances of the planets from
+the sun. Each one, after the first, is, very roughly, about double the
+distance from the sun of the one inside it. This holds good for all the
+first four, then there is a great gap where we might expect to find
+another planet, after which follow the four large planets. Now, this gap
+puzzled astronomers greatly; for though there seemed to be no reason why
+the planets should be at regular distances one outside the other, yet
+there the fact was, and that the series should be broken by a missing
+planet was annoying. So very careful search was made, and a thrill of
+excitement went all through the scientific world when it was known that
+a tiny planet had been discovered in the right place. But this was not
+the end of it, for within a few years three or four more tiny planets
+were observed not far from the first one, and, as years rolled on, one
+after another was discovered until now the number amounts to over six
+hundred and others are perpetually being added to<span class='pagenum'><a name="Page_43" id="Page_43">[Pg 43]</a></span> the list! Here was a
+new feature in the solar system, a band of tiny planets not one of which
+was to be compared in size with the least of those already known. The
+largest may be about as large as Europe, and others perhaps about the
+size of Wales, while there may be many that have only a few square miles
+of surface altogether, and are too small for us to see. To account for
+this strange discovery many theories were advanced.</p>
+
+<p>One was that there had been a planet&mdash;it might be about the size of
+Mars&mdash;which had burst up in a great explosion, and that these were the
+pieces&mdash;a very interesting and exciting idea, but one which proved to be
+impossible. The explanation now generally accepted is a little
+complicated, and to understand it we must go back for a bit.</p>
+
+<p>When we were talking of the earth and the moon we realized that once
+long ago the moon must have been a part of the earth, at a time when the
+earth was much larger and softer than she now is; to put it in the
+correct way, we should say when she was less dense. There is no need to
+explain the word 'dense,' for in its ordinary sense we use it every day,
+but in an astronomical sense it does not mean exactly the same thing.
+Everything is made up of minute particles or atoms, and when<span class='pagenum'><a name="Page_44" id="Page_44">[Pg 44]</a></span> these
+atoms are not very close together the body they compose is loose in
+texture, while if they are closer together the body is firmer. For
+instance, air is less dense than water, and water than earth, and earth
+than steel. You see at once by this that the more density a thing has
+the heavier it is; for as a body is attracted to another body by every
+atom or particle in it, so if it has more particles it will be more
+strongly attracted. Thus on the earth the denser things are really
+heavier. But 'weight' is only a word we use in connection with the
+earth; it means the earth's pulling power toward any particular thing at
+the surface, and if we were right out in space away from the earth, the
+pulling power of the earth would be less, and so the weight would be
+less; and as it would be impossible always to state just how far away a
+thing was from the earth, astronomers talk about density, which means
+the number of particles a body contains in proportion to other bodies.
+Thus the planet Jupiter is very much larger than the earth, but his
+density is less. That does not mean to say that if Jupiter were in one
+scale and the earth in the other he would weigh less, because he is so
+very much bigger he would outweigh the earth<span class='pagenum'><a name="Page_45" id="Page_45">[Pg 45]</a></span> still; his total <i>mass</i>
+would be greater than that of the earth, but it means that a piece of
+Jupiter the same size as a piece of the earth would weigh less under the
+same conditions.</p>
+
+<p>Now, before there were any planets at all or any sun, in the place of
+our solar system was a vast gaseous cloud called a nebula, which slowly
+rotated, and this rotation was the first impulse or force which God gave
+it. It was not at all dense, and as it rotated a part broke off, and
+inheriting the first impulse, went on rotating too. The impulse would
+have sent it off in a straight line, but the pull of gravity from the
+nebula held it in place, and it circled round; then the nebula, as it
+rotated, contracted a little, and occupied less space and grew denser,
+and presently a second piece was thrown off, to become in time another
+planet. The same process was repeated with Saturn, and then with the
+huge Jupiter. The nebula was always rotating and always contracting. And
+as it behaved, so did the planets in their turn; they spun round and
+cooled and contracted, and the moons were flung off from them, just as
+they&mdash;the planets&mdash;had been flung off from the parent nebula.</p>
+
+<p>Now, after the original nebula had parted with the mighty mass of
+Jupiter, it never again made an<span class='pagenum'><a name="Page_46" id="Page_46">[Pg 46]</a></span> effort so great, and for a long time
+the fragments that were detached were so small as hardly to be worth
+calling planets; they were the asteroids, little lumps and fragments
+that the nebula left behind. But as it still contracted in time there
+came Mars; and having recovered a little, the nebula with more energy
+got rid of the earth, and next Venus, and lastly little Mercury, the
+smallest of the eight planets. Then it contracted further, and perhaps
+you can guess what the remainder of it is&mdash;the sun; and by spinning in a
+plastic state the sun, like the earth, has become a globe, round and
+comparatively smooth; and its density is now too great to allow of its
+losing any more fragments, so, as far as we can see, the solar system is
+complete.</p>
+
+<p>This theory of the origin of the planets is called the nebula theory. We
+cannot prove it, but there are so many facts that can only be explained
+by it, we have strong reason for believing that something of the kind
+must have happened. When we come to speak of the starry heavens we shall
+see that there are many masses of glowing gas which are nebul&aelig; of the
+same sort, and which form an object-lesson in our own history.</p>
+
+<p>We have spoken rather lightly of the nebula rotating and throwing off
+planets; but we must not<span class='pagenum'><a name="Page_47" id="Page_47">[Pg 47]</a></span> think of all this as having happened in a
+short time. It is almost as impossible for the human mind to conceive
+the ages required for such slow changes as to grasp the great gulfs of
+space that separate us from the stars. We can only do it by comparison.
+You know what a second is, and how the seconds race past without ceasing
+day and night. It makes one giddy to picture the seconds there are in a
+year; yet if each one of those seconds was a year in itself, what then?
+That seems a stupendous time, but it is nothing compared with the time
+needed to form a nebula into a planetary system. If we had five thousand
+of such years, with every second in them a year, we should then only
+have counted one billion real years, and billions must have passed since
+the sun was a gaseous nebula filling the outermost bounds of our
+system!<span class='pagenum'><a name="Page_48" id="Page_48">[Pg 48]</a></span></p>
+
+
+
+<hr style="width: 65%;" />
+<h2><a name="CHAPTER_V" id="CHAPTER_V"></a>CHAPTER V</h2>
+
+<h3>FOUR SMALL WORLDS</h3>
+
+
+<p>What must the sun appear to Mercury, who is so much nearer to him than
+we are? To understand that we should have to imagine our sun increased
+to eight or nine times his apparent size, and pouring out far greater
+heat and light than anything that we have here, even in the tropics. It
+was at first supposed that Mercury must have an extra thick covering of
+clouds to protect him from this tremendous glare; but recent
+observations tend to prove that, far from this, he is singularly free
+from cloud. As this is so, no life as we know it could possibly exist on
+Mercury.</p>
+
+<p>His year&mdash;the time he takes to go round the sun and come back to the
+same place again&mdash;is eighty-eight days, or about one-quarter of ours. As
+his orbit is much more like an ellipse than a circle, it follows that he
+is much nearer to the sun at one time than at another&mdash;in fact, when he
+is nearest, the size of the sun must seem three and a half<span class='pagenum'><a name="Page_49" id="Page_49">[Pg 49]</a></span> times
+greater than when he is furthest away from it! Even at the best Mercury
+is very difficult to observe, and what we can learn about him is not
+much; but, as we have heard, his axis is supposed to be upright. If so
+his seasons cannot depend on the bend toward or away from the sun, but
+must be influenced solely by the changes in his distance from the sun,
+which are much greater than in our own ease. There is some reason to
+believe, too, that Mercury's day and year are the same length. This
+means that as the planet circles round the sun he turns once. If this is
+so the sun will shine on one half of the planet, producing an
+accumulated heat terrific to think of; while the other side is plunged
+in blackness. The side which faces the sun must be heated to a pitch
+inconceivable to us during the nearer half of the orbit&mdash;a pitch at
+which every substance must be at boiling-point, and which no life as we
+know it could possibly endure. Seen from our point of view, Mercury goes
+through all the phases of the moon, as he shines by the reflected light
+of the sun; but this point we shall consider more particularly in regard
+to Venus, as Venus is nearer to us and easier to study. For a long time
+astronomers had a fancy that there might be another planet even nearer
+to the sun<span class='pagenum'><a name="Page_50" id="Page_50">[Pg 50]</a></span> than Mercury, perhaps hidden from us by the great glare of
+the sun. They even named this imaginary planet Vulcan, and some thought
+they had seen it, but it is tolerably certain that Vulcan existed only
+in imagination. Mercury is the nearest planet to the sun, and also the
+smallest, of course excepting the asteroids. It is about three thousand
+miles in diameter, and as our moon is two thousand miles, it is not so
+much bigger than that. So far as we are concerned, it is improbable we
+shall ever know very much more about this little planet.</p>
+
+<p>But next we come to Venus, our beautiful bright neighbour, who
+approaches nearer to us than any other heavenly body except the moon.
+Alas! when she is nearest, she like Mercury, turns her dark side toward
+us, coming in between us and the sun, so that we cannot observe her at
+all.</p>
+
+<p>Everyone must have noticed Venus, however carelessly they have looked at
+the sky; but it is likely that far more people have seen her as an
+evening than a morning star, for most people are in bed when the sun
+rises, and it is only before sunrise or after sunset we can see Venus
+well. She is at her best from our point of view when she seems to us to
+be furthest from the sun, for then we can study her best, and at these
+times she<span class='pagenum'><a name="Page_51" id="Page_51">[Pg 51]</a></span> appears like a half or three-quarter moon, as we only see a
+part of the side from which the sunlight is reflected. She shines like a
+little silver lamp, excelling every other planet, even Jupiter, the
+largest of all. If we look at her even with the naked eye, we can see
+that she is elongated or drawn out, but her brilliance prevents us from
+seeing her shape exactly; to do this we must use a telescope.</p>
+
+<div class="figcenter" style="width: 650px;">
+<img src="images/i-074.jpg" width="650" height="647" alt="DIFFERENT PHASES OF VENUS." title="" />
+<span class="caption">DIFFERENT PHASES OF VENUS.</span>
+</div><p><span class='pagenum'><a name="Page_52" id="Page_52">[Pg 52]</a></span></p>
+
+<p>It is a curious fact that some planets shine much more brightly than
+others, without regard to their size&mdash;that is to say, the surface on
+which the sun's rays strike is of greater reflecting power in some than
+in others. One of the brightest things in Nature that we can imagine is
+a bank of snow in sunlight; it is so dazzling that we have to look away
+or wink hard at the sight; and the reflective power of the surface of
+Venus is as dazzling as if she were made of snow. This is probably
+because the light strikes on the upper surface of the clouds which
+surround her. In great contrast to this is the surface of Mercury, which
+reflects as dully as a mass of lead. Our own moon has not a high
+reflecting power, as will be easily understood if we imagine what the
+world would be if condemned to perpetual moonlight only. It would,
+indeed, be a sad deprivation if the mournful cold light of the moon,
+welcome enough as a change from sunlight, were to take the place of
+sunlight in the daytime.</p>
+
+<p>For a very long time astronomers could not discover what time Venus took
+in rotating on her own axis&mdash;that is to say, what the length of her day
+was. She is difficult to observe, and in order to find out the rotation
+it is necessary to note some fixed object on the surface which turns
+round with<span class='pagenum'><a name="Page_53" id="Page_53">[Pg 53]</a></span> the planet and comes back to the same place again, so that
+the time it takes in its journey can be measured. But the surface of
+Venus is always changing, so that it is impossible to judge at all
+certainly. Opinions differ greatly, some astronomers holding that
+Venus's day is not much longer than an earthly day, while others believe
+that the planet's day is equal to her year, just as in the case of
+Mercury. Venus's year is 225 days, or about seven and a half of our
+months, and if, indeed, her day and year are the same length, very
+peculiar effects would follow. For instance, terrible heat would be
+absorbed by the side of the planet facing the sun in the perpetual
+summer; and the cold which would be felt in the dreary winter's night
+would far exceed our bitterest Arctic climate. We cannot but fancy that
+any beings who might live on a planet of this kind must be different
+altogether from ourselves. Then, there is another point: even here on
+earth very strong winds are caused by the heating of the tropics; the
+hot air, being lighter than the cold air, rises, and the colder air from
+the poles rushes in to supply its place. This causes wind, but the winds
+which would be raised on Venus by the rush of air from the icy side of
+the planet to the hot one would be<span class='pagenum'><a name="Page_54" id="Page_54">[Pg 54]</a></span> tornadoes such as we could but
+faintly dream of. It is, of course, useless to speculate when we know so
+little, but in a subject so intensely interesting we cannot help
+guessing a little.</p>
+
+<p>Venus is only slightly smaller than the earth, and her density is not
+very unlike ours; therefore the pull of gravity must be pretty much
+there what it is here&mdash;that is to say, things will weigh at her surface
+about the same as they do here. Her orbit is nearly a circle, so that
+her distance from the sun does not vary much, and the heat will not be
+much greater from this cause at one time of the year than another.</p>
+
+<p>As her orbit is tilted up a little she does not pass between us and the
+sun at each revolution, but occasionally she does so, and this passing
+is called a transit. Many important facts have been learned by watching
+these transits. Mercury also has transits across the sun, but as she is
+so much smaller than Venus they are not of such great importance. It was
+by the close observation of Venus during her transits that the distance
+from the earth to the sun was first measured. Not until the year 2004
+will another transit of Venus occur.<span class='pagenum'><a name="Page_55" id="Page_55">[Pg 55]</a></span></p>
+
+<div class="figcenter" style="width: 650px;">
+<img src="images/i-078.jpg" width="650" height="649" alt="ORBITS OF MARS, THE EARTH, VENUS, AND MERCURY." title="" />
+<span class="caption">ORBITS OF MARS, THE EARTH, VENUS, AND MERCURY.</span>
+</div>
+
+<p>It is not difficult to imagine that the earth must appear a splendid
+spectacle from Venus, whence she is seen to great advantage. When
+nearest to us she must see us like a little moon, with markings as the
+continents and seas rotate, and these will change as they are obscured
+by the clouds rolling over them. At the North and South Poles will be
+glittering ice-caps, growing larger and smaller as<span class='pagenum'><a name="Page_56" id="Page_56">[Pg 56]</a></span> they turn toward or
+away from the sun. A brilliant spectacle!</p>
+
+
+
+<p>We might say with a sigh, 'If only we could see such a world!' Well, we
+can see a world&mdash;not indeed, so large as Venus, yet a world that comes
+almost as near to us as Venus does, and which, unlike her, is outside us
+in order from the sun, so that when it is nearest to us the full
+sunlight is on it. This is Mars, our neighbour on the other side, and of
+all the fascinating objects in the sky Mars is the most fascinating, for
+there, if anywhere, should we be likely to discover beings like
+ourselves!</p>
+
+<p>Mars takes rather more than half an hour longer to rotate than we do,
+and as he is so much smaller than the earth, this means that he moves
+round more slowly. His axis is bent at nearly the same angle as ours is.
+Mars is much smaller than the earth, his diameter is about twice that of
+the moon, and his density is about three-quarters that of the earth, so
+that altogether, with his smaller size and less density, anything
+weighing a hundred pounds here would only weigh some forty pounds on
+Mars; and if, by some miraculous agency, you were suddenly transported
+there, you would find yourself so light that you<span class='pagenum'><a name="Page_57" id="Page_57">[Pg 57]</a></span> could jump enormous
+distances with little effort, and skip and hop as if you were on
+springs.</p>
+
+<div class="figcenter" style="width: 650px;">
+<a name="map" id="map"></a>
+<img src="images/i-080-tb.jpg" width="650" height="378" alt="Memoirs of the British Astronomical Association.
+
+MAP OF MARS." title="" />
+<span class="caption">Memoirs of the British Astronomical Association.
+
+MAP OF MARS.<br /></span>
+<span class="link"><a href="images/i-080-full.jpg">View larger image</a></span>
+</div>
+
+<p>Look at the map of Mars, in which the surface appears to be cut up into
+land and water, continents and oceans. The men who first observed Mars
+with accuracy saw that some parts were of a reddish colour and others
+greenish, and arguing from our own world, they called the greenish parts
+seas and the reddish land. For a long while no one doubted that we
+actually looked on a world like our own, more especially as there was
+supposed to be a covering of atmosphere. The so-called land and water
+are much more cut up and mixed together than ours, it is true. Here and
+there is a large sea, like that marked 'Mare Australe,' but otherwise
+the water and the land are strangely intermingled. The red colour of the
+part they named land puzzled astronomers a good deal, for our land seen
+at the same distance would not appear so red, and they came at last to
+the conclusion that vegetation on Mars must be red instead of green! But
+after a while another disturbing fact turned up to upset their theories,
+and that was that they saw canals, or what they called canals, on Mars.
+These were long, straight, dark markings, such as you see on<span class='pagenum'><a name="Page_58" id="Page_58">[Pg 58]</a></span> the map.
+It is true that some people never saw these markings at all, and
+disbelieved in their existence; but others saw them clearly, and watched
+them change&mdash;first go fainter and then darker again. And quite recently
+a photograph has been obtained which shows them plainly, so they must
+have an existence, and cannot be only in the eye of the observer, as the
+most sceptical people were wont to suggest. But further than this, one
+astronomer announced that some of these lines appeared to be double, yet
+when he looked at them again they had grown single. It was like a
+conjuring trick. Great excitement was aroused by this, for if the canals
+were altered so greatly it really did look as if there were intelligent
+beings on Mars capable of working at them. In any case, if these are
+really canals, to make them would be a stupendous feat, and if they are
+artificial&mdash;that is, made by beings and not natural&mdash;they show a very
+high power of engineering. Imagine anyone on earth making a canal many
+miles wide and two thousand miles long! It is inconceivable, but that is
+the feat attributed to the Martians. The supposed doubling of the
+canals, as I say, caused a great deal of talk, and very few people could
+see that they were double at all. Even now the fact<span class='pagenum'><a name="Page_59" id="Page_59">[Pg 59]</a></span> is doubted, yet
+there seems every reason to believe it is true. They do not all appear
+to be double, and those that do are always the same ones, while others
+undoubtedly remain single all the time. But the canals do not exhaust
+the wonders of Mars. At each pole there is an ice-cap resembling those
+found at our own poles, and this tells us pretty plainly something about
+the climate of Mars, and that there is water there.</p>
+
+<p>This ice-cap melts when the pole which it surrounds is directed toward
+the sun, and sometimes in a hot summer it dwindles down almost to
+nothing, in a way that the ice-caps at the poles of the earth never do.
+A curious appearance has been noticed when it is melting: a dark shadow
+seems to grow underneath the edge of it and extends gradually, and as it
+extends the canals near it appear much darker and clearer than they did
+before, and then the canals further south undergo the same change. This
+looks as if the melting of the snow filled up the canals with water, and
+was a means of watering the planet by a system totally different from
+anything we know here, where our poles are surrounded by oceans, and the
+ice-caps do not in the least affect our water-supply. But, then, another
+strange fact had to be taken into con<span class='pagenum'><a name="Page_60" id="Page_60">[Pg 60]</a></span>sideration. These straight lines
+called canals ran out over the seas occasionally, and it was impossible
+to believe that if they were canals they could do that. Other things
+began to be discussed, such as the fact that the green parts of Mars did
+not always remain green. In what is the springtime of Mars they are so,
+but afterwards they become yellow, and still later in the season parts
+near the pole turn brown. Thus the idea that the greenish parts are seas
+had to be quite given up, though it appeared so attractive. The idea now
+generally believed is that the greenish parts are vegetation&mdash;trees and
+bushes and so on, and that the red parts are deserts of reddish sand,
+which require irrigation&mdash;that is to say, watering&mdash;before anything can
+be grown on them. The apparent doubling of the canals may be due to the
+green vegetation springing up along the banks. This might form two broad
+lines, while the canal itself would not be seen, and when the vegetation
+dies down, we should see only the trench of the canal, which would
+possibly appear faint and single. Therefore the arrangements on Mars
+appear to be a rich and a barren season on each hemisphere, the growth
+being caused by the melting of the polar ice-cap, which sends floods
+down even beyond the Equator. If we could<span class='pagenum'><a name="Page_61" id="Page_61">[Pg 61]</a></span> imagine the same thing on
+earth we should have to think of pieces of land lying drear and dry and
+dead in winter between straight canal-like ditches of vast size. A
+little water might remain in these ditches possibly, but not enough to
+water the surrounding land. Then, as summer progressed, we should hear,
+'The floods are coming,' and each deep, huge canal would be filled up
+with a tide of water, penetrating further and further. The water drawn
+up into the air would fall in dew or rain. Vegetation would spring up,
+especially near the canal banks, and instead of dreary wastes rich
+growths would cover the land, gradually dying down again in the winter.
+So far Mars seems in some important respects very different from the
+earth. He is also less favourably placed than we are, for being so much
+further from the sun, he receives very much less heat and light. His
+years are 687 of our days, or one year and ten and a half months, and
+his atmosphere is not so dense as ours. With this greater distance from
+the sun and less air we might suppose the temperature would be very cold
+indeed, and that the surface would be frost-bound, not only at the
+poles, but far down towards the Equator. Instead of this being so, as we
+have seen, the polar caps melt more than those on the earth. We can<span class='pagenum'><a name="Page_62" id="Page_62">[Pg 62]</a></span>
+only surmise there must be some compensation we do not know of that
+softens down the rigour of the seasons, and makes them milder than we
+should suppose possible.</p>
+
+<p>Of course, the one absorbing question is, Are there people on Mars? To
+this it is at present impossible to reply. We can only say the planet
+seems in every way fitted to support life, even if it is a little
+different from our earth. It is most certainly a living world, not a
+dead one like the moon, and as our knowledge increases we may some day
+be able to answer the question which so thrills us.</p>
+
+<p>Our opportunities for the observation of Mars vary very greatly, for as
+the earth's orbit lies inside that of Mars, we can best see him when we
+are between him and the sun. Of course, it must be remembered that the
+earth and the other planets are so infinitely small in regard to the
+space between them that there is no possibility of any one of them
+getting in such a position that it would throw a shadow on any other or
+eclipse it. The planets are like specks in space, and could not
+interfere with one another in this way. When Mars, therefore, is in a
+line with us and the sun we can see him best, but some of these times
+are better than others, for<span class='pagenum'><a name="Page_63" id="Page_63">[Pg 63]</a></span> this reason&mdash;the earth's orbit is nearly a
+circle, and that of Mars more of an ellipse.</p>
+
+<div class="figcenter" style="width: 650px;">
+<img src="images/i-088.jpg" width="650" height="648" alt="ORBITS OF THE EARTH AND MARS." title="" />
+<span class="caption">ORBITS OF THE EARTH AND MARS.</span>
+</div>
+
+<p>Look at the illustration and remember that Mars' year is not quite two
+of ours&mdash;that is to say, every time we swing round our orbit we catch
+him<span class='pagenum'><a name="Page_64" id="Page_64">[Pg 64]</a></span> up in a different place, for he will have progressed less than half
+his orbit while we go right round ours.</p>
+
+<p>Sometimes when we overtake him he may be at that part which is furthest
+away from us, or he may be at that part which is nearest to us, and if
+he is in the latter position we can see him best. Now at these, the most
+favourable times of all, he is still more than thirty-five millions of
+miles away&mdash;that is to say, one hundred and forty times as far as the
+moon, yet comparatively we can see him very well. He is coming nearer
+and nearer to us, and very soon will be nearer than he has been since
+1892, or fifteen years ago. Then many telescopes will be directed on
+him, and much may be learned about him.</p>
+
+<p>For a long time it was supposed that Mars had no moons, and when Dean
+Swift wrote 'Gulliver's Travels' he wanted to make the Laputans do
+something very clever, so he described their discovery of two moons
+attending Mars, and to make it quite absurd he said that when they
+observed these moons they found that one of them went round the planet
+in about ten hours. Now, as Mars takes more than twenty-four hours to
+rotate, this was considered ridiculous, for no moon known<span class='pagenum'><a name="Page_65" id="Page_65">[Pg 65]</a></span> then took
+less time to go round its primary world than the primary world took to
+turn on its own axis. Our own moon, of course, takes thirty times as
+long&mdash;that is a month contains thirty days. Then one hundred and fifty
+years later this jest of Dean Swift's came true, for two moons were
+really discovered revolving round Mars, and one of them does actually
+take less time to complete its orbit than the planet does to
+rotate&mdash;namely, a little more than seven hours! So the absurdity in
+'Gulliver's Travels' was a kind of prophecy!</p>
+
+<p>These two moons are very small, the outer one perhaps five or six miles
+in diameter, and the inner one about seven; therefore from Mars the
+outer one, Deimos, cannot look much more than a brilliant star, and the
+inner one would be but a fifth part the apparent width of our own moon.
+So Mars is not very well off, after all. Still, there is great variety,
+for it must be odd to see the same moon appearing three times in the
+day, showing all the different phases as it goes from new to full, even
+though it is small!</p>
+
+<p>Such wonderful discoveries have already been made that it is not too
+much to say that perhaps some day we may be able to establish some sort
+of communication with Mars, and if it be inhabited<span class='pagenum'><a name="Page_66" id="Page_66">[Pg 66]</a></span> by any intelligent
+beings, we may be able to signal to them; but it is almost impossible
+that any contrivance could bridge the gulf of airless space that
+separates us, and it is not likely that holiday trips to Mars will ever
+become fashionable!<span class='pagenum'><a name="Page_67" id="Page_67">[Pg 67]</a></span></p>
+
+
+
+<hr style="width: 65%;" />
+<h2><a name="CHAPTER_VI" id="CHAPTER_VI"></a>CHAPTER VI</h2>
+
+<h3>FOUR LARGE WORLDS</h3>
+
+
+<p>I have told you about the four lesser worlds of which our earth is one,
+and you know that beyond Mars, the last of them, there lies a vast
+space, in which are found the asteroids, those strange small planets
+circling near to each other, like a swarm of bees. After this there
+comes Jupiter, who is so enormous, so superb in size compared with us,
+that he might well serve as the sun of a little system of his own. You
+remember that we represented him by a football, while the earth was only
+a greengage plum. But Jupiter himself is far less in comparison with the
+sun than we are in comparison with him. He differs from the planets we
+have heard about up to the present in that he seems to glow with some
+heat that he does not receive from the sun. The illumination which makes
+him appear as a star to us is, of course, merely reflected sunlight, and
+what we see is the external covering, his envelope of cloud.</p>
+
+<p>There is every reason to believe that the great<span class='pagenum'><a name="Page_68" id="Page_68">[Pg 68]</a></span> bulk of Jupiter is
+still at a high temperature. We know that in the depths of the earth
+there is still plenty of heat, which every now and then makes its
+presence felt by bursting up through the vents we call volcanoes, the
+weak spots in the earth's crust; but our surface long ago cooled, for
+the outside of any body gets cool before the inside, as you may have
+found if ever you were trying to eat hot porridge, and circled round the
+edge of the plate with a spoon. A large body cools more slowly than a
+small one, and it is possible that Jupiter, being so much larger than we
+are, has taken longer to cool. One reason we have for thinking this is
+that he is so very light compared with his size&mdash;in other words, his
+density is so small that it is not possible he could be made of
+materials such as the earth is made of.</p>
+
+<p>As I said, when we study him through telescopes we see just the
+exterior, the outer envelope of cloud, and as we should expect, this
+changes continually, and appears as a series of belts, owing to the
+rotation of the planet. Jupiter's rotation is very rapid; though he is
+so much greater than the earth, he takes less than half the time the
+earth does to turn round&mdash;that is to say, only ten hours. His days and
+nights of five hours each seem short to us,<span class='pagenum'><a name="Page_69" id="Page_69">[Pg 69]</a></span> accustomed to measure
+things by our own estimates. But we must remember that everything is
+relative; that is to say, there is really no such thing as fast or slow;
+it is all by comparison. A spider runs fast compared with a snail, but
+either is terribly slow compared with an express train; and the speed of
+an express train itself is nothing to the velocity of light.</p>
+
+<p>In the same way there is nothing absolutely great or small; it is all by
+comparison. We say how marvellous it is that a little insect has all the
+mechanism of life in its body when it is so tiny, but if we imagine that
+insect magnified by a powerful microscope until it appears quite large,
+the marvel ceases. Again, imagine a man walking on the surface of the
+earth as seen from a great distance through a telescope: he would seem
+less than an insect, and we might ask how could the mechanism of life be
+compressed into anything so small? Thus, when we say enormous or tiny we
+must always remember we are only speaking by the measurements of our own
+standards.</p>
+
+<p>There is nothing very striking about Jupiter's orbit. He takes between
+eleven and twelve of our years to get round the sun, so you see, though
+his day is shorter, his year is longer than ours. And<span class='pagenum'><a name="Page_70" id="Page_70">[Pg 70]</a></span> this is not only
+because his path is much larger, but because by the law of gravity the
+more distant a planet is from the sun the more slowly it travels, so
+that while the earth speeds over eighteen miles Jupiter has only done
+eight. Of course, we must be careful to remember the difference between
+rotation and revolution. Jupiter rotates much quicker than the
+earth&mdash;that is to say, he turns round more quickly&mdash;but he actually gets
+over the ground more slowly. The sun appears much smaller to him than it
+does to us, and he receives considerably less light and heat. There are
+various spots on his surface, and one remarkable feature is a dark mark,
+which is called the 'great red spot.' If as we suppose what we see of
+the planet is merely the cloudy upper atmosphere, we should not expect
+to find anything permanent there, for the markings would change from day
+to day, and this they do with this exception&mdash;that this spot, dark red
+in colour, has been seen for many years, turning as the planet turned.
+It was first noticed in 1878, and was supposed to be some great mountain
+or excrescence peeping up through the clouds. It grew stronger and
+darker for several years, and then seemed to fade, and was not so easily
+seen, and though still remaining it is now pale. But, most startling<span class='pagenum'><a name="Page_71" id="Page_71">[Pg 71]</a></span>
+to say, it has shifted its position a little&mdash;that is, it takes a few
+seconds longer to get round the planet than it did at first. A few
+seconds, you will say, but that is nothing! It does not seem much, but
+it shows how marvellously accurate astronomers are. Discoveries of vast
+importance have been made from observing a few seconds' discrepancy in
+the time the heavenly bodies take in their journeys, and the fact that
+this spot takes a little longer in its rotation than it did at first
+shows that it cannot be attached to the body of the planet. It is
+impossible for it to be the summit of a mountain or anything of that
+sort. What can it be? No one has yet answered that question.</p>
+
+<div class="figcenter" style="width: 600px;">
+<img src="images/i-096.jpg" width="600" height="405" alt="JUPITER AND ONE OF HIS MOONS" title="" />
+<span class="caption">JUPITER AND ONE OF HIS MOONS</span>
+</div>
+
+<p>When we get to the chapter on the sun, we shall find curiosities
+respecting the spots there as well.</p>
+
+<p>Jupiter has seven moons, and four of these are comparatively large. They
+have the honour of having been the first heavenly bodies ever actually
+discovered, for the six large planets nearest the sun have been known so
+long that there is no record of their first discovery, and of course our
+own moon has always been known. Galileo, who invented the telescope,
+turned it on to the sky in 1610, when our King Charles I. was on the
+throne, and he saw these curious bodies which at first he could<span class='pagenum'><a name="Page_72" id="Page_72">[Pg 72]</a></span> not
+believe to be moons. The four which he saw vary in size from two
+thousand one hundred miles in diameter to nearly three thousand six
+hundred. You remember our own moon is two thousand miles across, so even
+the smallest is larger than she. They go round at about the same level
+as the planet's Equator, and therefore they cross right in front of him,
+and go behind him once in every revolution. Since then the other three
+have been discovered in the band of Jupiter's satellites&mdash;one a small
+moon closer to him than any of the first set, and two others further
+out. It was by observation of the first four, however, that very
+interesting results were obtained. Mathematicians calculated the time
+that these satellites ought to disappear behind Jupiter and reappear
+again, but they found that this did not happen exactly at the time
+predicted; sometimes the moons disappeared sooner<span class='pagenum'><a name="Page_73" id="Page_73">[Pg 73]</a></span> than they should have
+done, and sometimes later. Then this was discovered to have some
+relation to the distance of our earth from Jupiter. When he was at the
+far side of his immense orbit he was much more distant from us than when
+he was on the nearer side&mdash;in fact, the difference may amount to more
+than three hundred millions of miles. And it occurred to some clever man
+that the irregularities in time we noticed in the eclipses of the
+satellites corresponded with the distance of Jupiter from us. The
+further he drew away from us, the later were the eclipses, and as he
+came nearer they grew earlier. By a brilliant inspiration, this was
+attributed to the time light took to travel from them to us, and this
+was the first time anyone had been able to measure the velocity or speed
+of light. For all practical purposes, on the earth's surface we hold
+light to be instantaneous, and well we may, for light could travel more
+than eight times round the world in one second. It makes one's brain
+reel to think of such a thing. Then think how far Jupiter must be away
+from us at the furthest, when you hear that sometimes these eclipses
+were delayed seventeen minutes&mdash;minutes, not seconds&mdash;because it took
+that time for light to cross the gulf to us!</p>
+
+<div class="figcenter" style="width: 650px;">
+<img src="images/i-099.jpg" width="650" height="214" alt="JUPITER AND HIS PRINCIPAL MOONS." title="" />
+<span class="caption">JUPITER AND HIS PRINCIPAL MOONS.</span>
+</div>
+
+<p>Sound is very slow compared with light, and<span class='pagenum'><a name="Page_74" id="Page_74">[Pg 74]</a></span> that is why, if you watch a
+man hammering at a distance, the stroke he gives the nail does not
+coincide with the bang that reaches you, for light gets to you
+practically at once, and the sound comes after it. No sound can travel
+without air, as we have heard, therefore no sound reaches us across
+space. If the moon were to blow up into a million pieces we should see
+the amazing spectacle, but should hear nothing of it. Light travels
+everywhere throughout the universe, and by the use of this universal
+carrier we have learnt all that we know about the stars and planets.
+When the time that light takes to travel had been ascertained by means
+of Jupiter's satellites, a still more important problem could be
+solved&mdash;that was our own distance from the sun, which before had only
+been known approximately, and this was calculated to be ninety-two
+millions seven hundred thousand miles, though sometimes we are a little
+nearer and sometimes a little further away.</p>
+
+<p>Jupiter is marvellous, but beyond him lies the most wonderful body in
+the whole solar system. We have found curiosities on our way out: we
+have studied the problem of the asteroids, of the little moon that goes
+round Mars in less time than Mars himself rotates; we have considered
+the 'great<span class='pagenum'><a name="Page_75" id="Page_75">[Pg 75]</a></span> red spot' on Jupiter, which apparently moves independently
+of the planet; but nothing have we found as yet to compare with the
+rings of Saturn. May you see this amazing sight through a telescope one
+day!</p>
+
+<p>Look at the picture of this wonderful system, and think what it would be
+like if the earth were surrounded with similar rings! The first question
+which occurs to all of us is what must the sky look like from Saturn?
+What must it be to look up overhead and see several great hoops or
+arches extending from one horizon to another, reflecting light in
+different degrees of intensity? It would be as if we saw several immense
+rainbows, far larger than any earthly rainbow, and of pure light, not
+split into colours, extending permanently across the sky, and now and
+then broken by the black shadow of the planet itself as it came between
+them and the sun. However, we must begin at the beginning, and find out
+about Saturn himself before we puzzle ourselves over his rings. Saturn
+is not a very great deal less than Jupiter, though, so small are the
+other planets in comparison, that if Saturn and all the rest were rolled
+together, they would not make one mass so bulky as Jupiter! Saturn is
+so<span class='pagenum'><a name="Page_76" id="Page_76">[Pg 76]</a></span> light&mdash;in other words, his density is so small&mdash;that he is actually
+lighter than water. He is the lightest, in comparison with his size, of
+any of the planets. Therefore he cannot be made largely of solid land,
+as our earth is, but must be to a great extent, composed of air and
+gaseous vapour, like his mighty neighbour. He approaches at times as
+near to Jupiter as Jupiter does to us, and on these occasions he must
+present a splendid spectacle to Jupiter. He takes no less than
+twenty-nine and a half of our years to complete his stately march around
+the sun, and his axis is a little more bent than ours; but, of course,
+at his great distance from the sun, this cannot have the same effect on
+the seasons that it does with us. Saturn turns fast on his axis, but not
+so fast as Jupiter, and in turning his face, or what we call his
+surface, presents much the same appearance to us that we might expect,
+for it changes very frequently and looks like cloud belts.</p>
+
+<p>The marvellous feature about Saturn is, of course, the rings. There are
+three of these, lying one within the other, and separated by a fine line
+from each other. The middle one is much the broadest, probably about ten
+thousand miles in width, and the inner one, which is the darkest, was
+not discovered until some time after the others. As the planet swings in
+his orbit<span class='pagenum'><a name="Page_77" id="Page_77">[Pg 77]</a></span> the rings naturally appear very different to us at different
+times. Sometimes we can only see them edgewise, and then even in the
+largest telescope they are only like a streak of light, and this shows
+that they cannot be more than fifty or sixty miles in thickness. The one
+which is nearest to Saturn's surface does not approach him within ten
+thousand miles. Saturn has no less than ten satellites, in addition to
+the rings, so that his midnight sky must present a magnificent
+spectacle. The rings, which do not shine by their own light but by
+reflected sunlight, are solid enough to throw a shadow on the body of
+the planet, and themselves receive his shadow. Sometimes for days
+together a large part of Saturn must suffer eclipse beneath the
+encircling rings, but at other times, at night, when the rings are clear
+of the planet's body, so that the light is not cut off from them, they
+must appear as radiant arches of glory spanning the sky.</p>
+
+<p>The subject of these rings is so complicated by the variety of their
+changes that it is difficult for us even to think about it. It is one of
+the most marvellous of all the features of our planetary system. What
+are these rings? what are they made of? It has been positively proved
+that they cannot be made of continuous matter,<span class='pagenum'><a name="Page_78" id="Page_78">[Pg 78]</a></span> either liquid or solid,
+for the force of gravity acting on them from the planet would tear them
+to pieces. What, then, can they be? It is now pretty generally believed
+that they are composed of multitudes of tiny bodies, each separate, and
+circling separately round the great planet, as the asteroids circle
+round the sun. As each one is detached from its neighbour and obeys its
+own impulses, there is none of the strain and wrench there would be were
+they all connected. According to the laws which govern planetary bodies,
+those which are nearest to the planet will travel more quickly than
+those which are further away. Of course, as we look at them from so
+great a distance, and as they are moving, they appear to us to be
+continuous. It is conjectured that the comparative darkness of the
+inside ring is caused by the fact that there are fewer of the bodies
+there to reflect the sunlight. Then, in addition to the rings, enough
+themselves to distinguish him from all other planets, there are the ten
+moons of richly-endowed Saturn to be considered. It is difficult to
+gather much about these moons, on account of our great distance from
+them. The largest is probably twice the diameter of our own moon. One of
+them seems to be much brighter&mdash;that is to say, of higher reflecting
+power&mdash;on one side than<span class='pagenum'><a name="Page_79" id="Page_79">[Pg 79]</a></span> the other, and by distinguishing the sides
+and watching carefully, astronomers have come to the conclusion that it
+presents always the same face to Saturn in the same way as our own moon
+does to us; in fact, there is reason to think that all the moons of
+large planets do this.</p>
+
+<div class="figcenter" style="width: 650px;">
+<img src="images/i-106.jpg" width="650" height="445" alt="THE PLANET SATURN WITH TWO OF HIS MOONS." title="" />
+<span class="caption">THE PLANET SATURN WITH TWO OF HIS MOONS.</span>
+</div>
+
+<p>All the moons lie outside the rings, and some at a very great distance
+from Saturn, so that they can only appear small as seen from him. Yet at
+the worst they must be brighter than ordinary stars, and add greatly to
+the variations in the sky scenery of this beautiful planet. In
+connection with Saturn's moons there is another of those astonishing
+facts that are continually cropping up to remind us that, however much
+we know, there is such a vast deal of which we are still ignorant. So
+far in dealing with all the planets and moons in the solar system we
+have made no remark on the way they rotate or revolve, because they all
+go in the same direction, and that direction is called
+counter-clockwise, which means that if you stand facing a clock and turn
+your hand slowly round the opposite direction to that in which the hands
+go, you will be turning it in the same way that the earth rotates on its
+axis and revolves in its orbit. It is, perhaps, just as well to give
+here a word of caution. Rotating<span class='pagenum'><a name="Page_80" id="Page_80">[Pg 80]</a></span> of course means a planet's turning on
+its own axis, revolving means its course in its orbit round the sun.
+Mercury, Venus, Earth, Mars, Jupiter, and all their moons, as well as
+Saturn himself, rotate on their axes in this one
+direction&mdash;counter-clockwise&mdash;and revolve in the same direction as they
+rotate. Even the queer little moon of Mars, which runs round him quicker
+than he rotates, obeys this same rule. Nine of Saturn's moons follow
+this example, but one independent little one, which has been named
+Ph&oelig;be, and is far out from the planet, actually revolves in the
+opposite way. We cannot see how it rotates, but if, as we said just now,
+it turns the same face always to Saturn, then of course it rotates the
+wrong way too. A theory has been suggested to account for this curious
+fact, but it could not be made intelligible to anyone who has not
+studied rather high mathematics, so there we must just leave it, and put
+it in the cabinet of curiosities we have already collected on our way
+out to Saturn.</p>
+
+<p>For ages past men have known and watched the planets lying within the
+orbit of Saturn, and they had made up their minds that this was the
+limit of our system. But in 1781 a great astronomer named Herschel was
+watching the heavens through a tele<span class='pagenum'><a name="Page_81" id="Page_81">[Pg 81]</a></span>scope when he noticed one strange
+object that he was certain was no star. The vast distance of the stars
+prevents their having any definite outline, or what is called a disc.
+The rays dart out from them in all directions and there is no 'edge' to
+them, but in the case of the planets it is possible to see a disc with a
+telescope, and this object which attracted Herschel's attention had
+certainly a disc. He did not imagine he had discovered a new planet,
+because at that time the asteroids had not been found, and no one
+thought that there could be any more planets. Yet Herschel knew that
+this was not a star, so he called it a comet! He was actually the first
+who discovered it, for he knew it was not a fixed star, but it was after
+his announcement of this fact that some one else, observing it
+carefully, found it to be a real planet with an orbit lying outside that
+of Saturn, then the furthest boundary of the solar system. Herschel
+suggested calling it Georgius Sidus, in honour of George III., then
+King; but luckily this ponderous name was not adopted, and as the other
+planets had been called after the Olympian deities, and Uranus was the
+father of Saturn, it was called Uranus. It was subsequently found that
+this new planet had already been observed by other astro<span class='pagenum'><a name="Page_82" id="Page_82">[Pg 82]</a></span>nomers and
+catalogued as a star no less than seventeen times, but until Herschel's
+clear sight had detected the difference between it and the fixed stars
+no one had paid any attention to it. Uranus is very far away from the
+sun, and can only sometimes be seen as a small star by people who know
+exactly where to look for him. In fact, his distance from the sun is
+nineteen times that of the earth.</p>
+
+<p>Yet to show at all he must be of great size, and that size has actually
+been found out by the most delicate experiments. If we go back to our
+former comparison, we shall remember that if the earth were like a
+greengage plum, then Uranus would be in comparison about the size of one
+of those coloured balloons children play with; therefore he is much
+larger than the earth.</p>
+
+<p>In this far distant orbit the huge planet takes eighty-four of our years
+to complete one of his own. A man on the earth will have grown from
+babyhood to boyhood, from boyhood to the prime of life, and lived longer
+than most men, while Uranus has only once circled in his path.</p>
+
+<p>But in dealing with Uranus we come to another of those startling
+problems of which astronomy is full. So far we have dealt with planets
+which are more or less upright, which rotate with a rotation<span class='pagenum'><a name="Page_83" id="Page_83">[Pg 83]</a></span> like that
+of a top. Now take a top and lay it on one side on the table, with one
+of its poles pointing toward the great lamp we used for the sun and the
+other pointing away. That is the way Uranus gets round his path, on his
+side! He rotates the wrong way round compared with the planets we have
+already spoken of, but he revolves the same way round the sun that all
+the others do. It seems wonderful that even so much can be found out
+about a body so far from us, but we know more: we have discovered that
+Uranus is made of lighter material than the earth; his density is less.
+How can that be known? Well, you remember every body attracts every
+other body in proportion to the atoms it contains. If, therefore, there
+were any bodies near to Uranus, it could be calculated by his influence
+on them what was his own mass, which, as you remember, is the word we
+use to express what would be weight were it at the earth's surface; and
+far away as Uranus is, the bodies from which such calculations may be
+made have been discovered, for he has no less than four satellites, or
+moons. Considering now the peculiar position of the planet, we might
+expect to find these moons revolving in a very different way from
+others, and this is indeed the<span class='pagenum'><a name="Page_84" id="Page_84">[Pg 84]</a></span> case. They turn round the planet at
+about its Equator&mdash;that is to say, if you hold the top representing
+Uranus as was suggested just now, these moons would go above and below
+the planet in passing round it. Only we must remember there is really no
+such thing as above and below absolutely. We who are on one side of the
+world point up to the sky and down to the earth, while the people on the
+other side of the earth, say at New Zealand, also point up to the sky
+and down to the earth, but their pointings are directly the opposite of
+ours. So when we speak of moons going above and below that is only
+because, for the moment, we are representing Uranus as a top we hold in
+our hands, and so we speak of above and below as they are to us.</p>
+
+<p>It was Herschel who discovered these satellites, as well as the planet,
+and for these great achievements he occupies one of the grandest places
+in the r&ocirc;le of names of which England is proud. But he did much more
+than this: his improvements in the construction of telescopes, and his
+devotion to astronomy in many other ways, would have caused him to be
+remembered without anything else.</p>
+
+<p>Of Uranus's satellites one, the nearest, goes round in about two and a
+half days, and the one that is<span class='pagenum'><a name="Page_85" id="Page_85">[Pg 85]</a></span> furthest away takes about thirteen and a
+half days, so both have a shorter period than our moon.</p>
+
+<p>The discovery of Uranus filled the whole civilized world with wonder.
+The astronomers who had seen him, but missed finding out that he was a
+planet, must have felt bitterly mortified, and when he was discovered he
+was observed with the utmost accuracy and care. The calculations made to
+determine his path in the sky were the easier because he had been noted
+as a star in several catalogues previously, so that his position for
+some time past was known. Everybody who worked at astronomy began to
+observe him. From these facts mathematicians set to work, and, by
+abstruse calculations, worked out exactly the orbit in which he ought to
+move; then his movements were again watched, and behold he followed the
+path predicted for him; but there was a small difference here and there:
+he did not follow it exactly. Now, in the heavens there is a reason for
+everything, though we may not always be clever enough to find it out,
+and it was easily guessed that it was not by accident that Uranus did
+not precisely follow the path calculated for him. The planets all act
+and react on one another, as we know, according to their mass and their
+distance, and in the calcula<span class='pagenum'><a name="Page_86" id="Page_86">[Pg 86]</a></span>tions the pull of Jupiter on Saturn and of
+Saturn on Uranus were known and allowed for. But Uranus was pulled by
+some unseen influence also.</p>
+
+<p>A young Englishman named Adams, by some abstruse and difficult
+mathematical work far beyond the power of ordinary brains, found out not
+only the fact that there must be another planet nearly as large as
+Uranus in an orbit outside his, but actually predicted where such a
+planet might be seen if anyone would look for it. He gave his results to
+a professor of astronomy at Cambridge. Now, it seems an easy thing to
+say to anyone, 'Look out for a planet in such and such a part of the
+sky,' but in reality, when the telescope is turned to that part of the
+sky, stars are seen in such numbers that, without very careful
+comparison with a star chart, it is impossible to say which are fixed
+stars and which, if any, is an intruder. There happened to be no star
+chart of this kind for the particular part of the sky wanted, and thus a
+long time elapsed and the planet was not identified. Meantime a young
+Frenchman named Leverrier had also taken up the same investigation, and,
+without knowing anything of Adams' work, had come to the same
+conclusion. He sent his results to the Berlin Observatory, where a star
+chart such<span class='pagenum'><a name="Page_87" id="Page_87">[Pg 87]</a></span> as was wanted was actually just being made. By the use of
+this the Berlin astronomers at once identified this new member of our
+system, and announced to the astonished world that another large planet,
+making eight altogether, had been discovered. Then the English
+astronomers remembered that they too held in their hands the means for
+making this wonderful discovery, but, by having allowed so much time to
+elapse, they had let the honour go to France. However, the names of
+Adams and Leverrier will always be coupled together as the discoverers
+of the new planet, which was called Neptune. The marvel is that by pure
+reasoning the mind of man could have achieved such results.</p>
+
+<p>If the observation of Uranus is difficult, how much more that of
+Neptune, which is still further plunged in space! Yet by patience a few
+facts have been gleaned about him. He is not very different in size from
+Uranus. He also is of very slight density. His year includes one hundred
+and sixty-five of ours, so that since his discovery in 1846 he has only
+had time to get round less than a third of his path. His axis is even
+more tilted over than that of Uranus, so that if we compare Uranus to a
+top held horizontally, Neptune will be like a top with one end pointing<span class='pagenum'><a name="Page_88" id="Page_88">[Pg 88]</a></span>
+downwards. He rotates in this extraordinary position, in the same manner
+as Uranus&mdash;namely, the other way over from all the other planets, but he
+revolves, as they all do, counter-clockwise.</p>
+
+<p>Seen from Neptune the sun can only appear about as large as Venus
+appears to us at her best, and the light and heat received are but one
+nine-hundreth part of what he sends us. Yet so brilliant is sunshine
+that even then the light that falls on Neptune must be very
+considerable, much more than that which we receive from Venus, for the
+sun itself glows, and from Venus the light is only reflected. The sun,
+small as it must appear, will shine with the radiance of a glowing
+electric light. To get some idea of the brilliance of sunlight, sit near
+a screen of leaves on some sunny day when the sun is high overhead, and
+note the intense radiance of even the tiny rays which shine through the
+small holes in the leaves. The scintillating light is more glorious than
+any diamond, shooting out coloured rays in all directions. A small sun
+the apparent size of Venus would, therefore, give enough light for
+practical purposes to such a world as Neptune, even though to us a world
+so illuminated would seem to be condemned to a perpetual twilight.<span class='pagenum'><a name="Page_89" id="Page_89">[Pg 89]</a></span></p>
+
+
+
+<hr style="width: 65%;" />
+<h2><a name="CHAPTER_VII" id="CHAPTER_VII"></a>CHAPTER VII</h2>
+
+<h3>THE SUN</h3>
+
+
+<p>So far we have referred to the sun just so much as was necessary to show
+the planets rotating round him, and to acknowledge him as the source of
+all our light and heat; but we have not examined in detail this
+marvellous furnace that nourishes all the life on our planet and burns
+on with undiminished splendour from year to year, without thought or
+effort on our part. To sustain a fire on the earth much time and care
+and expense are necessary; fuel has to be constantly supplied, and men
+have to stoke the fire to keep it burning. Considering that the sun is
+not only vastly larger than all the fires on the earth put together, but
+also than the earth itself, the question very naturally occurs to us,
+Who supplies the fuel, and who does the stoking on the sun? Before we
+answer this we must try to get some idea of the size of this stupendous
+body. It is not the least use attempting to understand it by plain
+figures, for the figures<span class='pagenum'><a name="Page_90" id="Page_90">[Pg 90]</a></span> would be too great to make any impression on
+us&mdash;they would be practically meaningless; we must turn to some other
+method. Suppose, for instance, that the sun were a hollow ball; then, if
+the earth were set at the centre, the moon could revolve round her at
+the same distance she is now, and there would be as great a distance
+between the moon and the shell of the sun as there is between the moon
+and the earth. This gives us a little idea of the size of the sun.
+Again, if we go back to that solar system in which we represented the
+planets by various objects from a pea to a football, and set a lamp in
+the centre to do duty for the sun, what size do you suppose that lamp
+would have to be really to represent the sun in proportion to the
+planets? Well, if our greengage plum which did duty for the earth were
+about three-quarters of an inch in diameter we should want a lamp with a
+flame as tall as the tallest man you know, and even then it would not
+give a correct idea unless you imagined that man extending his arms
+widely, and you drew round him a circle and filled in all the circle
+with flame! If this glorious flame burnt clear and fair and bright,
+radiating beams of light all around, the little greengage plum would not
+have to be too near, or it would be shrivelled up as in the blast of a
+furnace. To place it at any<span class='pagenum'><a name="Page_91" id="Page_91">[Pg 91]</a></span>thing resembling the distance it is from the
+sun in reality you would have to walk away from the flaming light for
+about three hundred steps, and set it down there; then, after having
+done all this, you would have some little idea of the relative sizes of
+the sun and the earth, and of the distance between them.</p>
+
+<p>Of course, all the other planets would have to be at corresponding
+distances. On this same scale, Neptune, the furthest out, would be three
+miles from our artificial sun! It seems preposterous to think that some
+specks so small as to be quite invisible, specks that crawl about on
+that plum, have dared to weigh and measure the gigantic sun; but yet
+they have done it, and they have even decided what he is made of. The
+result of the experiments is that we know the sun to be a ball of
+glowing gas at a temperature so high that nothing we have on earth could
+even compare with it. Of his radiating beams extending in all directions
+few indeed fall on our little plum, but those that do are the source of
+all life, whether animal or vegetable. If the sun's rays were cut off
+from us, we should die at once. Even the coal we use to keep us warm is
+but sun's heat stored up ages ago, when the luxuriant tropical
+vegetation sprang up in the<span class='pagenum'><a name="Page_92" id="Page_92">[Pg 92]</a></span> warmth and then fell down and was buried in
+the earth. At night we are still enjoying the benefit of the sun's
+rays&mdash;that is, of those which are retained by our atmosphere; for if
+none remained even the very air itself would freeze, and by the next
+morning not one inhabitant would be left alive to tell the awful tale.
+Yet all this life and growth and heat we receive on the whole earth is
+but one part in two thousand two hundred millions of parts that go out
+in all directions into space. It has been calculated that the heat which
+falls on to all the planets together cannot be more than one part in one
+hundred millions and the other millions of parts seem to us to be simply
+wasted.</p>
+
+<p>For untold ages the sun has been pouring out this prodigal profusion of
+glory, and as we know that this cannot go on without some sort of
+compensation, we want to understand what keeps up the fires in the sun.
+It is true that the sun is so enormous that he might go on burning for a
+very long time without burning right away; but, then, even if he is
+huge, his expenditure is also huge. If he had been made of solid coal he
+would have been all used up in about six thousand years, burning at the
+pace he does. Now, we know that the ancient Egyptians kept careful note
+of the heavenly<span class='pagenum'><a name="Page_93" id="Page_93">[Pg 93]</a></span> bodies, and if the sun were really burning away he must
+have been very much larger in their time; but we have no record of this;
+on the contrary, all records of the sun even to five thousand years ago
+show that he was much the same as at present. It is evident that we must
+search elsewhere for an explanation. It has been suggested that his
+furnace is supplied by the number of meteors that fall into him. Meteors
+are small bodies of the same materials as the planets, and may be
+likened to the dust of the solar system. It is not difficult to
+calculate the amount of matter he would require on this assumption to
+keep him going, and the amount required is so great as to make it
+practically impossible that this is the source of his supply. We have
+seen that all matter influences all other matter, and the quantity of
+meteoric stuff that would be required to support the sun's expenditure
+would be enough to have a serious effect on Mercury, an effect that
+would certainly have been noticed. There can, therefore, be no such mass
+of matter near the sun, and though there is no doubt a certain number of
+meteors do fall into his furnaces day by day, it is not nearly enough to
+account for his continuous radiation. It seems after this as if nothing
+else could be suggested; but yet an answer has been found, an answer so<span class='pagenum'><a name="Page_94" id="Page_94">[Pg 94]</a></span>
+wonderful that it is more like a fairy tale than reality.</p>
+
+<p>To begin at the beginning, we must go back to the time when the sun was
+only a great gaseous nebula filling all the space included in the orbit
+of Neptune. This nebula was not in itself hot, but as it rotated it
+contracted. Now, heat is really only a form of energy, and energy and
+heat can be interchanged easily. This is a very startling thing when
+heard for the first time, but it is known as surely as we know anything
+and has been proved again and again. When a savage wants to make a fire
+he turns a piece of hard wood very very quickly between his
+palms&mdash;twiddles it, we should say expressively&mdash;into a hole in another
+piece of wood, until a spark bursts out. What is the spark? It is the
+energy of the savage's work turned to heat. When a horse strikes his
+iron-shod hoofs hard on the pavement you see sparks fly; that is caused
+by the energy of the horse's leg. When you pump hard at your bicycle you
+feel your pump getting quite hot, for part of the energy you are putting
+into your work is transformed into heat; and so on in numberless
+instances. No energetic action of any kind in this world takes place
+without some of the energy being turned into heat, though in many
+instances the<span class='pagenum'><a name="Page_95" id="Page_95">[Pg 95]</a></span> amount is so small as to be unnoticeable. Nothing falls
+to the ground without some heat being generated. Now, when this great
+nebula first began its remarkable career, by the action of gravity all
+the particles in it were drawn toward the centre; little by little they
+fell in, and the nebula became smaller. We are not now concerned with
+the origin of the planets&mdash;we leave that aside; we are only
+contemplating the part of the nebula which remained to become the sun.
+Now these particles being drawn inward each generated some heat, so as
+the nebula contracted its temperature rose. Throughout the ages, over
+the space of millions and millions of miles, it contracted and grew
+hotter. It still remained gaseous, but at last it got to an immense
+temperature, and is the sun as we know it. What then keeps it shining?
+It is still contracting, but slowly, so slowly that it is quite
+imperceptible to our finest instruments. It has been calculated that if
+it contracts two hundred and fifty feet in diameter in a year, the
+energy thus gained and turned into heat is quite sufficient to account
+for its whole yearly output. This is indeed marvellous. In comparison
+with the sun's size two hundred and fifty feet is nothing. It would take
+nine thousand years at this rate before any diminution could be<span class='pagenum'><a name="Page_96" id="Page_96">[Pg 96]</a></span> noticed
+by our finest instruments! Here is a source of heat which can continue
+for countless ages without exhaustion. Thus to all intents and purposes
+we may say the sun's shining is inexhaustible. Yet we must follow out
+the train of reasoning, and see what will happen in the end, in eras and
+eras of time, if nothing intervenes. Well, some gaseous bodies are far
+finer and more tenuous than others, and when a gaseous body contracts it
+is all the time getting denser; as it grows denser and denser it at last
+becomes liquid, and then solid, and then it ceases to contract, as of
+course the particles of a solid body cannot fall freely toward the
+centre, as those of a gaseous body can. Our earth has long ago reached
+this stage. When solid the action ceases, and the heat is no more kept
+up by this source of energy, therefore the body begins to cool&mdash;surface
+first, and lastly the interior; it cools more quickly the smaller it is.
+Our moon has parted with all her heat long ago, while the earth still
+retains some internally. In the sun, therefore, we have an object-lesson
+of the stages through which all the planets must have passed. They have
+all once been glowing hot, and some may be still hot even on the
+surface, as we have seen there is reason to believe is the case with
+Jupiter.<span class='pagenum'><a name="Page_97" id="Page_97">[Pg 97]</a></span></p>
+
+<p>By this marvellous arrangement for the continued heat of the sun we can
+see that the warmth of our planets is assured for untold ages. There is
+no need to fear that the sun will wear out by burning. His brightness
+will continue for ages beyond the thoughts of man.</p>
+
+<p>Besides this, a few other things have been discovered about him. He is,
+of course, exceptionally difficult to observe; for though he is so
+large, which should make it easy, he is so brilliant that anyone
+regarding him through a telescope without the precaution of prepared
+glasses to keep off a great part of the light would be blinded at once.
+One most remarkable fact about the sun is that his surface is flecked
+with spots, which appear sometimes in greater numbers and sometimes in
+less, and the reason and shape of these spots have greatly exercised
+men's minds. Sometimes they are large enough to be seen without a
+telescope at all, merely by looking through a piece of smoked or
+coloured glass, which cuts off the most overpowering rays. When they are
+visible like this they are enormous, large enough to swallow many earths
+in their depths. At other times they may be observed by the telescope,
+then they may be about five thousand miles across. Sometimes one spot
+can be followed<span class='pagenum'><a name="Page_98" id="Page_98">[Pg 98]</a></span> by an astronomer as it passes all across the sun,
+disappears at the edge, and after a lapse of time comes back again round
+the other edge. This first showed men that the sun, like all the
+planets, rotated on his axis, and gave them the means of finding out how
+long he took in doing so. But the spots showed a most surprising result,
+for they took slightly different times in making their journey round the
+sun, times which differed according to their position. For instance, a
+spot near the equator of the sun took twenty-five days to make the
+circuit, while one higher up or lower down took twenty-six days, and one
+further out twenty-seven; so that if these spots are, as certainly
+believed, actually on the surface, the conclusion is that the sun does
+not rotate all in one piece, but that some parts go faster than others.
+No one can really explain how this could be, but it is certainly more
+easily understood in the case of a body of gas than of a solid body,
+when it would be simply impossible to conceive. The spots seem to keep
+principally a little north and a little south of the equator; there are
+very few actually at it, and none found near the poles, but no reason
+for this distribution has been discovered. It has been noted that about
+every eleven years the greatest number of spots appears,<span class='pagenum'><a name="Page_99" id="Page_99">[Pg 99]</a></span> and that
+they become fewer again, mounting up in number to the next eleven years,
+and so on. All these curious facts show there is much yet to be solved
+about the sun. The spots were supposed for long to be eruptions bursting
+up above the surface, but now they are generally held to be deep
+depressions like saucers, probably caused by violent tempests, and it is
+thought that the inrush of cooler matter from above makes them look
+darker than the other parts of the sun's surface. But when we use the
+words 'cooler' and 'darker,' we mean only by comparison, for in reality
+the dark parts of the spots are brighter than electric light.</p>
+
+<div class="figcenter" style="width: 643px;">
+<a name="i128" id="i128"></a>
+<img src="images/i-128.jpg" width="643" height="650" alt="Royal Observatory, Greenwich.
+
+SUN-SPOTS." title="" />
+<span class="caption">Royal Observatory, Greenwich.
+
+SUN-SPOTS.</span>
+</div>
+
+<p>The fact that the spots are in reality depressions or holes is shown by
+their change of appearance as they pass over the face of the sun toward
+the edge; for the change of shape is exactly that which would be caused
+by foreshortening.</p>
+
+<p>It sounds odd to say that the best time for observing the sun is during
+a total eclipse, for then the sun's body is hidden by the moon. But yet
+to a certain extent this is true, and the reason is that the sun's own
+brilliance is our greatest hindrance in observing him, his rays are so
+dazzling that they light up our own atmosphere, which prevents us seeing
+the edges. Now, during a total eclipse,<span class='pagenum'><a name="Page_100" id="Page_100">[Pg 100]</a></span> when nearly all the rays are
+cut off, we can see marvellous things, which are invisible at other
+times. But total eclipses are few and far between, and so when one is
+approaching astronomers make great preparations beforehand.</p>
+
+<div class="figcenter" style="width: 447px;">
+<img src="images/i-132.jpg" width="447" height="650" alt="THE EARTH AS IT WOULD APPEAR IN COMPARISON WITH THE
+FLAMES SHOOTING OUT FROM THE SUN." title="" />
+<span class="caption">THE EARTH AS IT WOULD APPEAR IN COMPARISON WITH THE
+FLAMES SHOOTING OUT FROM THE SUN.</span>
+</div>
+
+
+<p>A total eclipse is not visible from all parts of the world, but only
+from that small part on which the shadow of the moon falls, and as the
+earth travels, this shadow, which is really a round spot, passes along,
+making a dark band. In this band astronomers choose the best
+observatories, and there they take up their stations. The dark body of
+the moon first appears to cut a little piece out of the side of the sun,
+and as it sails on, gradually blotting out more and more, eager
+telescopes follow it; at last it covers up the whole sun, and then a
+marvellous spectacle appears, for all round the edges of the black moon
+are seen glorious red streamers and arches and filaments of marvellous
+shapes, continually changing. These are thrown against a background of
+pale green light that surrounds the black moon and the hidden sun. In
+early days astronomers thought these wonderful coloured streamers
+belonged to the moon; but it was soon proved that they really are part
+of the sun, and are only invisible at ordinary times, because our
+atmosphere<span class='pagenum'><a name="Page_101" id="Page_101">[Pg 101]</a></span> is too bright to allow them to be seen. An instrument has
+now been invented to cut off most of the light of the sun, and when this
+is attached to a telescope these prominences, as they are called, can be
+seen at any time, so that there is no need to wait for an eclipse.</p>
+
+
+<p>What are these marvellous streamers and filaments? They are what they
+seem, eruptions of fiery matter discharged from the ever-palpitating sun
+thousands of miles into surrounding space. They are for ever shooting
+out and bursting and falling back, fireworks on a scale too enormous for
+us to conceive. Some of these brilliant flames extend for three hundred
+thousand miles, so that in comparison with one of them the whole world
+would be but a tiny ball, and this is going on day and night without
+cessation. Look at the picture where the artist has made a little black
+ball to represent the earth as she would appear if she could be seen in
+the midst of the flames shooting out from the sun. Do not make a mistake
+and think the earth really could be in this position; she is only shown
+there so that you may see how tiny she is in comparison with the sun.
+All the time you have lived and your father, and grandfather, and right
+back to the beginnings of English history, and far, far further into the
+dim ages, this stupendous<span class='pagenum'><a name="Page_102" id="Page_102">[Pg 102]</a></span> exhibition of energy and power has continued,
+and only of late years has anyone known anything about it; even now a
+mere handful of people do know, and the rest, who are warmed and fed and
+kept alive by the gracious beams of this great revolving glowing
+fireball, never give it a thought.</p>
+
+<p>I said just now a pale green halo surrounded the sun, extending far
+beyond the prominences; this is called the corona and can only be seen
+during an eclipse. It surrounds the sun in a kind of shell, and there is
+reason to believe that it too is made of luminous stuff ejected by the
+sun in its burning fury. It is composed of large streamers or filaments,
+which seem to shoot out in all directions; generally these are not much
+larger than the apparent width of the sun, but sometimes they extend
+much further. The puzzle is, this corona cannot be an atmosphere in any
+way resembling that of our earth; for the gravitational force of the
+sun, owing to its enormous size, is so great that it would make any such
+atmosphere cling to it much more densely near to the surface, while it
+would be thinner higher up, and the corona is not dense in any way, but
+thin and tenuous throughout. This makes it very difficult to explain; it
+is supposed that some kind of electrical force enters into the problem,
+but what it is exactly we are far from knowing yet.<span class='pagenum'><a name="Page_103" id="Page_103">[Pg 103]</a></span></p>
+
+
+
+<hr style="width: 65%;" />
+<h2><a name="CHAPTER_VIII" id="CHAPTER_VIII"></a>CHAPTER VIII</h2>
+
+<h3>SHINING VISITORS</h3>
+
+
+<p>Our solar system is set by itself in the midst of a great space, and so
+far as we have learnt about it in this book everything in it seems
+orderly: the planets go round the sun and the satellites go round the
+planets, in orbits more or less regular; there seems no place for
+anything else. But when we have considered the planets and the
+satellites, we have not exhausted all the bodies which own allegiance to
+the sun. There is another class, made up of strange and weird members,
+which flash in and out of the system, coming and going in all directions
+and at all times&mdash;sometimes appearing without warning, sometimes
+returning with a certain regularity, sometimes retiring to infinite
+depths of space, where no human eye will ever see them more. These
+strange visitors are called comets, and are of all shapes and sizes and
+never twice alike. Even as we watch them they grow and change, and then
+diminish in splendour. Some are so vast that men<span class='pagenum'><a name="Page_104" id="Page_104">[Pg 104]</a></span> see them as flaming
+signs in the sky, and regard them with awe and wonder; some cannot be
+seen at all without the help of the telescope. From the very earliest
+ages those that were large enough to be seen without glasses have been
+regarded with astonishment. Men used to think that they were signs from
+heaven foretelling great events in the world. Timid people predicted
+that the end of the world would come by collision with one of them.
+Others, again, fancifully likened them to fishes in that sea of space in
+which we swim&mdash;fishes gigantic and terrifying, endowed with sense and
+will.</p>
+
+<p>It is perhaps unnecessary to say that comets are no more alive than is
+our own earth, and as for causing the end of the world by collision,
+there is every reason to believe the earth has been more than once right
+through a comet's tail, and yet no one except scientific men even
+discovered it. These mysterious visitors from the outer regions of space
+were called comets from a Greek word signifying hair, for they often
+leave a long luminous trail behind, which resembles the filaments of a
+woman's hair. It is not often that one appears large and bright enough
+to be seen by the naked eye, and when it does it is not likely to be
+soon forgotten. In the year 1910 such a comet is expected, a comet<span class='pagenum'><a name="Page_105" id="Page_105">[Pg 105]</a></span>
+which at its former appearance compelled universal attention by its
+brilliancy and strangeness. At the time of the Norman Conquest of
+England a comet believed to be the very same one was stretching its
+glorious tail half across the sky, and the Normans seeing it, took it as
+a good omen, fancying that it foretold their success. The history of the
+Norman Conquest was worked in tapestry&mdash;that is to say, in what we
+should call crewels on a strip of linen&mdash;and in this record the comet
+duly appears. Look at him in the picture as the Normans fancied him. He
+has a red head with blue flames starting from it, and several tails. The
+little group of men on the left are pointing and chattering about him.
+We can judge what an impression this comet must have made to be recorded
+in such an important piece of work.</p>
+
+<div class="figcenter" style="width: 650px;">
+<img src="images/i-138-tb.jpg" width="650" height="343" alt="THE COMET IN THE BAYEUX TAPESTRY." title="" />
+<span class="caption">THE COMET IN THE BAYEUX TAPESTRY.<br />
+<span class="link"><a href="images/i-138-full.jpg">View larger image</a></span></span>
+</div>
+
+<p>But we are getting on too fast. We have yet to learn how anyone can know
+that the comet which appeared at the time of the Norman Conquest is the
+same as that which has come back again at different times, and above
+all, how anyone can tell that it will come again in the year 1910. All
+this involves a long story.</p>
+
+<p>Before the invention of telescopes of course only those comets could be
+seen which were of great size<span class='pagenum'><a name="Page_106" id="Page_106">[Pg 106]</a></span> and fine appearance. In those days men
+did not realize that our world was but one of a number and of no great
+importance except to ourselves, and they always took these blazing
+appearances in the heavens as a particular warning to the human race.
+But when astronomers, by the aid of the telescope, found that for one
+comet seen by the eye there were hundreds which no mortal eye unaided
+could see, this idea seemed, to say the least of it, unlikely. Yet even
+then comets were looked upon as capricious visitors from outer space;
+odd creatures drawn into our system by the attraction of the sun, who
+disappeared, never to return. It was Newton, the same genius who
+disclosed to us the laws of gravity, who first declared that comets
+moved in orbits, only that these orbits were far more erratic than any
+of those followed by the planets.</p>
+
+<p>So far we have supposed that the planets were all on what we should call
+a level&mdash;that is to say, we have regarded them as if they were floating
+in a sea of water around the sun; but this is only approximately
+correct, for the orbits of the planets are not all at one level. If you
+had a number of slender hoops or rings to represent the planetary
+orbits, you would have to tilt one a little this way and another a
+little that way, only never so far but that a line through<span class='pagenum'><a name="Page_107" id="Page_107">[Pg 107]</a></span> the centre
+of the hoop from one side to another could pass through the sun. The way
+in which the planetary orbits are tilted is slight in comparison with
+that of the orbits of comets, for these are at all sorts of angles&mdash;some
+turned almost sideways, and others slanting, and all of them are
+ellipses long drawn out and much more irregular than the planetary
+orbits; but erratic as they are, in every case a line drawn through the
+sun and extended both ways would touch each side of the orbits.</p>
+
+<p>A great astronomer called Halley, who was born in the time of the
+Commonwealth, was lucky enough to see a very brilliant comet, and the
+sight interested him so much that he made all the calculations necessary
+to find out just in what direction it was travelling in the heavens. He
+found out that it followed an ellipse which brought it very near to the
+sun at one part of its journey, and carried it far beyond the orbit of
+the earth, right out to that of Neptune, at the other. Then he began to
+search the records for other comets which had been observed before his
+time. He found that two particularly bright ones had been carefully
+noted&mdash;one about seventy-five years before that which he had seen, and
+the other seventy-five years before that again. Both these comets had
+been<span class='pagenum'><a name="Page_108" id="Page_108">[Pg 108]</a></span> watched so scientifically that the paths in which they had
+travelled could be computed. A brilliant inspiration came to Halley. He
+believed that instead of these three, his own and the other two, being
+different comets, they were the same one, which returned to the sun
+about every seventy-five years. This could be proved, for if this idea
+were correct, of course the comet would return again in another
+seventy-five years, unless something unforeseen occurred. But Halley was
+in the prime of life: he could not hope to live to see his forecast
+verified. The only thing he could do was to note down exact particulars,
+by means of which others who lived after him might recognize his comet.
+And so when the time came for its return, though Halley was in his
+grave, numbers of astronomers were watching eagerly to see the
+fulfilment of his prediction. The comet did indeed appear, and since
+then it has been seen once again, and now we expect it to come back in
+the year 1910, when you and I may see it for ourselves. When the
+identity of the comet was fully established men began to search further
+back still, to compare the records of other previous brilliant comets,
+and found that this one had been noticed many times before, and once as
+I said, at the time of the Norman Conquest. Halley's comet is<span class='pagenum'><a name="Page_109" id="Page_109">[Pg 109]</a></span> peculiar
+in many ways. For instance, it is unusual that so large and interesting
+a comet should return within a comparatively limited time. It is the
+smaller comets, those that can only be seen telescopically, that usually
+run in small orbits. The smallest orbits take about three and a half
+years to traverse, and some of the largest orbits known require a period
+of one hundred and ten thousand years. Between these two limits lies
+every possible variety of period. One comet, seen about the time
+Napoleon was born, was calculated to take two thousand years to complete
+its journey, and another, a very brilliant one seen in 1882, must
+journey for eight hundred years before it again comes near to the sun.
+But we never know what might happen, for at any moment a comet which has
+traversed a long solitary pathway in outer darkness may flash suddenly
+into our ken, and be for the first time noted and recorded, before
+flying off at an angle which must take it for ever further and further
+from the sun.</p>
+
+<p>Everything connected with comets is mysterious and most fascinating.
+From out of the icy regions of space a body appears; what it is we know
+not, but it is seen at first as a hairy or softly-glowing star, and it
+was thus that Herschel mistook Uranus<span class='pagenum'><a name="Page_110" id="Page_110">[Pg 110]</a></span> for a comet when he first
+discovered it. As it draws nearer the comet sends out some fan-like
+projections toward the sun, enclosing its nucleus in filmy wrappings
+like a cocoon of light, and it travels faster and faster. From its head
+shoots out a tail&mdash;it may be more than one&mdash;growing in splendour and
+width, and always pointing away from the sun. So enormous are some of
+these tails that when the comet's head is close to the sun the tail
+extends far beyond the orbit of the earth. Faster still and faster flies
+the comet, for as we have seen it is a consequence of the law of
+gravitation that the nearer planets are to the sun the faster they move
+in their orbits, and the same rule applies to comets too. As the comet
+dashes up to the sun his pace becomes something indescribable; it has
+been reckoned for some comets at three hundred miles a second! But
+behold, as the head flies round the sun the tail is always projected
+outwards. The nucleus or head may be so near to the sun that the heat it
+receives would be sufficient to reduce molten iron to vapour; but this
+does not seem to affect it: only the tail expands. Sometimes it becomes
+two or more tails, and as it sweeps round behind the head it has to
+cover a much greater space in the same time, and therefore it must
+travel even faster than<span class='pagenum'><a name="Page_111" id="Page_111">[Pg 111]</a></span> the head. The pace is such that no calculations
+can account for it, if the tail is composed of matter in any sense as we
+know it. Then when the sun is passed the comet sinks away again, and as
+it goes the tail dies down and finally disappears. The comet itself
+dwindles to a hairy star once more and goes&mdash;whither? Into space so
+remote that we cannot even dream of it&mdash;far away into cold more
+appalling than anything we could measure, the cold of absolute space.
+More and more slowly it travels, always away and away, until the sun, a
+short time back a huge furnace covering all the sky, is now but a faint
+star. Thus on its lonely journey unseen and unknown the comet goes.</p>
+
+<p>This comet which we have taken as an illustration is a typical one, but
+all are not the same. Some have no tails at all, and never develop any;
+some change utterly even as they are watched. The same comet is so
+different at different times that the only possible way of identifying
+it is by knowing its path, and even this is not a certain method, for
+some comets appear to travel at intervals along the same path!</p>
+
+<p>Now we come to the question that must have been in the mind of everyone
+from the beginning<span class='pagenum'><a name="Page_112" id="Page_112">[Pg 112]</a></span> of this chapter, What are comets? This question no
+one can answer definitely, for there are many things so puzzling about
+these strange appearances that it is difficult even to suggest an
+explanation. Yet a good deal is known. In the first place, we are
+certain that comets have very little density&mdash;that is to say, they are
+indescribably thin, thinner than the thinnest kind of gas; and air,
+which we always think so thin, would be almost like a blanket compared
+with the material of comets. This we judge because they exercise no sort
+of influence on any of the planetary bodies they draw near to, which
+they certainly would do if they were made of any kind of solid matter.
+They come sometimes very close to some of the planets. A comet was so
+near to Jupiter that it was actually in among his moons. The comet was
+violently agitated; he was pulled in fact right out of his old path, and
+has been going on a new one ever since; but he did not exercise the
+smallest effect on Jupiter, or even on the moons. And, as I said earlier
+in this chapter, we on the earth have been actually in the folds of a
+comet's tail. This astonishing fact happened in June, 1861. One evening
+after the sun had set a golden-yellow disc, surrounded with filmy
+wrappings, appeared<span class='pagenum'><a name="Page_113" id="Page_113">[Pg 113]</a></span> in the sky. The sun's light, diffused throughout
+our atmosphere, had prevented its being seen sooner. This was apparently
+the comet's head. It is described as 'though a number of light, hazy
+clouds were floating around a miniature full moon.' From this a cone of
+light extended far up into the sky, and when the head disappeared below
+the horizon this tail was seen to reach to the zenith. But that was not
+all. Strange shafts of light seemed to hang right overhead, and could
+only be accounted for by supposing that they were caused by another tail
+hanging straight above us, so that we looked up at it foreshortened by
+perspective. The comet's head lay between the earth and the sun, and its
+tail, which extended over many millions of miles, stretched out behind
+in such a way that the earth must have gone right through it. The fact
+that the comet exercised no perceptible influence on the earth at all,
+and that there were not even any unaccountable magnetic storms or
+displays of electricity, may reassure us so that if ever we do again
+come in contact with one of these extremely fine, thin bodies, we need
+not be afraid.</p>
+
+<p>There is another way in which we can judge of the wonderful tenuity or
+thinness of comets&mdash;that<span class='pagenum'><a name="Page_114" id="Page_114">[Pg 114]</a></span> is, that the smallest stars can be seen
+through their tails, even though those tails must be many thousands of
+miles in thickness. Now, if the tails were anything approaching the
+density of our own atmosphere, the stars when seen through them would
+appear to be moved out of their places. This sounds odd, and requires a
+word of explanation. The fact is that anything seen through any
+transparent medium like water or air is what is called refracted&mdash;that
+is to say, the rays coming from it look bent. Everyone is quite familiar
+with this in everyday life, though perhaps they may not have noticed it.
+You cannot thrust a stick into the water without seeing that it looks
+crooked. Air being less dense than water has not quite so strong a
+refracting power, but still it has some. We cannot prove it in just the
+same way, because we are all inside the atmosphere ourselves, and there
+is no possibility of thrusting a stick into it from the outside! The
+only way we know it is by looking at something which is 'outside'
+already, and we find plenty of objects in the sky. As a matter of fact,
+the stars are all a little pulled out of their places by being seen
+through the air, and though of course we do not notice this, astronomers
+know it and have to make allowance for it. The effect<span class='pagenum'><a name="Page_115" id="Page_115">[Pg 115]</a></span> is most
+noticeable in the case of the sun when he is going down, for the
+atmosphere bends his rays up, and though we see him a great glowing red
+ball on the horizon, and watch him, as we think, drop gradually out of
+sight, we are really looking at him for the last moment or two when he
+has already gone, for the rays are bent up by the air and his image
+lingers when the real sun has disappeared.</p>
+
+<div class="figcenter" style="width: 600px;">
+<img src="images/i-150.jpg" width="600" height="382" alt="A STICK THRUST INTO THE WATER APPEARS CROOKED." title="" />
+<span class="caption">A STICK THRUST INTO THE WATER APPEARS CROOKED.</span>
+</div>
+
+<p>Therefore in looking through the luminous stuff that forms a comet's
+tail astronomers might well expect to see the stars displaced, but not a
+sign of this appears. It is difficult to imagine, therefore, what the
+tail can be made of. The idea is that the sun exercises a sort of
+repulsive effect on certain elements found in the comet's head&mdash;that is
+to say, it pushes them away, and that as the head approaches the sun,
+these elements are driven out of it away from the sun in vapour. This
+action may have something to do with electricity, which is yet little
+understood; anyway, the effect is that, instead of attracting the matter
+toward itself, in which case we should see the comet's tails stretching
+toward the sun, the sun drives it away! In the chapter on the sun we had
+to imagine something of the same kind to account for the corona, and the
+corona and<span class='pagenum'><a name="Page_116" id="Page_116">[Pg 116]</a></span> the comet's tails may be really akin to each other, and
+could perhaps be explained in the same way. Now we come to a stranger
+fact still. Some comets go right through the sun's corona, and yet do
+not seem to be influenced by it in the smallest degree. This may not
+seem very wonderful at first perhaps, but if you remember that a dash
+through anything so dense as our atmosphere, at a pace much less than
+that at which a comet goes, is enough to heat iron to a white heat, and
+then make it fly off in vapour, we get a glimpse of the extreme fineness
+of the materials which make the corona.</p>
+
+<p>Here is Herschel's account of a comet that went very near the sun:</p>
+
+<p>'The comet's distance from the sun's centre was about the 160th part of
+our distance from it. All the heat we enjoy on this earth comes from the
+sun. Imagine the heat we should have to endure if the sun were to
+approach us, or we the sun, to one 160th part of its present distance.
+It would not be merely as if 160 suns were shining on us all at once,
+but, 160 times 160, according to a rule which is well known to all who
+are conversant with such matters. Now, that is 25,600. Only imagine a
+glare 25,600 times fiercer than that of the equatorial sunshine at noon
+day with the sun vertical. In such a heat<span class='pagenum'><a name="Page_117" id="Page_117">[Pg 117]</a></span> there is no substance we know
+of which would not run like water, boil, and be converted into smoke or
+vapour. No wonder the comet gave evidence of violent excitement, coming
+from the cold region outside the planetary system torpid and ice-bound.
+Already when arrived even in our temperate region it began to show signs
+of internal activity; the head had begun to develop, and the tail to
+elongate, till the comet was for a time lost sight of&mdash;not for days
+afterwards was it seen; and its tail, whose direction was reversed, and
+which could not possibly be the same tail it had before, had already
+lengthened to an extent of about ninety millions of miles, so that it
+must have been shot out with immense force in a direction away from the
+sun.'</p>
+
+<p>We remember that comets have sometimes more than one tail, and a theory
+has been advanced to account for this too. It is supposed that perhaps
+different elements are thrust away by the sun at different angles, and
+one tail may be due to one element and another to another. But if the
+comet goes on tail-making to a large extent every time it returns to the
+sun, what happens eventually? Do the tails fall back again into the head
+when out of reach of the sun's action? Such an idea is inconceivable;
+but if not, then every time a comet approaches the<span class='pagenum'><a name="Page_118" id="Page_118">[Pg 118]</a></span> sun he loses
+something, and that something is made up of the elements which were
+formerly in the head and have been violently ejected. If this be so we
+may well expect to see comets which have returned many times to the sun
+without tails at all, for all the tail-making stuff that was in the head
+will have been used up, and as this is exactly what we do see, the
+theory is probably true.</p>
+
+<p>Where do the comets come from? That also is a very large question. It
+used to be supposed they were merely wanderers in space who happened to
+have been attracted by our sun and drawn into his system, but there are
+facts which go very strongly against this, and astronomers now generally
+believe that comets really belong to the solar system, that their proper
+orbits are ellipses, and that in the case of those which fly off at such
+an angle that they can never return they must at some time have been
+pulled out of their original orbit by the influence of one of the
+planets.</p>
+
+<div class="figcenter" style="width: 499px;">
+<img src="images/i-156.jpg" width="499" height="600" alt="Royal Observatory, Cape of Good Hope.
+
+A GREAT COMET." title="" />
+<span class="caption">Royal Observatory, Cape of Good Hope.
+
+A GREAT COMET.</span>
+</div>
+
+<p>To get a good idea of a really fine comet, until we have the opportunity
+of seeing one for ourselves, we cannot do better than look at this
+picture of a comet photographed in 1901 at the Cape of Good Hope. It is
+only comparatively recently that photography has been applied to comets.
+When<span class='pagenum'><a name="Page_120" id="Page_120">[Pg 120]</a></span><span class='pagenum'><a name="Page_119" id="Page_119">[Pg 119]</a></span> Halley's comet appeared last time such a thing was not thought
+of, but when he comes again numbers of cameras, fitted up with all the
+latest scientific appliances, will be waiting to get good impressions of
+him.<span class='pagenum'><a name="Page_121" id="Page_121">[Pg 121]</a></span></p>
+
+
+
+<hr style="width: 65%;" />
+<h2><a name="CHAPTER_IX" id="CHAPTER_IX"></a>CHAPTER IX</h2>
+
+<h3>SHOOTING STARS AND FIERY BALLS</h3>
+
+
+<p>All the substances which we are accustomed to see and handle in our
+daily lives belong to our world. There are vegetables which grow in the
+earth, minerals which are dug out of it, and elementary things, such as
+air and water, which have always made up a part of this planet since man
+knew it. These are obvious, but there are other things not quite so
+obvious which also help to form our world. Among these we may class all
+the elements known to chemists, many of which have difficult names, such
+as oxygen and hydrogen. These two are the elements which make up water,
+and oxygen is an important element in air, which has nitrogen in it too.
+There are numbers and numbers of other elements perfectly familiar to
+chemists, of which many people never even hear the names. We live in the
+midst of these things, and we take them for granted and pay little
+attention to them; but when<span class='pagenum'><a name="Page_122" id="Page_122">[Pg 122]</a></span> we begin to learn about other worlds we at
+once want to know if these substances and elements which enter so
+largely into our daily lives are to be found elsewhere in the universe
+or are quite peculiar to our own world. This question might be answered
+in several ways, but one of the most practical tests would be if we
+could get hold of something which had not been always on the earth, but
+had fallen upon it from space. Then, if this body were made up of
+elements corresponding with those we find here, we might judge that
+these elements are very generally diffused throughout the bodies in the
+solar system.</p>
+
+<p>It sounds in the highest degree improbable that anything should come
+hurling through the air and alight on our little planet, which we know
+is a mere speck in a great ocean of space; but we must not forget that
+the power of gravity increases the chances greatly, for anything coming
+within a certain range of the earth, anything small enough, that is, and
+not travelling at too great a pace, is bound to fall on to it. And,
+however improbable it seems, it is undoubtedly true that masses of
+matter do crash down upon the earth from time to time, and these are
+called meteorites. When we think of the great expanse of the oceans, of
+the ice round the poles,<span class='pagenum'><a name="Page_123" id="Page_123">[Pg 123]</a></span> and of the desert wastes, we know that for
+every one of such bodies seen to fall many more must have fallen unseen
+by any human being. Meteors large enough to reach the earth are not very
+frequent, which is perhaps as well, and as yet there is no record of
+anyone's having been killed by them. Most of them consist of masses of
+stone, and a few are of iron, while various substances resembling those
+that we know here have been found in them. Chemists in analyzing them
+have also come across certain elements so far unknown upon earth, though
+of course there is no saying that these may not exist at depths to which
+man has not penetrated.</p>
+
+<p>A really large meteor is a grand sight. If it is seen at night it
+appears as a red star, growing rapidly bigger and leaving a trail of
+luminous vapour behind as it passes across the sky. In the daytime this
+vapour looks like a cloud. As the meteor hurls itself along there may be
+a deep continuous roar, ending in one supreme explosion, or perhaps in
+several explosions, and finally the meteor may come to the earth in one
+mass, with a force so great that it buries itself some feet deep in the
+soil, or it may burst into numbers of tiny fragments, which are
+scattered over a large area.<span class='pagenum'><a name="Page_124" id="Page_124">[Pg 124]</a></span> When a meteor is found soon after its fall
+it is very hot, and all its surface has 'run,' having been fused by
+heat. The heat is caused by the friction of our atmosphere. The meteor
+gets entangled in the atmosphere, and, being drawn by the attraction of
+the earth, dashes through it. Part of the energy of its motion is turned
+to heat, which grows greater and greater as the denser air nearer to the
+earth is encountered; so that in time all the surface of the meteor runs
+like liquid, and this liquid, rising to a still higher temperature, is
+blown off in vapour, leaving a new surface exposed. The vapour makes the
+trail of fire or cloud seen to follow the meteor. If the process went on
+for long the meteor would be all dissipated in vapour, and in any case
+it must reach the earth considerably reduced in size.</p>
+
+<p>Numbers and numbers of comparatively small ones disappear, and for every
+one that manages to come to earth there must be hundreds seen only as
+shooting stars, which vanish and 'leave not a wrack behind.' When a
+meteor is seen to fall it is traced, and, whenever possible, it is found
+and placed in a museum. Men have sometimes come across large masses of
+stone and iron with their surfaces fused with heat. These are in every
+way<span class='pagenum'><a name="Page_125" id="Page_125">[Pg 125]</a></span> like the recognized meteorites, except that no eye has noted their
+advent. As there can be no reasonable doubt that they are of the same
+origin as the others, they too are collected and placed in museums, and
+in any large museum you would be able to see both kinds&mdash;those which
+have been seen to come to earth and those which have been found
+accidentally.</p>
+
+<p>The meteors which appear very brilliant in their course across the sky
+are sometimes called fire-balls, which is only another name for the same
+thing. Some of these are brighter than the full moon, so bright that
+they cause objects on earth to cast a shadow. In 1803 a fiery ball was
+noticed above a small town in Normandy; it burst and scattered stones
+far and wide, but luckily no one was hurt. The largest meteorites that
+have been found on the earth are a ton or more in weight; others are
+mere stones; and others again just dust that floats about in the
+atmosphere before gently settling. Of course, meteors of this last kind
+could not be seen to fall like the larger ones, yet they do fall in such
+numbers that calculations have been made showing that the earth must
+catch about a hundred millions of meteors daily, having altogether a
+total weight of about a hundred tons. This sounds<span class='pagenum'><a name="Page_126" id="Page_126">[Pg 126]</a></span> enormous, but
+compared with the weight of the earth it is very small indeed.</p>
+
+<p>Now that we have arrived at the fact that strange bodies do come
+hurtling down upon us out of space, and that we can actually handle and
+examine them, the next question is, Where do they come from? At one time
+it was thought that they were fragments which had been flung off by the
+earth herself when she was subject to violent explosions, and that they
+had been thrown far enough to resist the impulse to drop down upon her
+again, and had been circling round the sun ever since, until the earth
+came in contact with them again and they had fallen back upon her. It is
+not difficult to imagine a force which would be powerful enough to
+achieve the feat of speeding something off at such a velocity that it
+passed beyond the earth's power to pull it back, but nothing that we
+have on earth would be nearly strong enough to achieve such a feat.
+Imaginative writers have pictured a projectile hurled from a cannon's
+mouth with such tremendous force that it not only passed beyond the
+range of the earth's power to pull it back, but so that it fell within
+the influence of the moon and was precipitated on to her surface! Such
+things must remain achievements in imagination only; it is not possible
+for them to be<span class='pagenum'><a name="Page_127" id="Page_127">[Pg 127]</a></span> carried out. Other ideas as to the origin of meteors
+were that they had been expelled from the moon or from the sun. It would
+need a much less force to send a projectile away from the moon than from
+the earth on account of its smaller size and less density, but the
+distance from the earth to the moon is not very great, and any
+projectile hurled forth from the moon would cross it in a comparatively
+short time. Therefore if the meteorites come from the moon, the moon
+must be expelling them still, and we might expect to see some evidence
+of it; but we know that the moon is a dead world, so this explanation is
+not possible. The sun, for its part, is torn by such gigantic
+disturbances that, notwithstanding its vast size, there is no doubt
+sufficient force there to send meteors even so far as the earth, but the
+chances of their encountering the earth would be small. Both these
+theories are now discarded. It is believed that the meteors are merely
+lesser fragments of the same kind of materials as the planets, circling
+independently round the sun; and a proof of this is that far more
+meteorites fall on that part of the earth which is facing forward in its
+journey than on that behind, and this is what we should expect if the
+meteors were scattered independently through space and it was by reason<span class='pagenum'><a name="Page_128" id="Page_128">[Pg 128]</a></span>
+of our movements that we came in contact with them. There is no need to
+explain this further. Everyone knows that in cycling or driving along a
+road where there is a good deal of traffic both ways the people we meet
+are more in number than those who overtake us, and the same result would
+follow with the meteors; that is to say, in travelling through space
+where they were fairly evenly distributed we should meet more than we
+should be overtaken by.</p>
+
+<p>You remember that it was suggested the sun's fuel might be obtained from
+meteors, and this was proved to be not possible, even though there are
+no doubt unknown millions of these strange bodies circling throughout
+the solar system.</p>
+
+<p>There are so many names for these flashing bodies that we may get a
+little confused: when they are seen in the sky they are meteors, or
+fire-balls; when they reach the earth they are called meteorites, and
+also aerolites. Then there is another class of the same bodies called
+shooting stars, and these are in reality only meteors on a smaller
+scale; but there ought to be no confusion in our thoughts, for all these
+objects are small bodies travelling round the sun, and caught by the
+earth's influence.</p>
+
+<p>When you watch the sky for some time on a<span class='pagenum'><a name="Page_129" id="Page_129">[Pg 129]</a></span> clear night, you will seldom
+fail to see at least one star flash out suddenly in a path of thrilling
+light and disappear, and you cannot be certain whether that star had
+been shining in the sky a minute before, or if it had appeared suddenly
+only in order to go out. The last idea is right. We must get rid at once
+of the notion that it would be possible for any fixed star to behave in
+this manner. To begin with, the fixed stars are many of them actually
+travelling at a great velocity at present, yet so immeasurably distant
+are they that their movement makes no perceptible difference to us. For
+one of them to appear to dash across the heavens as a meteor does would
+mean a velocity entirely unknown to us, even comparing it with the speed
+of light. No, these shooting stars are not stars at all, though they
+were so named, long before the real motions of the fixed stars were even
+dimly guessed at. As we have seen, they belong to the same class as
+meteors.</p>
+
+<p>I remember being told by a clergyman, years ago, that one night in
+November he had gone up to bed very late, and as he pulled up his blind
+to look at the sky, to his amazement he saw a perfect hail of shooting
+stars, some appearing every minute, and all darting in vivid trails of
+light, longer or shorter,<span class='pagenum'><a name="Page_130" id="Page_130">[Pg 130]</a></span> though all seemed to come from one point. So
+marvellous was the sight that he dashed across the village street,
+unlocked the church door, and himself pulled the bell with all his
+might. The people in that quiet country village had long been in bed,
+but they huddled on their clothes and ran out of their pretty thatched
+cottages, thinking there must be a great fire, and when they saw the
+wonder in the sky they were amazed and cried out that the world must be
+coming to an end. The clergyman knew better than that, and was able to
+reassure them, and tell them he had only taken the most effectual means
+of waking them so that they might not miss the display, for he was sure
+as long as they lived they would never see such another sight. A star
+shower of this kind is certainly well worth getting up to see, but
+though uncommon it is not unique. There are many records of such showers
+having occurred in times gone by, and when men put together and examined
+the records they found that the showers came at regular intervals. For
+instance, every year about the same time in November there is a star
+shower, not comparable, it is true, with the brilliant one the clergyman
+saw, but still noticeable, for more shooting stars are seen then than at
+other times, and once in every thirty-<span class='pagenum'><a name="Page_131" id="Page_131">[Pg 131]</a></span>three years there is a specially
+fine one. It happened in fact to be one of these that the village people
+were wakened up to see.</p>
+
+<p>Not all at once, but gradually, the mystery of these shower displays was
+solved. It was realized that the meteors need not necessarily come from
+one fixed place in the sky because they seemed to us to do so, for that
+was only an effect of perspective. If you were looking down a long,
+perfectly straight avenue of tree-trunks, the avenue would seem to close
+in, to get narrower and narrower at the far end until it became a point;
+but it would not really do so, for you would know that the trees at the
+far end were just the same distance from each other as those between
+which you were standing. Now, two meteors starting from the same
+direction at a distance from each other, and keeping parallel, would
+seem to us to start from a point and to open out wider and wider as they
+approached, but they would not really do so; it would only be, as in the
+case of the avenue, an effect of perspective. If a great many meteors
+did the same thing, they would appear to us all to start from one point,
+whereas really they would be on parallel lines, only as they rushed to
+meet us or we rushed to meet them this effect would be produced.
+There<span class='pagenum'><a name="Page_132" id="Page_132">[Pg 132]</a></span>fore the first discovery was that these meteors were thousands and
+thousands of little bodies travelling in lines parallel to each other,
+like a swarm of little planets. To judge that their path was not a
+straight line but a circle or ellipse was the next step, and this was
+found to be the case. From taking exact measurements of their paths in
+the sky an astronomer computed they were really travelling round the sun
+in a lengthened orbit, an ellipse more like a comet's orbit than that of
+a planet. But next came the puzzling question, Why did the earth
+apparently hit them every year to some extent, and once in thirty-three
+years seem to run right into the middle of them? This also was answered.
+One has only to imagine a swarm of such meteors at first hastening
+busily along their orbit, a great cluster all together, then, by the
+near neighbourhood of some planet, or by some other disturbing causes,
+being drawn out, leaving stragglers lagging behind, until at last there
+might be some all round the path, but only thinly scattered, while the
+busy, important cluster that formed the nucleus was still much thicker
+than any other part. Now, if the orbit that the meteors followed cut the
+orbit or path of the earth at one point, then every time the earth came
+to what we may call the level crossing<span class='pagenum'><a name="Page_133" id="Page_133">[Pg 133]</a></span> she must run into some of the
+stragglers, and if the chief part of the swarm took thirty-three years
+to get round, then once in about thirty-three years the earth must
+strike right into it. This would account for the wonderful display. So
+long drawn-out is the thickest part of the swarm that it takes a year to
+pass the points at the level crossing. If the earth strikes it near the
+front one year, she may come right round in time to strike into the rear
+part of the swarm next year, so that we may get fine displays two years
+running about every thirty-three years. The last time we passed through
+the swarm was in 1899, and then the show was very disappointing. Here in
+England thick clouds prevented our seeing much, and there will not be
+another chance for us to see it at its best until 1932.</p>
+
+<p>These November meteors are called Leonids, because they <i>seem</i> to come
+from a group of stars named Leo, and though the most noticeable they are
+not the only ones. A shower of the same kind occurs in August too, but
+the August meteors, called Perseids, because they seem to come from
+Perseus, revolve in an orbit which takes a hundred and forty-two years
+to traverse! So that only every one hundred and forty-second year could
+we hope to see a good display. When all these<span class='pagenum'><a name="Page_134" id="Page_134">[Pg 134]</a></span> facts had been gathered
+up, it seemed without doubt that certain groups of meteors travelled in
+company along an elliptical orbit. But there remained still something
+more&mdash;a bold and ingenious theory to be advanced. It was found that a
+comet, a small one, only to be seen with the telescope, revolved in
+exactly the same orbit as the November meteors, and another one, larger,
+in exactly the same orbit as the August ones; hence it could hardly be
+doubted that comets and meteors had some connection with each other,
+though what that connection is exactly no one knows. Anyway, we can have
+no shadow of doubt when we find the comet following a marked path, and
+the meteors pursuing the same path in his wake, that the two have some
+mysterious affinity. There are other smaller showers besides these of
+November and August, and a remarkable fact is known about one of them.
+This particular stream was found to be connected with a comet named
+Biela's Comet, that had been many times observed, and which returned
+about every seven years to the sun. After it had been seen several
+times, this astonishing comet split in two and appeared as two comets,
+both of which returned at the end of the next seven years. But on the
+next two occasions when they were expected they never came at all,<span class='pagenum'><a name="Page_135" id="Page_135">[Pg 135]</a></span> and
+the third time there came instead a fine display of shooting stars, so
+it really seemed as if these meteors must be the fragments of the lost
+comet.</p>
+
+<p>It is very curious and interesting to notice that in these star showers
+there is no certain record of any large meteorite reaching the earth;
+they seem to be made up of such small bodies that they are all
+dissipated in vapour as they traverse our air.<span class='pagenum'><a name="Page_136" id="Page_136">[Pg 136]</a></span></p>
+
+
+
+<hr style="width: 65%;" />
+<h2><a name="CHAPTER_X" id="CHAPTER_X"></a>CHAPTER X</h2>
+
+<h3>THE GLITTERING HEAVENS</h3>
+
+
+<p>On a clear moonless night the stars appear uncountable. You see them
+twinkling through the leafless trees, and covering all the sky from the
+zenith, the highest point above your head, down to the horizon. It seems
+as if someone had taken a gigantic pepper-pot and scattered them far and
+wide so that some had fallen in all directions. If you were asked to
+make a guess as to how many you can see at one time, no doubt you would
+answer 'Millions!' But you would be quite wrong, for the number of stars
+that can be seen at once without a telescope does not exceed two
+thousand, and this, after the large figures we have been dealing with,
+appears a mere trifle. With a telescope, even of small power, many more
+are revealed, and every increase in the size of the telescope shows more
+still; so that it might be supposed the universe is indeed illimitable,
+and that we are only prevented from seeing beyond a certain point by our
+limited<span class='pagenum'><a name="Page_137" id="Page_137">[Pg 137]</a></span> resources. But in reality we know that this cannot be so. If
+the whole sky were one mass of stars, as it must be if the number of
+them were infinite, then, even though we could not distinguish the
+separate items, we should see it bright with a pervading and diffused
+light. As this is not so, we judge that the universe is not unending,
+though, with all our inventions, we may never be able to probe to the
+end of it. We need not, indeed, cry for infinity, for the distances of
+the fixed stars from us are so immeasurable that to atoms like ourselves
+they may well seem unlimited. Our solar system is set by itself, like a
+little island in space, and far, far away on all sides are other great
+light-giving suns resembling our own more or less, but dwindled to the
+size of tiny stars, by reason of the great void of space lying between
+us and them. Our sun is, indeed, just a star, and by no means large
+compared with the average of the stars either. But, then, he is our own;
+he is comparatively near to us, and so to us he appears magnificent and
+unique. Judging from the solar system, we might expect to find that
+these other great suns which we call stars have also planets circling
+round them, looking to them for light and heat as we do to our sun.
+There is no reason to doubt that in some instances the<span class='pagenum'><a name="Page_138" id="Page_138">[Pg 138]</a></span> conjecture is
+right, and that there may be other suns with attendant planets. It is
+however a great mistake to suppose that because our particular family in
+the solar system is built on certain lines, all the other families must
+be made on the same pattern. Why, even in our own system we can see how
+very much the planets differ from each other: there are no two the same
+size; some have moons and some have not; Saturn's rings are quite
+peculiar to himself, and Uranus and Neptune indulge in strange vagaries.
+So why should we expect other systems to be less varied?</p>
+
+<p>As science has advanced, the idea that these faraway suns must have
+planetary attendants as our sun has been discarded. The more we know the
+more is disclosed to us the infinite variety of the universe. For
+instance, so much accustomed are we to a yellow sun that we never think
+of the possibility of there being one of another colour. What would you
+say then to a ruby sun, or a blue one; or to two suns of different
+colours, perhaps red and green, circling round each other; or to two
+such suns each going round a dark companion? For there are dark bodies
+as well as shining bodies in the sky. These are some of the marvels of
+the starry sky, marvels quite as<span class='pagenum'><a name="Page_139" id="Page_139">[Pg 139]</a></span> absorbing as anything we have found in
+the solar system.</p>
+
+<p>It requires great care and patience and infinite labour before the very
+delicate observations which alone can reveal to us anything of the
+nature of the fixed stars can be accomplished. It is only since the
+improvement in large telescopes that this kind of work has become
+possible, and so it is but recently men have begun to study the stars
+intimately, and even now they are baffled by indescribable difficulties.
+One of these is our inability to tell the distance of a thing by merely
+looking at it unless we also know its size. On earth we are used to
+seeing things appear smaller the further they are from us, and by long
+habit can generally tell the real size; but when we turn to the stars,
+which appear so much alike, how are we to judge how far off they are?
+Two stars apparently the same size and close together in the sky may
+really be as far one from another as the earth is from the nearest; for
+if the further one were very much larger than the nearer, they would
+then appear the same size.</p>
+
+<p>At first it was natural enough to suppose that the big bright stars of
+what we call the first magnitude were the nearest to us, and the less
+bright the<span class='pagenum'><a name="Page_140" id="Page_140">[Pg 140]</a></span> next nearest, and so on down to the tiny ones, only revealed
+by the telescope, which would be the furthest away of all; but research
+has shown that this is not correct. Some of the brightest stars may be
+comparatively near, and some of the smallest may be near also. The size
+is no test of distance. So far as we have been able to discover, the
+star which seems nearest <i>is</i> a first magnitude one, but some of the
+others which outshine it must be among the infinitely distant ones. Thus
+we lie in the centre of a jewelled universe, and cannot tell even the
+size of the jewels which cover its radiant robe.</p>
+
+<p>I say 'lie,' but that is really not the correct word. So far as we have
+been able to find out, there is no such thing as absolute rest in the
+universe&mdash;in fact, it is impossible; for even supposing any body could
+be motionless at first, it would be drawn by the attraction of its
+nearest neighbours in space, and gradually gain a greater and greater
+velocity as it fell toward them. Even the stars we call 'fixed' are all
+hurrying along at a great pace, and though their distance prevents us
+from seeing any change in their positions, it can be measured by
+suitable instruments. Our sun is no exception to this universal rule.
+Like all his compeers, he is hurrying<span class='pagenum'><a name="Page_141" id="Page_141">[Pg 141]</a></span> busily along somewhere in
+obedience to some impulse of which we do not know the nature; and as he
+goes he carries with him his whole cort&egrave;ge of planets and their
+satellites, and even the comets. Yes, we are racing through space with
+another motion, too, besides those of rotation and revolution, for our
+earth keeps up with its master attractor, the sun. It is difficult, no
+doubt, to follow this, but if you think for a moment you will remember
+that when you are in a railway-carriage everything in that carriage is
+really travelling along with it, though it does not appear to move. And
+the whole solar system may be looked at as if it were one block in
+movement. As in a carriage, the different bodies in it continue their
+own movements all the time, while sharing in the common movement. You
+can get up and change your seat in the train, and when you sit down
+again you have not only moved that little way of which you are
+conscious, but a great way of which you are not conscious unless you
+look out of the window. Now in the case of the earth's own motion we
+found it necessary to look for something which does not share in that
+motion for purposes of comparison, and we found that something in the
+sun, who shows us very clearly we are turning on our axis.<span class='pagenum'><a name="Page_142" id="Page_142">[Pg 142]</a></span> But in the
+case of the motion of the solar system the sun is moving himself, so we
+have to look beyond him again and turn to the stars for confirmation.
+Then we find that the stars have motions of their own, so that it is
+very difficult to judge by them at all. It is as if you were bicycling
+swiftly towards a number of people all walking about in different
+directions on a wide lawn. They have their movements, but they all also
+have an apparent movement, really caused by you as you advance toward
+them; and what astronomers had to do was to separate the true movements
+of the stars from the false apparent movement made by the advance of the
+sun. This great problem was attacked and overcome, and it is now known
+with tolerable certainty that the sun is sweeping onward at a pace of
+about twelve miles a second toward a fixed point. It really matters very
+little to us where he is going, for the distances are so vast that
+hundreds of years must elapse before his movement makes the slightest
+difference in regard to the stars. But there is one thing which we can
+judge, and that is that though his course appears to be in a straight
+line, it is most probably only a part of a great curve so huge that the
+little bit we know seems straight.<span class='pagenum'><a name="Page_143" id="Page_143">[Pg 143]</a></span></p>
+
+<p>When we speak of the stars, we ought to keep quite clearly in our minds
+the fact that they lie at such an incredible distance from us that it is
+probable we shall never learn a great deal about them. Why, men have not
+even yet been able to communicate with the planet Mars, at its nearest
+only some thirty-five million miles from us, and this is a mere nothing
+in measuring the space between us and the stars. To express the
+distances of the stars in figures is really a waste of time, so
+astronomers have invented another way. You know that light can go round
+the world eight times in a second; that is a speed quite beyond our
+comprehension, but we just accept it. Then think what a distance it
+could travel in an hour, in a day; and what about a year? The distance
+that light can travel in a year is taken as a convenient measure by
+astronomers for sounding the depths of space. Measured in this way light
+takes four years and four months to reach us from the nearest star we
+know of, and there are others so much more distant that hundreds&mdash;nay,
+thousands&mdash;of years would have to be used to convey it. Light which has
+been travelling along with a velocity quite beyond thought, silently,
+unresting, from the time when the Britons lived and ran half naked on<span class='pagenum'><a name="Page_144" id="Page_144">[Pg 144]</a></span>
+this island of ours, has only reached us now, and there is no limit to
+the time we may go back in our imaginings. We see the stars, not as they
+are, but as they were. If some gigantic conflagration had happened a
+hundred years ago in one of them situated a hundred light-years away
+from us, only now would that messenger, swifter than any messenger we
+know, have brought the news of it to us. To put the matter in figures,
+we are sure that no star can lie nearer to us than twenty-five billions
+of miles. A billion is a million millions, and is represented by a
+figure with twelve noughts behind it, so&mdash;1,000,000,000,000; and
+twenty-five such billions is the least distance within which any star
+can lie. How much farther away stars may be we know not, but it is
+something to have found out even that. On the same scale as that we took
+in our first example, we might express it thus: If the earth were a
+greengage plum at a distance of about three hundred of your steps from
+the sun, and Neptune were, on the same scale, about three miles away,
+the nearest fixed star could not be nearer than the distance measured
+round the whole earth at the Equator!</p>
+
+<p>All this must provoke the question, How can anyone find out these
+things? Well, for a long<span class='pagenum'><a name="Page_145" id="Page_145">[Pg 145]</a></span> time the problem of the distances of the stars
+was thought to be too difficult for anyone to attempt to solve it, but
+at last an ingenious method was devised, a method which shows once more
+the triumph of man's mind over difficulties. In practice this method is
+extremely difficult to carry out, for it is complicated by so many other
+things which must be made allowance for; but in theory, roughly
+explained, it is not too hard for anyone to grasp. The way of it is
+this: If you hold up your finger so as to cover exactly some object a
+few feet distant from you, and shut first one eye and then the other,
+you will find that the finger has apparently shifted very considerably
+against the background. The finger has not really moved, but as seen
+from one eye or the other, it is thrown on a different part of the
+background, and so appears to jump; then if you draw two imaginary
+lines, one from each eye to the finger, and another between the two
+eyes, you will have made a triangle. Now, all of you who have done a
+little Euclid know that if you can ascertain the length of one side of a
+triangle, and the angles at each end of it, you can form the rest of the
+triangle; that is to say, you can tell the length of the other two
+sides. In this instance the base line, as it is called&mdash;that is to say
+the line lying between<span class='pagenum'><a name="Page_146" id="Page_146">[Pg 146]</a></span> the two eyes&mdash;can easily be measured, and the
+angles at each end can be found by an instrument called a sextant, so
+that by simple calculation anyone could find out what distance the
+finger was from the eye. Now, some ingenious man decided to apply this
+method to the stars. He knew that it is only objects quite near to us
+that will appear to shift with so small a base line as that between the
+eyes, and that the further away anything is the longer must the base
+line be before it makes any difference. But this clever man thought that
+if he could only get a base line long enough he could easily compute the
+distance of the stars from the amount that they appeared to shift
+against their background. He knew that the longest base line he could
+get on earth would be about eight thousand miles, as that is the
+diameter of the earth from one side to the other; so he carefully
+observed a star from one end of this immense base line and then from the
+other, quite confident that this plan would answer. But what happened?
+After careful observations he discovered that no star moved at all with
+this base line, and that it must be ever so much longer in order to make
+any impression. Then indeed the case seemed hopeless, for here we are
+tied to the earth and we cannot get<span class='pagenum'><a name="Page_147" id="Page_147">[Pg 147]</a></span> away into space. But the astronomer
+was nothing daunted. He knew that in its journey round the sun the earth
+travels in an orbit which measures about one hundred and eighty-five
+millions of miles across, so he resolved to take observations of the
+stars when the earth was at one side of this great circle, and again,
+six months later, when she had travelled to the other side. Then indeed
+he would have a magnificent base line, one of one hundred and
+eighty-five millions of miles in length. What was the result? Even with
+this mighty line the stars are found to be so distant that many do not
+move at all, not even when measured with the finest instruments, and
+others move, it may be, the breadth of a hair at a distance of several
+feet! But even this delicate measure, a hair's-breadth, tells its own
+tale; it lays down a limit of twenty-five billion miles within which no
+star can lie!</p>
+
+<p>This system which I have explained to you is called finding the star's
+parallax, and perhaps it is easier to understand when we put it the
+other way round and say that the hair's-breadth is what the whole orbit
+of the earth would appear to have shrunk to if it were seen from the
+distance of these stars!</p>
+
+<p>Many, many stars have now been examined, and of<span class='pagenum'><a name="Page_148" id="Page_148">[Pg 148]</a></span> them all our nearest
+neighbour seems to be a bright star seen in the Southern Hemisphere. It
+is in the constellation or star group called Centaurus, and is the
+brightest star in it. In order to designate the stars when it is
+necessary to refer to them, astronomers have invented a system. To only
+the very brightest are proper names attached; others are noted according
+to the degree of their brightness, and called after the letters of the
+Greek alphabet: alpha, beta, gamma, delta, etc. Our own word 'alphabet'
+comes, you know, from the first two letters of this Greek series. As
+this particular star is the brightest in the constellation Centaurus, it
+is called Alpha Centauri; and if ever you travel into the Southern
+Hemisphere and see it, you may greet it as our nearest neighbour in the
+starry universe, so far as we know at present.<span class='pagenum'><a name="Page_149" id="Page_149">[Pg 149]</a></span></p>
+
+
+
+<hr style="width: 65%;" />
+<h2><a name="CHAPTER_XI" id="CHAPTER_XI"></a>CHAPTER XI</h2>
+
+<h3>THE CONSTELLATIONS</h3>
+
+
+<p>From the very earliest times men have watched the stars, felt their
+mysterious influence, tried to discover what they were, and noted their
+rising and setting. They classified them into groups, called
+constellations, and gave such groups the names of figures and animals,
+according to the positions of the stars composing them. Some of these
+imaginary figures seem to us so wildly ridiculous that we cannot
+conceive how anyone could have gone so far out of their way as to invent
+them. But they have been long sanctioned by custom, so now, though we
+find it difficult to recognize in scattered groups of stars any likeness
+to a fish or a ram or a bear; we still call the constellations by their
+old names for convenience in referring to them.<span class='pagenum'><a name="Page_150" id="Page_150">[Pg 150]</a></span></p>
+
+<div class="figcenter" style="width: 445px;">
+<img src="images/i-190-tb.jpg" width="445" height="650" alt="CONSTELLATIONS NEAR THE POLE STAR." title="" />
+<span class="caption">CONSTELLATIONS NEAR THE POLE STAR.</span><br />
+<span class="link"><a href="images/i-190-full.jpg">View larger image</a></span>
+</div>
+
+<p>Supposing the axis of the earth were quite upright, straight up and down
+in regard to the plane at which the earth goes round the sun, then we
+should always see the same set of stars from the Northern and the same
+set of stars from the Southern Hemispheres all the year round. But as
+the axis is tilted slightly, we can, during our nights in the winter in
+the Northern Hemisphere, see more of the sky to the south than we can in
+the summer; and in the Southern Hemisphere just the reverse is the case,
+far more stars to the north can be seen in the winter than in the
+summer. But always, whether it is winter or summer, there is one fixed
+point in each hemisphere round which all the other stars seem to swing,
+and this is the point immediately over the North or the South Poles.
+There is, luckily, a bright star almost at the point at which the North
+Pole would seem to strike the sky were it infinitely lengthened. This is
+not one of the brightest stars in the sky, but quite bright enough to
+serve the purpose, and if we stand with our faces towards it, we can be
+sure we are looking due north. How can we discover this star for
+ourselves in the sky? Go out on any starlight night when the sky is
+clear, and see if you can find a very conspicuous set of seven stars
+called the Great Bear. I shall not describe the Great Bear, because
+every child ought to know it already, and if they don't, they can ask
+the first grown-up person they meet, and they will certainly be told.
+(See <a href='#map'><b>map</b></a>.)</p>
+
+
+
+<p>Having found the Great Bear, you have only to draw an imaginary line
+between the two last stars forming the square on the side away from the
+tail, and carry it on about three times as far as the distance between
+those two stars, and you will come straight to the Pole Star. The two
+stars in the Great Bear which help one to find it are called the
+Pointers, because they point to it.</p>
+
+<p>The Great Bear is one of the constellations known from the oldest times;
+it is also sometimes called Charles's Wain, the Dipper, or the Plough.
+It is always easily seen in England, and seems to swing round the Pole
+Star as if held by an invisible rope tied to the Pointers. Besides the
+Great Bear there is, not far from it, the Little Bear, which is really
+very like it, only smaller and harder to find. The Pole Star is the last
+star in its tail; from it two small stars lead away parallel to the
+Great Bear, and they bring the eye to a small pair which form one side
+of a square just like that in the Great Bear. But the whole of the
+Little Bear is turned the opposite way from the Great Bear, and the tail
+points in the opposite direction. And when you come to think of it,
+it<span class='pagenum'><a name="Page_151" id="Page_151">[Pg 151]</a></span> is very ridiculous to have called these groups Bears at all, or to
+talk about tails, for bears have no tails! So it would have been better
+to have called them foxes or dogs, or almost any other animal rather
+than bears.</p>
+
+<p>Now, if you look at the sky on the opposite side of the Pole Star from
+the Great Bear, you will see a clearly marked capital W made up of five
+or six bright stars. This is called Cassiopeia, or the Lady's Chair.</p>
+
+<p>In looking at Cassiopeia you cannot help noticing that there is a zone
+or broad band of very many stars, some exceedingly small, which
+apparently runs right across the sky like a ragged hoop, and Cassiopeia
+seems to be set in or on it. This band is called the Milky Way, and
+crosses not only our northern sky, but the southern sky too, thus making
+a broad girdle round the whole universe. It is very wonderful, and no
+one has yet been able to explain it. The belt is not uniform and even,
+but it is here and there broken up into streamers and chips, having the
+same appearance as a piece of ribbon which has been snipped about by
+scissors in pure mischief; or it may be compared to a great river broken
+up into many channels by rocks and obstacles in its course.</p>
+
+<p>The Milky Way is mainly made up of thousands<span class='pagenum'><a name="Page_152" id="Page_152">[Pg 152]</a></span> and thousands of small
+stars, and many more are revealed by the telescope; but, as we see in
+Cassiopeia, there are large bright stars in it too, though, of course,
+these may be infinitely nearer to us, and may only appear to us to be in
+the Milky Way because they are between us and it.</p>
+
+<p>Now, besides the few constellations that I have mentioned, there are
+numbers of others, some of which are difficult to discover, as they
+contain no bright stars. But there are certain constellations which
+every one should know, because in them may be found some of the
+brightest stars, those of the first magnitude. Magnitude means size, and
+it is really absurd for us to say a star is of the first magnitude
+simply because it appears to us to be large, for, as I have explained
+already, a small star comparatively near to us might appear larger than
+a greater one further away. But the word 'magnitude' was used when men
+really thought stars were large or small according to their appearance,
+and so it is used to this day. They called the biggest and brightest
+first magnitude stars. Of these there are not many, only some twenty, in
+all the sky. The next brightest&mdash;about the brightness of the Pole Star
+and the stars in the Great Bear&mdash;are of the second magnitude, and so<span class='pagenum'><a name="Page_153" id="Page_153">[Pg 153]</a></span>
+on, each magnitude containing stars less and less bright. When we come
+to stars of the sixth magnitude we have reached the limit of our sight,
+for seventh magnitude stars can only be seen with a telescope. Now that
+we understand what is meant by the magnitude, we can go back to the
+constellations and try to find some more.</p>
+
+<p>If you draw an imaginary line across the two stars forming the backbone
+of the Bear, starting from the end nearest the tail, and continue it
+onward for a good distance, you will come to a very bright star called
+Capella, which you will know, because near it are three little ones in a
+triangle. Now, Capella means a goat, so the small ones are called the
+kids. In winter Capella gets high up into the sky, and then there is to
+be seen below her a little cluster called the Pleiades. There is nothing
+else like this in the whole sky. It is formed of six stars, as it
+appears to persons of ordinary sight, and these stars are of the sixth
+magnitude, the lowest that can be seen by the naked eye. But though
+small, they are set so close together, and appear so brilliant,
+twinkling like diamonds, that they are one of the most noticeable
+objects in the heavens. A legend tells that there were once seven stars
+in the Pleiades clearly<span class='pagenum'><a name="Page_154" id="Page_154">[Pg 154]</a></span> visible, and that one has now disappeared. This
+is sometimes spoken of as 'the lost Pleiad,' but there does not seem to
+be any foundation for the story. In old days people attached particular
+holiness or luck to the number seven, and possibly, when they found that
+there were only six stars in this wonderful group, they invented the
+story about the seventh.</p>
+
+<div class="figcenter" style="width: 440px;">
+<img src="images/i-196-tb.jpg" width="440" height="650" alt="ORION AND HIS NEIGHBOURS." title="" />
+<span class="caption">ORION AND HIS NEIGHBOURS.</span><br />
+<span class="link"><a href="images/i-196-full.jpg">View larger image</a></span>
+</div>
+
+<p>As the Pleiades rise, a beautiful reddish star of the first magnitude
+rises beneath them. It is called Aldebaran, and it, as well as the
+Pleiades, forms a part of the constellation of Taurus the bull. In
+England we can see in winter below Aldebaran the whole of the
+constellation of Orion, one of the finest of all the constellations,
+both for the number of the bright stars it contains and for the extent
+of the sky it covers. Four bright stars at wide distances enclose an
+irregular four-sided space in which are set three others close together
+and slanting downwards. Below these, again, are another three which seem
+to fall from them, but are not so bright. The figure of Orion as drawn
+in the old representations of the constellations is a very magnificent
+one. The three bright stars form his belt, and the three smaller ones
+the hilt of his sword hanging from it.</p>
+
+
+
+<p>If you draw an imaginary line through the stars<span class='pagenum'><a name="Page_155" id="Page_155">[Pg 155]</a></span> forming the belt and
+prolong it downwards slantingly, you will see, in the very height of
+winter, the brightest star in all the sky, either in the Northern or
+Southern Hemisphere. This is Sirius, who stands in a class quite by
+himself, for he is many times brighter than any other first magnitude
+star. He never rises very high above the horizon here, but on crisp,
+frosty nights may be seen gleaming like a big diamond between the
+leafless twigs and boughs of the rime-encrusted trees. Sirius is the Dog
+Star, and it is perhaps fortunate that, as he is placed, he can be seen
+sometimes in the southern and sometimes in the northern skies, so that
+many more people have a chance of looking at his wonderful brilliancy,
+than if he had been placed near the Pole star. In speaking of the
+supreme brightness of Sirius among the stars, we must remember that
+Venus and Jupiter, which outrival him, are not stars, but planets, and
+that they are much nearer to us. Sirius is so distant that the measures
+for parallax make hardly any impression on him, but, by repeated
+experiments, it has now been proved that light takes more than eight
+years to travel from him to us. So that, if you are eight years old, you
+are looking at Sirius as he was when you were a baby!</p>
+
+<p>Not far from the Pleiades, to the left as you face<span class='pagenum'><a name="Page_156" id="Page_156">[Pg 156]</a></span> them, are to be
+found two bright stars nearly the same size; these are the Heavenly
+Twins, or Gemini.</p>
+
+<p>Returning now to the Great Bear, we find, if we draw a line through the
+middle and last stars of his tail, and carry it on for a little
+distance, we come fairly near to a cluster of stars in the form of a
+horseshoe; there is only one fairly bright one in it, and some of the
+others are quite small, but yet the horseshoe is distinct and very
+beautiful to look at. This is the Northern Crown. The very bright star
+not far from it is another first-class star called Arcturus.</p>
+
+<p>To the left of the Northern Crown lies Hercules, which is only mentioned
+because near it is the point to which the sun with all his system
+appears at present to be speeding.</p>
+
+<p>For other fascinating constellations, such as Leo or the Lion, Andromeda
+and Perseus, and the three bright stars by which we recognize Aquila the
+Eagle, you must wait awhile, unless you can get some one to point them
+out.</p>
+
+<p>Those which you have noted already are enough to lead you on to search
+for more.</p>
+
+<p>Perhaps some of you who live in towns and can see only a little strip of
+sky from the nursery or schoolroom windows have already found this
+chapter dull, and if so you may skip the rest of it and go on to<span class='pagenum'><a name="Page_157" id="Page_157">[Pg 157]</a></span> the
+next. For the others, however, there is one more thing to know before
+leaving the subject, and that is the names of the string of
+constellations forming what is called the Zodiac. You may have heard the
+rhyme:</p>
+
+<p>
+<span style="margin-left: 1em;">'The Ram, the Bull, the Heavenly Twins,</span><br />
+<span style="margin-left: 1em;">And next the Crab, the Lion shines,</span><br />
+<span style="margin-left: 1em;">The Virgin and the Scales;</span><br />
+<span style="margin-left: 1em;">The Scorpion, Archer, and He-goat,</span><br />
+<span style="margin-left: 1em;">The Man that holds the watering-pot,</span><br />
+<span style="margin-left: 1em;">The Fish with glittering tails.'</span><br />
+</p>
+
+<p>This puts in a form easy to remember the signs of the constellations
+which lie in the Zodiac, an imaginary belt across the whole heavens. It
+is very difficult to explain the Zodiac, but I must try. Imagine for a
+moment the earth moving round its orbit with the sun in the middle. Now,
+as the earth moves the sun will be seen continually against a different
+background&mdash;that is to say, he will appear to us to move not only across
+our sky in a day by reason of our rotation, but also along the sky,
+changing his position among the stars by reason of our revolution. You
+will say at once that we cannot see the stars when the sun is there, and
+no more we can. But the stars are there all the same, and every month
+the sun seems to have moved on<span class='pagenum'><a name="Page_158" id="Page_158">[Pg 158]</a></span> into a new constellation, according to
+astronomers' reckoning. If you count up the names of the constellations
+in the rhyme, you will find that there are just twelve, one for each
+month, and at the end of the year the sun has come round to the first
+one again. The first one is Aries the Ram, and the sun is seen projected
+or thrown against that part of the sky where Aries is, in April, when we
+begin spring; this is the first month to astronomers, and not January,
+as you might suppose. Perhaps you will learn to recognize all the
+constellations in the Zodiac one day; a few of them, such as the Bull
+and the Heavenly Twins, you know already if you have followed this
+chapter.<span class='pagenum'><a name="Page_159" id="Page_159">[Pg 159]</a></span></p>
+
+
+
+<hr style="width: 65%;" />
+<h2><a name="CHAPTER_XII" id="CHAPTER_XII"></a>CHAPTER XII</h2>
+
+<h3>WHAT THE STARS ARE MADE OF</h3>
+
+
+<p>How can we possibly tell what the stars are made of? If we think of the
+vast oceans of space lying between them and us, and realize that we can
+never cross those oceans, for in them there is no air, it would seem to
+be a hopeless task to find out anything about the stars at all. But even
+though we cannot traverse space ourselves, there is a messenger that
+can, a messenger that needs no air to sustain him, that moves more
+swiftly than our feeble minds can comprehend, and this messenger brings
+us tidings of the stars&mdash;his name is Light. Light tells us many
+marvellous things, and not the least marvellous is the news he gives us
+of the workings of another force, the force of gravitation. In some ways
+gravitation is perhaps more wonderful than light, for though light
+speeds across airless space, it is stopped at once by any opaque
+substance&mdash;that is to say, any substance not transparent, as you know
+very well by your own<span class='pagenum'><a name="Page_160" id="Page_160">[Pg 160]</a></span> shadows, which are caused by your bodies stopping
+the light of the sun. Light striking on one side of the earth does not
+penetrate through to the other, whereas gravitation does. You remember,
+of course, what the force of gravitation is, for we read about that very
+early in this book. It is a mysterious attraction existing between all
+matter. Every atom pulls every other atom towards itself, more or less
+strongly according to distance. Now, solid matter itself makes no
+difference to the force of gravitation, which acts through it as though
+it were not there. The sun is pulling the earth toward itself, and it
+pulls the atoms on the far side of the earth just as strongly as it
+would if there were nothing lying between it and them. Therefore, unlike
+light, gravitation takes no heed of obstacles in the way, but acts in
+spite of them. The gravitation of the earth holds you down just the
+same, though you are on the upper floor of a house, with many layers of
+wood and plaster between you and it. It cannot pull you down, for the
+floor holds you up, but it is gravitation that keeps your feet on the
+ground all the same. A clever man made up a story about some one who
+invented a kind of stuff which stopped the force of gravitation going
+through it, just as a solid body<span class='pagenum'><a name="Page_161" id="Page_161">[Pg 161]</a></span> stops light; when this stuff was made,
+of course, it went right away off into space, carrying with it anyone
+who stood on it, as there was nothing to hold it to the earth! That was
+only a story, and it is not likely anyone could invent such stuff, but
+it serves to make clear the working of gravitation. These two tireless
+forces, light and gravitation, run throughout the whole universe, and
+carry messages of tremendous importance for those who have minds to
+grasp them. Without light we could know nothing of these distant worlds,
+and without understanding the laws of gravity we should not be able to
+interpret much that light tells us.</p>
+
+<p>To begin with light, what can we learn from it? We turn at once to our
+own great light-giver, the sun, to whom we owe not only all life, but
+also all the colour and beauty on earth. It is well known to men of
+science that colour lies in the light itself, and not in any particular
+object. That brilliant blue cloak of yours is not blue of itself, but
+because of the light that falls on it. If you cannot believe this, go
+into a room lighted only by gas, and hey, presto! the colour is changed
+as if it were a conjuring trick. You cannot tell now by looking at the
+cloak whether it is blue or green! Therefore<span class='pagenum'><a name="Page_162" id="Page_162">[Pg 162]</a></span> you must admit that as the
+colour changes with the change of light it must be due to light, and not
+to any quality belonging to the material of the cloak. But, you may
+protest, if the colour is solely due to light, and light falls on
+everything alike, why are there so many colours? That is a very fair
+question. If the light that comes from the sun were of only one
+colour&mdash;say blue or red&mdash;then everything would be blue or red all the
+world over. Some doors in houses are made with a strip of red or blue
+glass running down the sides. If you have one in your house like that,
+go and look through it, and you will see an astonishing world made up of
+different tones of the same colour. Everything is red or blue, according
+to the colour of the glass, and the only difference in the appearance of
+objects lies in the different shades, whether things are light or dark.
+This is a world as it might appear if the sun's rays were only blue or
+only red. But the sun's light is not of one colour only, fortunately for
+us; it is of all the colours mixed together, which, seen in a mass, make
+the effect of white light. Now, objects on earth are only either seen by
+the reflected light of the sun or by some artificial light. They have no
+light of their own. Put them in the dark and they do not shine at all;
+you cannot see<span class='pagenum'><a name="Page_163" id="Page_163">[Pg 163]</a></span> them. It is the sun's light striking on them that makes
+them visible. But all objects do not reflect the light equally, and this
+is because they have the power of absorbing some of the rays that strike
+on them and not giving them back at all, and only those rays that are
+given back show to the eye. A white thing gives back all the rays, and
+so looks white, for we have the whole of the sun's light returned to us
+again. But how about a blue thing? It absorbs all the rays except the
+blue, so that the blue rays are the only ones that come back or rebound
+from it again to meet our eyes, and this makes us see the object blue;
+and this is the case with all the other colours. A red object retains
+all rays except the red, which it sends back to us; a yellow object
+gives back only the yellow rays, and so on. What an extraordinary and
+mysterious fact! Imagine a brilliant flower-garden in autumn. Here we
+have tall yellow sunflowers with velvety brown centres, clustering pink
+and crimson hollyhocks, deep red and bright yellow peonies, slender
+fairy-like Japanese anemones, great bunches of mauve Michaelmas daisies,
+and countless others, and mingled with all these are many shades of
+green. Yet it is the light of the sun alone that falling on all these
+varied objects, makes that<span class='pagenum'><a name="Page_164" id="Page_164">[Pg 164]</a></span> glorious blaze of colour; it seems
+incredible. It may be difficult to believe, but it is true beyond all
+doubt. Each delicate velvety petal has some quality in it which causes
+it to absorb certain of the sun's rays and send back the others, and its
+colour is determined by those it sends back.</p>
+
+<p>Well then how infinitely varied must be the colours hidden in the sun's
+light, colours which, mixed all together, make white light! Yes, this is
+so, for all colours that we know are to be found there. In fact, the
+colours that make up sunlight are the colours to be seen in the rainbow,
+and they run in the same order. Have you ever looked carefully at a
+rainbow? If not, do so at the next chance. You will see it begins by
+being dark blue at one end, and passes through all colours until it gets
+to red at the other.</p>
+
+<p>We cannot see a rainbow every day just when we want to, but we can see
+miniature rainbows which contain just the same colours as the real ones
+in a number of things any time the sun shines. For instance, in the
+cut-glass edge of an inkstand or a decanter, or in one of those
+old-fashioned hanging pieces of cut-glass that dangle from the
+chandelier or candle-brackets. Of course you have often seen these
+colours reflected on the wall, and<span class='pagenum'><a name="Page_165" id="Page_165">[Pg 165]</a></span> tried to get them to shine upon your
+face. Or you have caught sight of a brilliant patch of colour on the
+wall and looked around to see what caused it, finally tracing it to some
+thick edge of shining glass standing in the sunlight. Now, the cut-glass
+edge shows these colours to you because it breaks up the light that
+falls upon it into the colours it is made of, and lets each one come out
+separately, so that they form a band of bright colours instead of just
+one ray of white light.</p>
+
+<p>This is perhaps a little difficult to understand, but I will try to
+explain. When a ray of white light falls on such a piece of glass, which
+is known as a prism, it goes in as white light at one side, but the
+three-cornered shape of the glass breaks it up into the colours it is
+made of, and each colour comes out separately at the other side&mdash;namely,
+from blue to red&mdash;like a little rainbow, and instead of one ray of white
+light, we have a broad band of all the colours that light is made of.</p>
+
+<p>Who would ever have thought a pretty plaything like this could have told
+us what we so much wanted to know&mdash;namely, what the sun and the stars
+are made of? It seems too marvellous to be true, yet true it is that for
+ages and ages light has been carrying its silent messages to our eyes,
+and<span class='pagenum'><a name="Page_166" id="Page_166">[Pg 166]</a></span> only recently men have learnt to interpret them. It is as if some
+telegraph operator had been going steadily on, click, click, click, for
+years and years, and no one had noticed him until someone learnt the
+code of dot and dash in which he worked, and then all at once what he
+was saying became clear. The chief instrument in translating the message
+that the light brings is simply a prism, a three-cornered wedge of
+glass, just the same as those hanging lustres belonging to the
+chandeliers. When a piece of glass like this is fixed in a telescope in
+such a way that the sun's rays fall on it, then there is thrown on to a
+piece of paper or any other suitable background a broad coloured band of
+lovely light like a little rainbow, and this is called the sun's
+spectrum, and the instrument by which it is seen is called a
+spectroscope. But this in itself could tell us little; the message it
+brings lies in the fact that when it has passed through the telescope,
+so that it is magnified, it is crossed by hundreds of minute black
+lines, not placed evenly at all, but scattered up and down. There may be
+two so close together that they look like one, and then three far apart,
+and then some more at different distances. When this remarkable
+appearance was examined carefully it was found that in sunlight the
+lines that appeared were always exactly the<span class='pagenum'><a name="Page_167" id="Page_167">[Pg 167]</a></span> same, in the same places,
+and this seemed so curious that men began to seek for an explanation.</p>
+
+<p>Someone thought of an experiment which might teach us something about
+the matter, and instead of letting sunlight fall on the prism, he made
+an artificial light by burning some stuff called sodium, and then
+allowed the band of coloured light to pass through the telescope; when
+he examined the spectrum that resulted, he found that, though numbers of
+lines to be found in the sun's spectrum were missing, there were a few
+lines here exactly matching a few of the lines in the sun's spectrum;
+and this could not be the result of chance only, for the lines are so
+mathematically exact, and are in themselves so peculiarly distributed,
+that it could only mean that they were due to the same cause. What could
+this signify, then, but that away up there in the sun, among other
+things, stuff called sodium, very well known to chemists on earth, is
+burning? After this many other substances were heated white-hot so as to
+give out light, in order to discover if the lines to be seen in their
+spectra were also to be found in the sun's spectrum. One of these was
+iron, and, astonishing to say, all the many little thread-like lines
+that appeared in its spectrum were reproduced to a hair's-breadth, among
+others, in the sun's spectrum.<span class='pagenum'><a name="Page_168" id="Page_168">[Pg 168]</a></span> So we have found out beyond all
+possibility of doubt some of the materials of which the sun is made. We
+know that iron, sodium, hydrogen, and numerous other substances and
+elements, are all burning away there in a terrific furnace, to which any
+furnace we have on earth is but as the flicker of a match.</p>
+
+<div class="figcenter" style="width: 465px;">
+<img src="images/i-212.jpg" width="465" height="700" alt="THE SPECTRUM OF THE SUN AND SIRIUS." title="" />
+<span class="caption">THE SPECTRUM OF THE SUN AND SIRIUS.</span>
+</div>
+
+<p>It was not, of course, much use applying this method to the planets, for
+we know that the light which comes from them to us is only reflected
+sunlight, and this, indeed, was proved by means of the spectroscope. But
+the stars shine by their own light, and this opened up a wide field for
+inquiry. The difficulty was, of course, to get the light of one star
+separated from all the rest, because the light of one star is very faint
+and feeble to cast a spectrum at all. Yet by infinite patience
+difficulties were overcome. One star alone was allowed to throw its
+light into the telescope; the light passed through a prism, and showed a
+faint band of many colours, with the expected little black lines cutting
+across it more or less thickly. Examinations have thus been made of
+hundreds of stars. In the course of them some substances as yet unknown
+to us on earth have been encountered, and in some stars one
+element&mdash;hydrogen&mdash;is much stronger than in others; but, on the whole,
+speaking broadly, it has been satisfactorily shown that the stars are
+made<span class='pagenum'><a name="Page_169" id="Page_169">[Pg 169]</a></span> on the same principles as our own sun, so that the reasoning of
+astronomers which had argued them to be suns was proved.</p>
+
+
+
+<p>We have here in the picture the spectrum of the sun and the spectrum of
+Arcturus. You can see that the lines which appear in the band of light
+belonging to Sirius are also in the band of light belonging to the sun,
+together with many others. This means that the substances flaming out
+and sending us light from the far away star are also giving out light
+from our own sun, and that the sun and Sirius both contain the same
+elements in their compositions.</p>
+
+<p>This, indeed, seems enough for the spectroscope to have accomplished; it
+has interpreted for us the message light brings from the stars, so that
+we know beyond all possibility of mistake that these glowing, twinkling
+points of light are brilliant suns in a state of intense heat, and that
+in them are burning elements with which we ourselves are quite familiar.
+But when the spectroscope had done that, its work was not finished, for
+it has not only told us what the stars are made of, but another thing
+which we could never have known without it&mdash;namely, if they are moving
+toward us or going away from us.<span class='pagenum'><a name="Page_170" id="Page_170">[Pg 170]</a></span></p>
+
+
+
+<hr style="width: 65%;" />
+<h2><a name="CHAPTER_XIII" id="CHAPTER_XIII"></a>CHAPTER XIII</h2>
+
+<h3>RESTLESS STARS</h3>
+
+
+<p>You remember we have already remarked upon the difficulty of telling how
+far one star lies behind another, as we do not know their sizes. It is,
+to take another similar case, easy enough to tell if a star moves to one
+side or the other, but very difficult by ordinary observation to tell if
+it is advancing toward us or running away from us, for the only means we
+have of judging is if it gets larger or smaller, and at that enormous
+distance the fact whether it advances or recedes makes no difference in
+its size. Now, the spectroscope has changed all this, and we can tell
+quite as certainly if a star is coming toward us as we can if it moves
+to one side. I will try to explain this. You know, perhaps, that sound
+is caused by vibration in the air. The noise, whatever it is, jars the
+air and the vibrations strike on our ears. It is rather the same thing
+as the result of throwing a stone into a pond: from the centre of the
+splash<span class='pagenum'><a name="Page_171" id="Page_171">[Pg 171]</a></span> little wavelets run out in ever-widening circles; so through the
+air run ever-widening vibrations from every sound. The more vibrations
+there are in a second the shriller is the note they make. In a high note
+the air-vibrations follow one another fast, pouring into one's ear at a
+terrific speed, so that the apparatus in the ear which receives them
+itself vibrates fiercely and records a high note, while a lower note
+brings fewer and slower vibrations in a second, and the ear is not so
+much disturbed. Have you ever noticed that if a railway engine is
+sweeping-toward you and screaming all the time, its note seems to get
+shriller and shriller? That is because the engine, in advancing, sends
+the vibrations out nearer to you, so more of them come in a second, and
+thus they are crowded up closer together, and are higher and higher.</p>
+
+<p>Now, light is also caused by waves, but they are not the same as sound
+waves. Light travels without air, whereas sound we know cannot travel
+without air, and is ever so much slower, and altogether a grosser,
+clumsier thing than light. But yet the waves or rays which make light
+correspond in some ways to the vibrations of sound. What corresponds to
+the treble on the piano is the blue end of the spectrum in light, and
+the bass is the red end. Now, when<span class='pagenum'><a name="Page_172" id="Page_172">[Pg 172]</a></span> we are looking at the spectrum of
+any body which is advancing swiftly toward us, something of the same
+effect is observed as in the case of the shrieking engine. Take any star
+and imagine that that star is hastening toward us at a pace of three
+hundred miles a second, which is not at all an unusual rate for a star;
+then, if we examine the band of light, the spectrum, of such a star, we
+shall observe an extraordinary fact&mdash;all these little lines we have
+spoken of are shoved up toward the treble or blue end of the spectrum.
+They still remain just the same distances from each other, and are in
+twos and threes or single, so that the whole set of lines is unaltered
+as a set, but everyone of them is shifted a tiny fraction up toward the
+blue end of the spectrum, just a little displaced. Now if, instead of
+advancing toward us, this same star had been rushing away from us at a
+similar pace, all these lines would have been moved a tiny bit toward
+the red or bass end of the spectrum. This is known to be certainly true,
+so that by means of the spectroscope we can tell that some of these
+great sun-stars are advancing toward us and some receding from us,
+according to whether the multitudes of little lines in the spectrum are
+shifted slightly to the blue or the red end.<span class='pagenum'><a name="Page_173" id="Page_173">[Pg 173]</a></span></p>
+
+<p>You remember that it has been surmised that the pace the sun moves with
+his system is about twelve miles a second. This seems fast enough to us,
+who think that one mile a minute is good time for an express train, but
+it is slow compared with the pace of many of the stars. As I have said,
+some are travelling at a rate of between two hundred and three hundred
+miles a second; and it is due to the spectroscope that we know not only
+whether a star is advancing toward us or receding from us, but also
+whether the pace is great or not; it even tells us what the pace is, up
+to about half a mile a second, which is very marvellous. It is a curious
+fact that many of the small stars show greater movement than the large
+ones, which mayor may not mean that they are nearer to us.</p>
+
+<p>It may be taken as established that there is no such thing as absolute
+rest in the universe: everything, stars and nebul&aelig; alike, are moving
+somewhere; in an infinite variety of directions, with an infinite
+variety of speed they hasten this way and that. It would be impossible
+for any to remain still, for even supposing it had been so 'in the
+beginning,' the vast forces at work in the universe would not let it
+remain so. Out of space would come the persistent call of gravitation:
+atoms would<span class='pagenum'><a name="Page_174" id="Page_174">[Pg 174]</a></span> cry silently to atoms. There could be no perfect equality
+of pull on all sides; from one side or another the pull would be the
+stronger. Slowly the inert mass would obey and begin falling toward it;
+it might be an inch at a time, but with rapid increase, until at last it
+also was hastening some whither in this universe which appears to us to
+be infinite.</p>
+
+<p>It must be remembered that these stars, even when moving at an enormous
+pace, do not change their places in the sky when regarded by ordinary
+observers. It would take thousands of years for any of the
+constellations to appear at all different from what they are now, even
+though the stars that compose them are moving in different directions
+with a great velocity, for a space of many millions of miles, at the
+distance of most of the stars, would be but as the breadth of a fine
+hair as seen by us on earth. So thousands of years ago men looked up at
+the Great Bear, and saw it apparently the same as we see it now; yet for
+all that length of time the stars composing it have been rushing in this
+direction and that at an enormous speed, but do not appear to us on the
+earth to alter their positions in regard to each other. I know of
+nothing that gives one a more overwhelming sense<span class='pagenum'><a name="Page_175" id="Page_175">[Pg 175]</a></span> of the mightiness of
+the universe and the smallness of ourselves than this fact. From age to
+age men look on changeless heavens, yet this apparently stable universe
+is fuller of flux and reflux than is the restless ocean itself, and the
+very wavelets on the sea are not more numerous nor more restless than
+the stars that bestrew the sky.<span class='pagenum'><a name="Page_176" id="Page_176">[Pg 176]</a></span></p>
+
+
+
+<hr style="width: 65%;" />
+<h2><a name="CHAPTER_XIV" id="CHAPTER_XIV"></a>CHAPTER XIV</h2>
+
+<h3>THE COLOURS OF THE STARS</h3>
+
+
+<p>Has it ever occurred to you that the stars are not all of the same
+colour? It is true that, just glancing at them casually, you might say
+they are all white; but if you examine them more carefully you cannot
+help seeing that some shine with a steely blue light, while others are
+reddish or yellowish. These colours are not easy to distinguish with the
+naked eye, and might not attract any attention at all unless they were
+pointed out; yet when attention is drawn to the fact, it is impossible
+to deny the redness of some, such as Aldebaran. But though we may admit
+this, we might add that the colours are so very faint and inconspicuous,
+that they might be, after all, only the result of imagination.</p>
+
+<p>To prove that the colours are constant and real we must use a telescope,
+and then we need have no further doubt of their reality, for instead of
+disappearing, the colours of some stars stand out<span class='pagenum'><a name="Page_177" id="Page_177">[Pg 177]</a></span> quite vividly beyond
+the possibility of mistake. Red stars are a bright red, and they are the
+most easily seen of all, though the other colours, blue and yellow and
+green, are seen very decidedly by some people. The red stars have been
+described by various observers as resembling 'a drop of blood on a black
+field,' 'most magnificent copper-red,' 'most intense blood-red,' and
+'glowing like a live coal out of the darkness of space.' Some people see
+them as a shining red, like that of a glowing cloud at sunset. Therefore
+there can be no doubt that the colours are genuine enough, and are
+telling us some message. This message we are able to read, for we have
+begun to understand the language the stars speak to us by their light
+since the invention of the spectroscope. The spectroscope tells us that
+these colours indicate different stages in the development of the stars,
+or differences of constitution&mdash;that is to say, in the elements of which
+they are made. Our own sun is a yellow star, and other yellow stars are
+akin to him; while red and blue and green stars contain different
+elements, or elements in different proportions.</p>
+
+<p>Stars do not always remain the same colours for an indefinite time; one
+star may change slowly<span class='pagenum'><a name="Page_178" id="Page_178">[Pg 178]</a></span> from yellow to white, and another from red to
+yellow; and there are instances of notable changes, such as that of the
+brilliant white Sirius, who was stated in old times by many different
+observers to be a red star. All this makes us think, and year by year
+thought leads us on to knowledge, and knowledge about these distant suns
+increases. But though we know a good deal now, there are still many
+questions we should like to ask which we cannot expect to have answered
+for a long time yet, if ever.</p>
+
+<p>The star colours have some meanings which we cannot even guess; we can
+only notice the facts regarding them. For instance, blue stars are never
+known to be solitary&mdash;they always have a companion, but why this should
+be so passes our comprehension. What is it in the constitution of a blue
+star which holds or attracts another? Whatever it may be, it is
+established by repeated instances that blue stars do not stand alone. In
+the constellation of Cygnus there are two stars, a blue and a yellow
+one, which are near enough to each other to be seen in the same
+telescope at the same time, and yet in reality are separated by an
+almost incredible number of billions of miles. But as we know that a
+blue star is never seen alone, and<span class='pagenum'><a name="Page_179" id="Page_179">[Pg 179]</a></span> that it has often as its companion a
+yellowish or reddish star, it is probable that these two, situated at an
+enormous distance from one another, are yet in some mysterious way
+dependent on each other, and are not merely seen together because they
+happen to fall in the same field of view.</p>
+
+<p>Many double stars show most beautifully contrasted colours: among them
+are pairs of yellow and rose-red, golden and azure, orange and purple,
+orange and lilac, copper-colour and blue, apple-green and cherry-red,
+and so on. In the Southern Hemisphere there is a cluster containing so
+many stars of brilliant colours that Sir John Herschel named it 'the
+Jewelled Cluster.'</p>
+
+<p>I expect most of you have seen an advertisement of Pear's Soap, in which
+you are asked to stare at some red letters, and then look away to some
+white surface, such as a ceiling, when you will see the same letters in
+green. This is because green is the complementary or contrasting colour
+to red, and the same thing is the case with blue and yellow. When any
+one colour of either of these pairs is seen, it tends to make the other
+appear by reaction, and if the eye gazed hard at blue instead of red, it
+would next see yellow, and not green. Now, many people to whom this
+curious fact is known argue<span class='pagenum'><a name="Page_180" id="Page_180">[Pg 180]</a></span> that perhaps the colours of the double
+stars are not real, but the effect of contrast only; for instance, they
+say a red star near a companion white one would tend to make the
+companion appear green, and so, of course, it would. But this does not
+account for the star colours, which are really inherent in the stars
+themselves, as may be proved by cutting off the light of one star, and
+looking only at the other, when its colour still appears unchanged.
+Another argument equally strong against the contrast theory is that the
+colours of stars in pairs are by no means always those which would
+appear if the effect was only due to complementary colours. It is not
+always blue and yellow or red and green pairs that we see, though these
+are frequent, but many others of various kinds, such as copper and blue,
+and ruddy and blue.</p>
+
+<p>We have therefore come to the conclusion that there are in this
+astonishing universe numbers of gloriously coloured suns, some of which
+apparently lie close together. What follows? Why, we want to know, of
+course, if these stars are really pairs connected with each other, or if
+they only appear so by being in the same line of sight, though one is
+infinitely more distant than the other. And that question also has been
+answered. There are now<span class='pagenum'><a name="Page_181" id="Page_181">[Pg 181]</a></span> known thousands of cases in which stars,
+hitherto regarded as single, have been separated into two, or even more,
+by the use of a telescope. Of these thousands, some hundreds have been
+carefully investigated, and the result is that, though there are
+undoubtedly some in which the connexion is merely accidental, yet in by
+far the greater number of cases the two stars thus seen together have
+really some connexion which binds them to one another; they are
+dependent on one another. This has been made known to us by the working
+of the wonderful law of gravitation, which is obeyed throughout the
+whole universe. We know that by the operation of this law two mighty
+suns will be drawn toward each other with a certain pull, just as surely
+as we know that a stone let loose from the hand will fall upon the
+earth; so by noting the effect of two mighty suns upon each other many
+facts about them may be found out. By the most minute and careful
+measurements, by the use of the spectroscope, and by every resource
+known to science, astronomers have, indeed, actually found out with a
+near approach to exactness how far some of these great suns lie from
+each other, and how large they are in comparison with one another.</p>
+
+<p>The very first double star ever discovered was<span class='pagenum'><a name="Page_182" id="Page_182">[Pg 182]</a></span> one which you have
+already seen, the middle one in the tail of the Great Bear. If you look
+at it you will be delighted to find that you can see a wee star close to
+it, and you will think you are looking at an example of a double star
+with your very own eyes; but you will be wrong, for that wee star is
+separated by untold distances from the large one to which it seems so
+near. In fact, any stars which can be seen to be separate by the naked
+eye must lie immeasurably far apart, however tiny seems the space
+between them. Such stars may possibly have some connexion with each
+other, but, at any rate in this case, such a connexion has not been
+proved. No, the larger star itself is made up of two others, which can
+only be seen apart in a telescope. Since this discovery double stars
+have been plentifully found in every part of the sky. The average space
+between such double stars as seen from our earth is&mdash;what do you think?
+It is the width of a single hair held up thirty-six feet from our eyes!
+This could not, of course, be seen without the use of a telescope or
+opera-glasses. It serves to give some impression of star distances when
+we think that the millions and millions of miles lying between those
+stars have shrunk to that hair's-breadth seen from our point of view.<span class='pagenum'><a name="Page_183" id="Page_183">[Pg 183]</a></span></p>
+
+<p>Twin stars circle together round a common centre of gravity, and are
+bound by the laws of gravitation just as the planets are. Our sun is a
+solitary star, with no companion, and therefore such a state of things
+seems to us to be incredible. Fancy two gigantic suns, one topaz-yellow
+and the other azure-blue, circling around in endless movement! Where in
+such a system would there be room for the planets? How could planets
+exist under the pull of two suns in opposite directions? Still more
+wonders are unfolded as the inquiry proceeds. Certain irregularities in
+the motions of some of these twin systems led astronomers to infer that
+they were acted upon by another body, though this other body was not
+discernible. In fact, though they could not see it, they knew it must be
+there, just as Adams and Leverrier knew of the existence of Neptune,
+before ever they had seen him, by the irregularities in the movements of
+Uranus. As the results showed, it was there, and was comparable in size
+to the twin suns it influenced, and yet they could not see it. So they
+concluded this third body must be dark, not light-giving like its
+companions. We are thus led to the strange conclusion that some of these
+systems are very complicated, and are formed not only of shining suns,
+but of<span class='pagenum'><a name="Page_184" id="Page_184">[Pg 184]</a></span> huge dark bodies which cannot be called suns. What are they,
+then? Can they be immense planets? Is it possible that life may there
+exist? No fairy tale could stir the imagination so powerfully as the
+thought of such systems including a planetary body as large or larger
+than its sun or suns. If indeed life exists there, what a varied scene
+must be presented day by day! At one time both suns mingling their
+flashing rays may be together in the sky; at another time only one
+appears, a yellow or blue sun, as the case may be. The surface of such
+planets must undergo weird transformations, the foliage showing one day
+green, the next yellow, and the next blue; shadows of azure and orange
+will alternate! But fascinating as such thoughts are, we can get no
+further along that path.</p>
+
+<p>To turn from fancy to facts, we find that telescope and spectroscope
+have supplied us with quite enough matter for wonder without calling
+upon imagination. We have discovered that many of the stars which seem
+to shine with a pure single light are double, and many more consist not
+only of two stars, but of several, some of which may be dark bodies. The
+Pole Star was long known to be double, and is now discovered to have a
+third<span class='pagenum'><a name="Page_185" id="Page_185">[Pg 185]</a></span> member in its system. These multiple systems vary from one
+another in almost every case. Some are made up of a mighty star and a
+comparatively small one; others are composed of stars equal in
+light-giving power&mdash;twin suns. Some progress swiftly round their orbits,
+some go slowly; indeed, so slowly that during the century they have been
+under observation only the very faintest sign of movement has been
+detected; and in other systems, which we are bound to suppose double,
+the stars are so slow in their movements that no progress seems to have
+been made at all.</p>
+
+<p>The star we know as the nearest to us in the heavens, Alpha Centauri, is
+composed of two very bright partners, which take about eighty-seven
+years to traverse their orbit. They sometimes come as near to each other
+as Saturn is to the sun. In the case of Sirius astronomers found out
+that he had a companion by reason of his irregularities of movement
+before they discovered that companion, which is apparently a very small
+star, only to be discerned with good telescopes. But here, again, it
+would be unwise to judge only by what we see. Though the star appears
+small, we know by the influence it exercises on Sirius that it is very
+nearly the same size as he is. Thus we judge that it is poor in<span class='pagenum'><a name="Page_186" id="Page_186">[Pg 186]</a></span>
+light-giving property; in fact, its shining power is much less than that
+of its companion, though its size is so nearly equal. This is not
+wonderful, for Sirius's marvellous light-giving power is one of the
+wonders of the universe; he shines as brilliantly as twenty-nine or
+thirty of our suns!</p>
+
+<p>In some cases the dark body which we cannot see may even be larger than
+the shining one, through which alone we can know anything of it. Here we
+have a new idea, a hint that in some of these systems there may be a
+mighty earth with a smaller sun going round it, as men imagined our sun
+went around the earth before the real truth was found out.</p>
+
+<p>So we see that, when we speak of the stars as suns comparable with our
+sun, we cannot think of them all as being exactly on the same model.
+There are endless varieties in the systems; there are solitary suns like
+ours which may have a number of small planets going round them, as in
+the solar system; but there are also double suns going round each other,
+suns with mighty dark bodies revolving round them which may be planets,
+and huge dark bodies with small suns too. Every increase of knowledge
+opens up new wonders, and the world in which we live is but one kind of
+world amid an infinite number.<span class='pagenum'><a name="Page_187" id="Page_187">[Pg 187]</a></span></p>
+
+<p>In this chapter we have learnt an altogether new fact&mdash;the fact that the
+hosts of heaven comprise not only those shining stars we are accustomed
+to see, but also dark bodies equally massive, and probably equally
+numerous, which we cannot see. In fact, the regions of space may be
+strewn with such dark bodies, and we could have no possible means of
+discovering them unless they were near enough to some shining body to
+exert an influence upon it. It is not with his eyes alone, or with his
+senses, man knows of the existence of these great worlds, but often
+solely by the use of the powers of his mind.<span class='pagenum'><a name="Page_188" id="Page_188">[Pg 188]</a></span></p>
+
+
+
+<hr style="width: 65%;" />
+<h2><a name="CHAPTER_XV" id="CHAPTER_XV"></a>CHAPTER XV</h2>
+
+<h3>TEMPORARY AND VARIABLE STARS</h3>
+
+
+<p>It is a clear night, nearly all the world is asleep, when an astronomer
+crosses his lawn on his way to his observatory to spend the dark hours
+in making investigations into profound space. His brilliant mind,
+following the rays of light which shoot from the furthest star, will
+traverse immeasurable distances, while the body is forgotten. Just
+before entering the observatory he pauses and looks up; his eye catches
+sight of something that arrests him, and he stops involuntarily. Yet any
+stranger standing beside him, and gazing where he gazes, would see
+nothing unusual. There is no fiery comet with its tail stretching across
+from zenith to horizon, no flaming meteor dashing across the darkened
+sky. But that there is something unusual to be seen is evident, for the
+astronomer breathes quickly, and after another earnest scrutiny of the
+object which has attracted him, he rushes into the observatory, searches
+for a star-chart, and<span class='pagenum'><a name="Page_189" id="Page_189">[Pg 189]</a></span> examines attentively that part of the sky at
+which he has been gazing. He runs his finger over the chart: here and
+there are the well-known stars that mark that constellation, but here?
+In that part there is no star marked, yet he knows, for his own eyes
+have told him but a few moments ago, that here there is actually blazing
+a star, not large, perhaps, but clear enough to be seen without a
+telescope&mdash;a star, maybe, which no eye but his has yet observed!</p>
+
+<p>He hurries to his telescope, and adjusts it so as to bring the stranger
+into the field of view. A new star! Whence has it come? What does it
+mean?</p>
+
+<p>By the next day at the latest the news has flown over the wires, and all
+the scientific world is aware that a new star has been detected where no
+star ever was seen before. Hundreds of telescopes are turned on to it;
+its spectrum is noted, and it stands revealed as being in a state of
+conflagration, having blazed up from obscurity to conspicuousness. Night
+after night its brilliance grows, until it ranks with the brightest
+stars in heaven, and then it dies down and grows dim, gradually
+sinking&mdash;sinking into the obscurity from whence it emerged so briefly,
+and its place in the sky knows it no more. It may be<span class='pagenum'><a name="Page_190" id="Page_190">[Pg 190]</a></span> there still, but
+so infinitely faint and far away that no power at our command can reveal
+it to us. And the amazing part of it is that this huge disaster, this
+mighty conflagration, is not actually happening as it is seen, but has
+happened many hundreds of years ago, though the message brought by the
+light carrier has but reached us now.</p>
+
+<p>There have not been a great many such outbursts recorded, though many
+may have taken place unrecorded, for even in these days, when trained
+observers are ceaselessly watching the sky, 'new' stars are not always
+noticed at once. In 1892 a new star appeared, and shone for two months
+before anyone noticed it. This particular one never rose to any very
+brilliant size. I twas situated in the constellation of Auriga, and was
+noticed on February 1. It remained fairly bright until March 6, when it
+began to die down; but it has now sunk so low that it can only be seen
+in the very largest telescopes.</p>
+
+<p>Photography has been most useful in recording these stars, for when one
+is noticed it has sometimes been found that it has been recorded on a
+photographic plate taken some time previously, and this shows us how
+long it has been visible. More and more photography becomes the useful<span class='pagenum'><a name="Page_191" id="Page_191">[Pg 191]</a></span>
+handmaid of astronomers, for the photographic prepared plate is more
+sensitive to rays of light than the human eye, and, what is more useful
+still, such plates retain the rays that fall upon them, and fix the
+impression. Also on a plate these rays are cumulative&mdash;that is to say,
+if a very faint star shines continuously on a plate, the longer the
+plate is exposed, within certain limits, the clearer will the image of
+that star become, for the light rays fall one on the top of the other,
+and tend to enforce each other, and so emphasize the impression, whereas
+with our eyes it is not the same thing at all, for if we do not see an
+object clearly because it is too faint, we do not see it any better,
+however much we may stare at the place where it ought to be. This is
+because each light ray that reaches our eye makes its own impression,
+and passes on; they do not become heaped on each other, as they do on a
+photographic plate.</p>
+
+<p>One variable star in Perseus, discovered in 1901, rose to such
+brilliancy that for one night it was queen of the Northern Hemisphere,
+outshining all the other first-class stars.</p>
+
+<p>It rose into prominence with wonderful quickness, and sank equally fast.
+At its height it outshone our sun eight thousand times! This<span class='pagenum'><a name="Page_192" id="Page_192">[Pg 192]</a></span> star was
+so far from us that it was reckoned its light must take about three
+hundred years to reach us, consequently the great conflagration, or
+whatever caused the outburst, must have taken place in the reign of
+James I., though, as it was only seen here in 1901, it was called the
+new star of the new century.</p>
+
+<p>When these new stars die down they sometimes continue to shine faintly
+for a long time, so that they are visible with a telescope, but in other
+cases they may die out altogether. We know very little about them, and
+have but small opportunity for observing them, and so it is not safe to
+hazard any theories to account for their peculiarities. At first men
+supposed that the great flame was made by a violent collision between
+two bodies coming together with great velocity so that both flared up,
+but this speculation has been shown by the spectroscope to be
+improbable, and now it is supposed by some people that two stars
+journeying through space may pass through a nebulous region, and thus
+may flare up, and such a theory is backed up by the fact that a very
+great number of such stars do seem to be mixed up in some strange way
+with a nebulous haze.</p>
+
+<p>All these new stars that we have been discussing<span class='pagenum'><a name="Page_193" id="Page_193">[Pg 193]</a></span> so far have only
+blazed up once and then died down, but there is another class of stars
+quite as peculiar, and even more difficult to explain, and these are
+called variable stars. They get brighter and brighter up to a certain
+point, and then die down, only to become bright once more, and these
+changes occur with the utmost regularity, so that they are known and can
+be predicted beforehand. This is even more unaccountable than a sudden
+and unrepeated outburst, for one can understand a great flare-up, but
+that a star should flare and die down with regularity is almost beyond
+comprehension. Clearly we must look further than before for an
+explanation. Let us first examine the facts we know. Variable stars
+differ greatly from each other. Some are generally of a low magnitude,
+and only become bright for a short time, while others are bright most of
+the time and die down only for a short time. Others become very bright,
+then sink a little bit, but not so low as at first; then they become
+bright again, and, lastly, go right down to the lowest point, and they
+keep on always through this regular cycle of changes. Some go through
+the whole of these changes in three days, and others take much longer.
+The periods, as the intervals between<span class='pagenum'><a name="Page_194" id="Page_194">[Pg 194]</a></span> the complete round of changes are
+called, vary, in fact, between three days and six hundred! It may seem
+impossible that changes covering so long as six hundred days could be
+known and followed, but there is nothing that the patience of
+astronomers will not compass.</p>
+
+<p>One very well-known variable star you can see for yourselves, and as an
+ounce of observation is worth a pound of hearsay, you might take a
+little trouble to find it. Go out on any clear starlight night and look.
+Not very far from Cassiopeia (W.), to the left as you face it, are three
+bright stars running down in a great curve. These are in the
+constellation called Perseus, and a little to the right of the middle
+and lowest one is the only variable star we can see in the sky without a
+telescope.</p>
+
+<p>This is Algol. For the greater part of three days he is a bright star of
+about the second magnitude, then he begins to fade, and for four and a
+half hours grows steadily dimmer. At the dimmest he remains for about
+twenty minutes, and then rises again to his ordinary brightness in three
+and a half hours. How can we explain this? You may possibly be able to
+suggest a reason. What do you say to a dark body revolving round Algol,
+or, rather, revolving with him round a common centre<span class='pagenum'><a name="Page_195" id="Page_195">[Pg 195]</a></span> of gravity? If
+such a thing were indeed true, and if such a body happened to pass
+between us and Algol at each revolution, the light of Algol would be cut
+off or eclipsed in proportion to the size of such a body. If the dark
+body were the full size of Algol and passed right between him and us, it
+would cut off all the light, but if it were not quite the same size, a
+little would still be seen. And this is really the explanation of the
+strange changes in the brightness of Algol, for such a dark body as we
+are imagining does in reality exist. It is a large dark body, very
+nearly as large as Algol himself, and if, as we may conjecture, it is a
+mighty planet, we have the extraordinary example of a planet and its sun
+being nearly the same size. We have seen that the eclipse happens every
+three days, and this means, of course, that the planetary body must go
+round its sun in that time, so as to return again to its position
+between us and him, but the thing is difficult to believe. Why, the
+nearest of all our planets to the sun, the wee Mercury, takes
+eighty-seven days to complete its orbit, and here is a mighty body
+hastening round its sun in three! To do this in the time the large dark
+planet must be very near to Algol; indeed, astronomers have calcu<span class='pagenum'><a name="Page_196" id="Page_196">[Pg 196]</a></span>lated
+that the surfaces of the two bodies are not more than about two million
+miles apart, and this is a trifle when we consider that we ourselves are
+more than forty-six times as far as that from the sun. At this distance
+Algol, as observed from the planet, will fill half the sky, and the heat
+he gives out must be something stupendous. Also the effects of
+gravitation must be queer indeed, acting on two such huge bodies so
+close together. If any beings live in such a strange world, the pull
+which draws them to their mighty sun must be very nearly equal to the
+pull which holds them to their own globe; the two together may
+counteract each other, but the effect must be strange!</p>
+
+<p>From irregularities in the movements of Algol it has been judged that
+there may be also in the same system another dark body, but of it
+nothing has been definitely ascertained.</p>
+
+<p>But all variable stars need not necessarily be due to the light being
+intercepted by a dark body. There are cases where two bright stars in
+revolving round each other produce the same effect; for when seen side
+by side the two stars give twice as much light as when one is hidden
+behind the other, and as they are seen alternately side by side and in
+line, they seem to alter regularly in lustre.<span class='pagenum'><a name="Page_197" id="Page_197">[Pg 197]</a></span></p>
+
+
+
+<hr style="width: 65%;" />
+<h2><a name="CHAPTER_XVI" id="CHAPTER_XVI"></a>CHAPTER XVI</h2>
+
+<p>STAR CLUSTERS AND NEBUL&AElig;</p>
+
+
+<p>Could you point out any star cluster in the sky? You could if you would
+only think for a minute, for one has been mentioned already. This is the
+cluster known as the Pleiades, and it is so peculiar and so different
+from anything else, that many people recognize the group and know where
+to look for it even before they know the Great Bear, the favourite
+constellation in the northern sky, itself. The Pleiades is a real star
+cluster, and the chief stars in it are at such enormous distances from
+one another that they can be seen separately by the eye unaided, whereas
+in most clusters the stars appear to be so close together that without a
+telescope they make a mere blur of brightness. For a long time it was
+supposed that the stars composing the Pleiades could not really be
+connected because of the great distances between them; for, as you know,
+even a hair's-breadth apparently between stars signifies in reality many
+millions of miles.<span class='pagenum'><a name="Page_198" id="Page_198">[Pg 198]</a></span></p>
+
+<p>Light travelling from the Pleiades to us, at that incomprehensible pace
+of which you already know, takes a hundred and ninety years to reach us!
+At this incredibly remote distance lies the main part of the cluster
+from us; but it is more marvellous still that we have every reason to
+believe that the outlying stars of this cluster are as far from the
+central ones as the nearest star we know of, Alpha Centauri, is from us!
+Little wonder was it, then, that men hesitated to ascribe to the
+Pleiades any real connection with each other, and supposed them to be
+merely an assemblage of stars which seemed to us to lie together.</p>
+
+<p>With the unaided eye we see comparatively few stars in the Pleiades. Six
+is the usual number to be counted, though people with very good sight
+have made out fourteen. Viewed through the telescope, however, the scene
+changes: into this part of space stars are crowded in astonishing
+profusion; it is impossible to count them, and with every increase in
+the power of the telescope still more are revealed. Well over a thousand
+in this small space seems no exaggerated estimate. Now, it is impossible
+to say how many of these really belong to the group, and how many are
+seen there accidentally, but observations of the most prominent ones
+have<span class='pagenum'><a name="Page_199" id="Page_199">[Pg 199]</a></span> shown that they are all moving in exactly the same direction at
+the same pace. It would be against probability to conceive that such a
+thing could be the result of mere chance, considering the infinite
+variety of star movements in general, and so we are bound to believe
+that this wonderful collection of stars is a real group, and not only an
+apparent one.</p>
+
+<p>So splendid are the great suns that illuminate this mighty system, that
+at least fifty or sixty of them far surpass our own sun in brilliancy.
+Therefore when we look at that tiny sparkling group we must in
+imagination picture it as a vast cluster of mighty stars, all controlled
+and swayed by some dominant impulse, though separated by spaces enough
+to make the brain reel in thinking of them. If these suns possess also
+attendant planets, what a galaxy of worlds, what a universe within a
+universe is here!</p>
+
+<p>Other star clusters there are, not so conspicuous as the Pleiades, and
+most of these can only be seen through a telescope, so we may be
+thankful that we have one example so splendid within our own vision.
+There are some clusters so far and faintly shining that they were at
+first thought to be nebul&aelig;, and not stars at all; but the telescope
+gradually revealed the fact that many of these are made up<span class='pagenum'><a name="Page_200" id="Page_200">[Pg 200]</a></span> of stars,
+and so people began to think that all faint shining patches of nebulous
+light were really star clusters, which would be resolved into stars if
+only we had better telescopes. Since the invention of the spectroscope,
+however, fresh light has been thrown on the matter, for the spectrum
+which is shown by some of the nebulous patches is not the same as that
+shown by stars, and we know that many of these strange appearances are
+not made up of infinitely distant stars.</p>
+
+<p>We are talking here quite freely about nebul&aelig; because we have met one
+long ago when we discussed the gradual evolution of our own system, and
+we know quite well that a nebula is composed of luminous faintly-glowing
+gas of extreme fineness and thinness. We see in the sky at the present
+time what we may take to be object-lessons in our own history, for we
+see nebul&aelig; of all sorts and sizes, and in some stars are mixed up, and
+in others stars are but dimly seen, so that it does not require a great
+stretch of the imagination to picture these stars as being born,
+emerging from the swaddling bands of filmy webs that have enwrapped
+them; and other nebul&aelig; seem to be gas only, thin and glowing, with no
+stars at all to be found in it. We still know very little about these
+mysterious appearances,<span class='pagenum'><a name="Page_201" id="Page_201">[Pg 201]</a></span> but the work of classifying and resolving them
+is going on apace. Nebul&aelig; are divided into several classes, but the
+easiest distinction to remember is that between white nebul&aelig; and green
+nebul&aelig;. This is not to say that we can see some coloured green, but that
+green appears in the spectrum of some of the nebul&aelig;, while the spectrum
+of a white nebula is more like that of a star.</p>
+
+<p>It is fortunate for us that in the sky we can see without a telescope
+one instance of each of the several objects of interest that we have
+referred to.</p>
+
+<p>We have been able to see one very vivid example of a variable star; we
+have seen one very beautiful example of a star cluster; and it remains
+to look for one very good example of a white nebula.</p>
+
+<p>Just as in finding Algol you were doing a little bit of practical work,
+proving something of which you had read, so by seeing this nebula you
+will remember more about nebul&aelig; in general than by reading many chapters
+on the subject. This particular nebula is in Andromeda, and is not far
+from Algol; and it is not difficult to find. It is the only one that can
+be well seen without a telescope, and was known to the ancients; it is
+believed to have been mentioned in a book of the tenth century!<span class='pagenum'><a name="Page_202" id="Page_202">[Pg 202]</a></span></p>
+
+<div class="figcenter" style="width: 634px;">
+<img src="images/i-248.jpg" width="634" height="650" alt="Dr. Max Wolf.
+
+THE GREAT NEBULA IN ANDROMEDA." title="" />
+<span class="caption"><i>Dr. Max Wolf.</i><br />
+
+THE GREAT NEBULA IN ANDROMEDA.</span>
+</div>
+
+
+<p>If you take an imaginary line down from the two left-hand stars of
+Cassiopeia, and follow it carefully, you will come before long to a
+rather faint star, and close to it is the nebula.</p>
+
+<p>When you catch sight of it you will, perhaps, at first be disappointed,
+for all you will see is a soft blur of white, as if someone had laid a
+dab of luminous paint on the sky with a finger; but as you gaze at it
+night after night and realize its unchangeableness, realize also that it
+is a mass of glowing gas, an island in space, infinitely distant,
+unsupported and inexplicable, something of the wonder of it will creep
+over you.</p>
+
+
+<p>Thousands of telescopic nebul&aelig; are now known, and have been examined,
+and they are of all shapes. Roughly, they have been divided up into
+several classes&mdash;those that seem to us to be round and those that are
+long ovals, like this one in Andromeda; but these may, of course, be
+only round ones seen edgewise by us; others are very irregular, and
+spread over an enormous part of the sky. The most remarkable of these is
+that in Orion, and if you look very hard at the middle star in the
+sword-hilt of Orion, you may be able to make out a faint mistiness.
+This, when seen through a telescope, becomes a wonderful and
+far-spreading<span class='pagenum'><a name="Page_203" id="Page_203">[Pg 203]</a></span> nebula, with brighter and darker parts like gulfs in
+it, and dark channels. It has been sometimes called the Fish-mouth
+Nebula, from a fanciful idea as to its shape. Indeed, so extraordinarily
+varied are these curious structures, that they have been compared with
+numbers of different objects. We have some like brushes, others
+resembling fans, rings, spindles, keyholes; others like animals&mdash;a fish,
+a crab, an owl, and so on; but these suggestions are imaginative, and
+have nothing to do with the real problem. In <i>The System of the Stars</i>
+Miss Clerke says: 'In regarding these singular structures we seem to see
+surges and spray-flakes of a nebulous ocean, bewitched into sudden
+immobility; or a rack of tempest-driven clouds hanging in the sky,
+momentarily awaiting the transforming violence of a fresh onset.
+Sometimes continents of pale light are separated by narrow straits of
+comparative darkness; elsewhere obscure spaces are hemmed in by luminous
+inlets and channels.'</p>
+
+<p>One curious point about the Orion Nebula is that the star which seems to
+be in the midst of it resolves itself under the telescope into not one
+but six, of various sizes.</p>
+
+<p>Nebul&aelig; are in most cases too enormously remote from the earth for us to
+have any possible means of<span class='pagenum'><a name="Page_204" id="Page_204">[Pg 204]</a></span> computing the distance; but we may take it
+that light must journey at least a thousand years to reach us from them,
+and in many cases much more. Therefore, if at the time of the Norman
+Conquest a nebula had begun to grow dim and fade away, it would, for all
+intents and purposes, still be there for us, and for those that come
+after us for several generations, though all that existed of it in
+reality would be its pale image fleeting onward through space in all
+directions in ever-widening circles.</p>
+
+<p>That nebul&aelig; do sometimes change we have evidence: there are cases in
+which some have grown indisputably brighter during the years they have
+been under observation, and some nebul&aelig; that have been recorded by
+careful observers seem to have vanished. When we consider that these
+strange bodies fill many, many times the area of our whole solar system
+to the outermost bounds of Neptune's orbit, it is difficult to imagine
+what force it is that acts on them to revive or quench their light. That
+that light is not the direct result of heat has long been known; it is
+probably some form of electric excitement causing luminosity, very much
+as it is caused in the comets. Indeed, many people have been tempted to
+think of the nebul&aelig;<span class='pagenum'><a name="Page_205" id="Page_205">[Pg 205]</a></span> as the comets of the universe, and in some points
+there are, no doubt, strong resemblances between the two. Both shine in
+the same way, both are so faint and thin that stars can be seen through
+them; but the spectroscope shows us that to carry the idea too far would
+be wrong, as there are many differences in constitution.</p>
+
+<p>We have seen that there are dark stars as well as light stars; if so,
+may there not be dark nebul&aelig; as well as light ones? It may very well be
+so. We have seen that there are reasons for supposing our own system to
+have been at first a cool dark nebula rotating slowly. The heavens may
+be full of such bodies, but we could not discern them. Their thinness
+would prevent their hiding any stars that happened to be behind them. No
+evidence of their existence could possibly be brought to us by any
+channel that we know.</p>
+
+<p>It is true that, besides the dark rifts in the bright nebul&aelig;, which may
+themselves be caused by a darker and non-luminous gas, there are also
+strange rifts in the Milky Way, which at one time were conjectured to be
+due to a dark body intervening between us and the starry background.
+This idea is now quite discarded; whatever may cause them, it is not
+that. One of the most startling<span class='pagenum'><a name="Page_206" id="Page_206">[Pg 206]</a></span> of these rifts is that called the
+Coal-Sack, in the Southern Hemisphere, and it occurs in a part of the
+sky otherwise so bright that it is the more noticeable. No possible
+explanation has yet been suggested to account for it.</p>
+
+<p>Thus it may be seen that, though much has been discovered, much remains
+to be discovered. By the patient work of generations of astronomers we
+have gained a clear idea of our own position in the universe. Here are
+we on a small globe, swinging round a far mightier and a self-luminous
+globe, in company with seven other planets, many of which, including
+ourselves, are attended by satellites or moons. Between the orbits of
+these planets is a ring or zone of tiny bodies, also going round the
+sun. Into this system flash every now and then strange luminous
+bodies&mdash;some coming but once, never to return; others returning again
+and again.</p>
+
+<p>Far out in space lies this island of a system, and beyond the gulfs of
+space are other suns, with other systems: some may be akin to ours and
+some quite different. Strewn about at infinite distances are star
+clusters, nebul&aelig;, and other mysterious objects.</p>
+
+<p>The whole implies design, creation, and the<span class='pagenum'><a name="Page_207" id="Page_207">[Pg 207]</a></span> working of a mighty
+intelligence; and yet there are small, weak creatures here on this
+little globe who refuse to believe in a God, or who, while acknowledging
+Him, would believe themselves to know better than He.</p>
+
+<h4>THE END</h4>
+
+<p class="center">BILLING AND SONS, LTD., PRINTERS, GUILDFORD</p>
+
+
+
+
+
+
+
+
+<pre>
+
+
+
+
+
+End of Project Gutenberg's The Children's Book of Stars, by G.E. Mitton
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+The Project Gutenberg EBook of The Children's Book of Stars, by G.E. Mitton
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
+
+
+Title: The Children's Book of Stars
+
+Author: G.E. Mitton
+
+Release Date: May 17, 2009 [EBook #28853]
+
+Language: English
+
+Character set encoding: ASCII
+
+*** START OF THIS PROJECT GUTENBERG EBOOK THE CHILDREN'S BOOK OF STARS ***
+
+
+
+
+Produced by Siobhan Hillman, Brenda Lewis, Janet Blenkinship
+and the Online Distributed Proofreading Team at
+http://www.pgdp.net
+
+
+
+
+
+
+
+
+
+  THE CHILDREN'S BOOK OF STARS
+
+  THE CHILDREN'S BOOK OF STARS
+
+  MITTON
+
+  A.&C. BLACK
+
+  [Illustration: THE MOON-CHILD MUST KEEP ON RUNNING ROUND HER. P. 11.]
+
+
+  THE CHILDREN'S BOOK OF STARS
+
+  +-------------------------------------------------+
+  |  BY THE SAME AUTHOR                             |
+  |                                                 |
+  |  CHILDREN'S BOOK OF LONDON                      |
+  |                                                 |
+  |  CONTAINING 12 FULL-PAGE ILLUSTRATIONS          |
+  |  IN COLOUR BY JOHN WILLIAMSON                   |
+  |  PRICE =6s.=                                    |
+  |                                                 |
+  |  'The stories are told in a way that is bound   |
+  |  to rivet attention, and the historical sketches|
+  |  will leave a lasting impression on the minds   |
+  |  of young readers which will be very useful     |
+  |  when their studies in history become more      |
+  |  advanced.'--_Scotsman._                        |
+  |                                                 |
+  |                                                 |
+  |  ANIMAL AUTOBIOGRAPHIES                         |
+  |                                                 |
+  |  THE DOG                                        |
+  |                                                 |
+  |  WITH 12 FULL-PAGE ILLUSTRATIONS IN             |
+  |  COLOUR BY J. WILLIAMSON                        |
+  |                                                 |
+  |  PRICE =6s.=                                    |
+  |                                                 |
+  |  'A true life history, written "out of the      |
+  |  fulness of first-hand knowledge" by an author  |
+  |  who is thoroughly acquainted with all the      |
+  |  ways of "the friend of man."'--_Glasgow        |
+  |  Herald._                                       |
+  |                                                 |
+  |  'The story is admirably told in clear and      |
+  |  fascinating language.'--_Freeman's Journal._   |
+  |                                                 |
+  |  A. & C. BLACK. SOHO SQUARE. LONDON, W.         |
+  |                                                 |
+  +-------------------------------------------------+
+
+  AGENTS
+
+  AMERICA  THE MACMILLAN COMPANY
+           64 & 66 FIFTH AVENUE, NEW YORK
+
+  CANADA   THE MACMILLAN COMPANY OF CANADA, LTD.
+           27 RICHMOND STREET WEST, TORONTO
+
+  INDIA   MACMILLAN & COMPANY, LTD.
+          MACMILLAN BUILDING, BOMBAY
+          309 BOW BAZAAR STREET, CALCUTTA
+
+
+  THE
+  CHILDREN'S BOOK
+  OF
+  STARS
+
+  BY
+
+  G. E. MITTON
+
+  AUTHOR OF
+  'THE CHILDREN'S BOOK OF LONDON,' 'ANIMAL AUTOBIOGRAPHIES:
+  THE DOG,' ETC.
+
+  LONDON
+  ADAM AND CHARLES BLACK
+  1907
+
+ _Published September, 1907_
+
+
+
+
+PREFACE
+
+
+It was the intention of the late Agnes Clerke to write the preface to
+this 'Children's Book of Stars.' Miss Clerke took a warm and sympathetic
+interest in the authoress and her work, but her lamented death occurred
+before this kindly intention could be fulfilled.
+
+I cannot pretend to write adequately as her substitute, but I could not
+resist the appeal made to me by the author, in the name and for the sake
+of her dear friend and mine, to write a few words of introduction.
+
+I am in no way responsible either for the plan or for any portion of
+this work, but I can commend it as a book, written in a simple and
+pleasant style, calculated to awaken the interest of intelligent
+children, and to enable parents otherwise ignorant or astronomy to
+answer many of those puzzling questions which such children often put.
+
+  DAVID GILL.
+
+
+
+
+AUTHOR'S NOTE
+
+
+This little work is the outcome of many suggestions on the part of
+friends who were anxious to teach their small children something of the
+marvels of the heavens, but found it exceedingly difficult to get hold
+of a book wherein the intense fascination of the subject was not lost in
+conventional phraseology--a book in which the stupendous facts were
+stated in language simple enough to be read aloud to a child without
+paraphrase.
+
+Whatever merit there may be in the present work is due entirely to my
+friend Agnes Clerke, the well-known writer on astronomy; the faults are
+all my own. She gave me the impetus to begin by her warm encouragement,
+and she helped me to continue by hearing every chapter read as it was
+written, and by discussing its successor and making suggestions for it.
+Thus she heard the whole book in MS. A week after the last chapter had
+been read to her I started on a journey lasting many months, and while
+I was in the Far East the news reached me of her death, by which the
+world is the poorer. For her sake, as he has stated, her friend Sir
+David Gill, K.C.B., kindly undertook to supply the missing preface.
+
+  G. E. MITTON.
+
+
+
+
+CONTENTS
+
+
+  CHAPTER I                                                         PAGE
+
+  THE EARTH                                                            1
+
+  CHAPTER II
+
+  HANGING IN SPACE                                                    13
+
+  CHAPTER III
+
+  THE SHINING MOON                                                    21
+
+  CHAPTER IV
+
+  THE EARTH'S BROTHERS AND SISTER                                     32
+
+  CHAPTER V
+
+  FOUR SMALL WORLDS                                                   48
+
+  CHAPTER VI
+
+  FOUR LARGE WORLDS                                                   67
+
+  CHAPTER VII
+
+  THE SUN                                                             89
+
+  CHAPTER VIII
+
+  SHINING VISITORS                                                   103
+
+  CHAPTER IX
+
+  SHOOTING STARS AND FIERY BALLS                                     120
+
+  CHAPTER X
+
+  THE GLITTERING HEAVENS                                             135
+
+  CHAPTER XI
+
+  THE CONSTELLATIONS                                                 148
+
+  CHAPTER XII
+
+  WHAT THE STARS ARE MADE OF                                         159
+
+  CHAPTER XIII
+
+  RESTLESS STARS                                                     170
+
+  CHAPTER XIV
+
+  THE COLOURS OF THE STARS                                           176
+
+  CHAPTER XV
+
+  TEMPORARY AND VARIABLE STARS                                       188
+
+  CHAPTER XVI
+
+  STAR CLUSTERS AND NEBULAE                                           197
+
+
+
+
+ILLUSTRATIONS
+
+PRINTED IN COLOUR
+
+  THE MOON-CHILD MUST KEEP ON  RUNNING ROUND HER           _Frontispiece_
+
+                                                             FACING PAGE
+
+  THE EARTH AND MOON HANGING IN SPACE                                 16
+
+  THE ENGLISH SUMMER AND WINTER                                       40
+
+  JUPITER AND ONE OF HIS MOONS                                        70
+
+  THE PLANET SATURN AND TWO OF HIS MOONS                              78
+
+  FLAMES FROM THE SUN                                                100
+
+  THE COMET IN THE BAYEUX TAPESTRY                                   104
+
+  A STICK THRUST INTO THE WATER APPEARS CROOKED                      114
+
+  CONSTELLATIONS NEAR THE POLE STAR                                  150
+
+  ORION AND HIS NEIGHBOURS                                           154
+
+  THE SPECTRUM OF THE SUN AND SIRIUS                                 168
+
+
+
+
+ILLUSTRATIONS
+
+IN BLACK AND WHITE
+
+
+                                                                    PAGE
+
+  THE MOON                                                   _facing_ 24
+
+  AN ECLIPSE OF THE MOON                                              28
+
+  AN ECLIPSE OF THE SUN                                               29
+
+  THE MOON RAISING THE TIDES                                          30
+
+  COMPARATIVE SIZES OF THE PLANETS                                    35
+
+  DIFFERENT PHASES OF VENUS                                           51
+
+  ORBITS OF MARS, THE EARTH, VENUS, AND MERCURY                       55
+
+  MAP OF MARS                                                _facing_ 56
+
+  ORBITS OF THE EARTH AND MARS                                        63
+
+  JUPITER AND HIS PRINCIPAL MOONS                                     72
+
+  SUN-SPOTS                                                  _facing_ 98
+
+  A GREAT COMET                                                 "    118
+
+  THE GREAT NEBULA IN ANDROMEDA                                 "    202
+
+
+THE CHILDREN'S BOOK OF STARS
+
+
+
+
+CHAPTER I
+
+THE EARTH
+
+
+It is a curious fact that when we are used to things, we often do not
+notice them, and things which we do every day cease to attract our
+attention. We find an instance of this in the curious change that comes
+over objects the further they are removed from us. They grow smaller and
+smaller, so that at a distance a grown-up person looks no larger than a
+doll; and a short stick planted in the ground only a few feet away
+appears as long as a much longer one at ten times the distance. This
+process is going on all round us every minute: houses, trees, buildings,
+animals, all seem larger or smaller in proportion to their distance from
+us. Sometimes I have seen a row of raindrops hanging on a bar by the
+window. When the sun catches one of them, it shines so brilliantly that
+it is as dazzling as a star; but my sense tells me it is a raindrop, and
+not a star at all. It is only because it is so near it seems as bright
+and important as a mighty star very, very far away.
+
+We are so much accustomed to this fact that we get into a habit of
+judging the distance of things by their size. If we see two lights
+shining on a dark night, and one is much larger than the other, we think
+that the bright one must be nearer to us; yet it need not necessarily be
+so, for the two lights might possibly be at the same distance from us,
+and one be large and the other small. There is no way in which we can
+tell the truth by just looking at them. Now, if we go out on any fine
+moonlight night and look up at the sky, we shall see one object there
+apparently much larger than any other, and that is the moon, so the
+question that occurs to us at once is, Is the moon really very much
+larger than any of the stars, or does it only seem so because it is very
+much nearer to us? As a matter of fact, the moon is one of the smallest
+objects in view, only, as it is our nearest neighbour, it appears very
+conspicuous. Having learned this, we shall probably look about to see
+what else there is to attract attention, and we may notice one star
+shining very brilliantly, almost like a little lamp, rather low down in
+the sky, in that part of it where the sun has lately set. It is so
+beautifully bright that it makes all the others look insignificant in
+comparison, yet it is not really large compared with the others, only,
+as it comes nearer to us than anything else in the sky except the moon,
+it looks larger than it has any right to do in comparison with the
+others.
+
+After this we might jump to the conclusion that all the bright large
+stars are really small and near to us, and all the faintly shining ones
+large and far away. But that would not be true at all, for some bright
+ones are very far away and some faint ones comparatively near, so that
+all we can do is to learn about them from the people who have studied
+them and found out about them, and then we shall know of our own
+knowledge which of them seem bright only because they are nearer than
+the others, and which are really very, very brilliant, and so still
+shine brightly, though set in space at an almost infinite distance from
+us.
+
+The sun, as we all know, appears to cross the sky every day; he gets up
+in the east and drops down in the west, and the moon does the same,
+only the moon is unlike the sun in this, that it changes its shape
+continually. We see a crescent moon growing every night larger and
+larger, until it becomes full and fat and round, and then it grows
+thinner and thinner, until it dies away; and after a little while it
+begins again, and goes through all the same changes once more. I will
+tell you why this is so further on, when we have a chapter all about the
+moon.
+
+If you watch the stars quietly for at least five minutes, you will see
+that they too are moving steadily on in the same way as the sun and
+moon. Watch one bright star coming out from behind a chimney-pot, and
+after about five minutes you will see that it has changed its place. Yet
+this is not true of all, for if we watch carefully we shall find that
+some, fairly high up in the sky, do not appear to move at all. The few
+which are moving so slowly that they seem to us to stand still are at a
+part of the sky close to the Pole Star, so called because it is always
+above the North Pole of the earth. I will explain to you how to find it
+in the sky for yourselves later on, but now you can ask anyone to point
+it out. Watch it. It appears to be fixed in one place, while the other
+stars are swinging round it in circles. In fact, it is as if we on the
+earth were inside a great hollow globe or ball, which continually turned
+round, with the Pole Star near the top of the globe; and you know that
+if you put your finger on the spot at the top of a spinning globe or
+ball, you can hold it there while all the rest of the ball runs round.
+Now, if you had to explain things to yourself, you would naturally
+think: 'Here is the great solid earth standing still, and the sun and
+moon go round it; the stars are all turning round it too, just as if
+they were fixed on to the inside of a hollow globe; we on the earth are
+in the middle looking up at them; and this great globe is slowly
+wheeling round us night by night.'
+
+In the childhood of the world men believed that this was really
+true--that the earth was the centre of the universe, that the sun and
+moon and all the hosts of heaven were there solely to light and benefit
+us; but as the world grew wiser the wonders of creation were fathomed
+little by little. Some men devoted their whole lives to watching the
+heavens, and the real state of things was gradually revealed to them.
+The first great discovery was that of the daily movement of the earth,
+its rotation on its own axis, which makes it appear as if all these
+shining things went round it. It is indeed a very difficult matter to
+judge which of two objects is moving unless we can compare them both
+with something outside. You must have noticed this when you are sitting
+in a train at a station, and there is another train on the other side of
+yours. For if one of the trains moves gently, either yours or the other,
+you cannot tell which one it is unless you look at the station platform;
+and if your position remains the same in regard to that, you know that
+your train is still standing, while the other one beside it has begun to
+move. And I am quite sure that there is no one of us who has not, at one
+time or another, stood on a bridge and watched the water running away
+underneath until we felt quite dizzy, and it seemed as if the water were
+standing still and the bridge, with ourselves on it, was flying swiftly
+away backwards. It is only when we turn to the banks and find them
+standing still, that we realize the bridge is not moving, and that it is
+the running water that makes it seem to do so. These everyday instances
+show us how difficult it is to judge whether we are moving or an outside
+object unless we have something else to compare with it. And the
+marvellous truth is that, instead of the sun and moon and stars rolling
+round the earth, it is the earth that is spinning round day by day,
+while the sun and the stars are comparatively still; and, though the
+moon does move, yet when we see her get up in the east and go down in
+the west that is due to our own movement and not to hers.
+
+The earth turns completely round once in a day and night. If you take an
+orange and stick a knitting-needle through it, and hold it so that the
+needle is not quite straight up but a little slanting, and then twirl it
+round, you will get quite a good idea of the earth, though of course
+there is no great pole like a gigantic needle stuck through it, that is
+only to make it easy for you to hold it by. In spinning the orange you
+are turning it as the earth turns day by day, or, as astronomers express
+it, as it rotates on its axis.
+
+There is a story of a cruel Eastern King who told a prisoner that he
+must die if he did not answer three questions correctly, and the
+questions were very difficult; this is one of them:
+
+'How long would it take a man to go round the earth if he never stopped
+to eat or drink on the way?'
+
+And the prisoner answered promptly: 'If he rose with the sun and kept
+pace with it all day, and never stopped for a moment to eat or drink, he
+would take just twenty-four hours, Your Royal Highness.' For in those
+days it was supposed that the sun went round the earth.
+
+Everyone is so remarkably clever nowadays that I am sure there will be
+someone clever enough to object that, if what I have said is true, there
+would be a great draught, for the air would be rushing past us. But, as
+a matter of fact, the air goes with us too. If you are inside a railway
+carriage with the windows shut you do not feel the rush of air, because
+the air in the carriage travels with you; and it is the same thing on
+the earth. The air which surrounds the earth clings to it and goes round
+with it, so there is no continuous breeze from this cause.
+
+But the spinning round on its own axis is not the earth's only movement,
+for all the time it is also moving on round the sun, and once in a whole
+year it completes its journey and comes back to the place from whence it
+started. Thus the turning round like a top or rotating on its axis makes
+the day and night, and the going in a great ring or revolving round the
+sun makes the years.
+
+Our time is divided into other sections besides days and years. We have,
+for instance, weeks and months. The weeks have nothing to do with the
+earth's movements; they are only made by man to break up the months;
+but the months are really decided by something over which we have no
+control. They are due to the moon, and, as I have said already, the moon
+must have a chapter to herself, so we won't say any more about the
+months here.
+
+If any friend of ours goes to India or New Zealand or America, we look
+upon him as a great traveller; yet every baby who has lived one year on
+the earth has travelled millions of miles without the slightest effort.
+Every day of our lives we are all flung through space without knowing it
+or thinking of it. It is as if we were all shut up in a comfortable
+travelling car, and were provided with so many books and pictures and
+companions that we never cared to look out of the windows, so that hour
+by hour as we were carried along over miles of space we never gave them
+a thought. Even the most wonderful car ever made by man rumbles and
+creaks and shakes, so that we cannot help knowing it is moving; but this
+beautiful travelling carriage of ours called the earth makes never a
+creak or groan as she spins in her age-long journey. It is always
+astonishing to me that so few people care to look out of the window as
+we fly along; most of them are far too much absorbed in their little
+petty daily concerns ever to lift their eyes from them. It is true that
+sometimes the blinds are down, for the sky is thickly covered with
+clouds, and we cannot see anything even if we want to. It is true also
+that we cannot see much of the scenery in the daytime, for the sun
+shining on the air makes a veil of blue glory, which hides the stars;
+but on clear nights we can see on every side numbers of stars quite as
+interesting and beautiful as any landscape; and yet millions of people
+never look up, never give a thought to the wonderful scenery through
+which their car is rushing.
+
+By reason of the onward rush of the earth in space we are carried over a
+distance of at least eighteen miles every second. Think of it: as we
+draw a breath we are eighteen miles away in space from the point we were
+at before, and this goes on unceasingly day and night. These astonishing
+facts make us feel how small and feeble we are, but we can take comfort
+in the thought that though our bodies are insignificant, the brain of
+man, which has discovered these startling facts, must in itself be
+regarded as one of the most marvellous of all the mysteries amid which
+we live.
+
+Well, we have arrived at some idea of our earth's position; we know
+that the earth is turning round day by day, and progressing round the
+sun year by year, and that all around lie the sentinel stars, scattered
+on a background of infinite space. If you take an older boy or girl and
+let him or her stand in the middle to represent the sun, then a smaller
+one would be the earth, and the smallest of all the moon; only in truth
+we could never get anyone large enough to represent the sun fairly, for
+the biggest giant that ever lived would be much too small in proportion.
+The one representing the sun must stand in the middle, and turn slowly
+round and round. Then let the earth-child turn too, and all the time she
+is spinning like a top she must be also hastening on in a big ring round
+the sun; but she must not go too fast, for the little moon-child must
+keep on running round her all the time. And the moon-child must keep her
+face turned always to the earth, so that the earth never sees her back.
+That is an odd thing, isn't it? We have never seen the other side of the
+moon, which goes round us, always presenting the same face to us.
+
+The earth is not the only world going round the sun; she has many
+brothers and a sister; some are nearer to the sun than she is, and some
+are further away, but all circle round the great central light-giver in
+rings lying one outside the other. These worlds are called planets, and
+the earth is one of them, and one of the smaller ones, too, nothing so
+great and important as we might have imagined.
+
+
+
+
+CHAPTER II
+
+HANGING IN SPACE
+
+
+If you are holding something in your hand and you let it go, what
+happens? It falls to the ground, of course. Now, why should it do so?
+You will say: 'How could it do anything else?' But that is only because
+you are hampered by custom. Try to shake yourself free, and think, Why
+should it go down instead of up or any other way? The first man who was
+clever enough to find some sort of an answer to this question was the
+great philosopher Sir Isaac Newton, though he was not quite the first to
+be puzzled by it. After years of study he discovered that every thing
+attracts every other thing in proportion to their masses (which is what
+you know as weight) and their distance from each other. In more
+scientific language, we should say every _body_ instead of every
+_thing_, for the word body does not only mean a living body, but every
+lump or mass of matter in the universe. The earth is a body in this
+sense, and so is the table or anything else you could name. Now as the
+earth is immeasurably heavier than anything that is on it, it pulls
+everything toward itself with such force that the little pulls of other
+things upon each other are not noticed. The earth draws us all toward
+it. It is holding us down to it every minute of the day. If we want to
+move we have to exert another force in order to overcome this attraction
+of the earth, so we exert our own muscles and lift first one foot and
+then the other away from the earth, and the effort we make in doing this
+tires us. All the while you are walking or running you are exercising
+force to lift your feet away from the ground. The pull of the earth is
+called gravitation. Just remember that, while we go on to something else
+which is almost as astonishing.
+
+We know that nothing here on earth continues to move for ever;
+everything has to be kept going. Anything left to itself has a tendency
+to stop. Why is this? This is because here in the world there is
+something that fights against the moving thing and tries to stop it,
+whether it be sent along the ground or thrown up in the air. You know
+what friction is, of course. If you rub your hands along any rough
+substance you will quickly feel it, but on a smooth substance you feel
+it less. That is why if you send a stone spinning along a carpet or a
+rough road it stops comparatively soon, whereas if you use the same
+amount of force and send it along a sheet of ice it goes on moving much
+longer. This kind of resistance, which we call friction, is one of the
+causes which is at work to bring things to a standstill; and another
+cause is the resistance of the air, which is friction in another form.
+It may be a perfectly still day, yet if you are bicycling you are
+breaking through the air all the time, just as you would be through
+water in swimming, only the resistance of the air is less than that of
+water. As the friction or the resistance of the air, or both combined,
+gradually lessens the pace of the stone you sent off with such force,
+the gravitation of the earth begins to be felt. When the stone first
+started the force you gave to it was enough to overcome the gravitation
+force, but as the stone moves more slowly the earth-pull asserts itself,
+and the stone drops down to the ground and lies still upon the surface.
+Now, if there were no friction, and therefore no resistance, there would
+be no reason why anything once set moving should not go on moving for
+ever. The force you give to any object you throw is enough to overcome
+gravitation; and it is only when the first force has been diminished by
+friction that the earth asserts its authority and pulls the moving
+object toward it. If it were possible to get outside the air and out of
+reach of the pull of the earth, we might fling a ball off into space,
+and it would go on in a straight line until something pulled it to
+itself by the force of gravity.
+
+Gravitation affects everything connected with the earth; even our air is
+held to the earth by gravitation. It grows thinner and thinner as we get
+further away from the earth. At the top of a high mountain the air is so
+thin that men have difficulty in breathing, and at a certain height they
+could not breathe at all. As they cannot breathe in very fine air, it is
+impossible for them to tell by personal experiment exactly where the air
+ends; but they have tried to find out in other ways, and though
+different men have come to different conclusions on the subject, it is
+safe to say that at about two hundred miles above the earth there is
+nothing that could be called air. Thus we can now picture our spinning
+earth clothed in a garment of air that clings closely about her, and
+grows thinner and thinner until it melts away altogether, for there is
+no air in space.
+
+[Illustration: THE EARTH AND MOON HANGING IN SPACE]
+
+Now in the beginning God made the world, and set it off by a first
+impulse. We know nothing about the details, though further on you shall
+hear what is generally supposed to have taken place; we only know that,
+at some remote age, this world, probably very different from what it is
+now, together with the other planets, was sent spinning off into space
+on its age-long journey. These planets were not sent off at random, but
+must have had some particular connection with each other and with the
+sun, for they all belong to one system or family, and act and react on
+each other. Now, if they had been at rest and not in movement, they
+would have fallen right into the sun, drawn by the force of gravitation;
+then they would have been burned up, and there would have been an end of
+them. But the first force had imparted to them the impulse to go on in a
+straight line, so when the sun pulled the result was a movement between
+the two: the planets did not continue to move in a straight line,
+neither did they fall on to the sun, but they went on a course between
+the two--that is, a circle--for the sun never let them get right away
+from him, but compelled them to move in circles round him. There is a
+very common instance of this kind of thing which we can see, or perhaps
+feel, every day. If you try to sit still on a bicycle you tumble off,
+because the earth pulls you down to itself; but if, by using the force
+of your own muscles, you give the bicycle a forward movement this
+resists the earth-pull, and the result is the bicycle runs along the
+ground. It does not get right away from the earth, not even two or three
+feet above ground; it is held to the earth, but still it goes forward
+and does not fall over, for the movement is made up of the earth-pull,
+which holds it to the ground, and the forward movement, which propels it
+along. Then again, as another instance, if you tie a ball to a string
+and whirl it round you, so long as you keep on whirling it will not fall
+to the ground, but the moment you stop down it drops, for there is
+nothing to fight against the pull of gravitation. Thus we can picture
+the earth and all the planets as if they were swinging round the sun,
+held by invisible strings. It is the combination of two forces that
+keeps them in their places--the first force and the sun's pull. It is
+very wonderful to think of. Here we are swinging in space on a ball that
+seems only large to us because we are so much smaller ourselves; there
+is nothing above or below it but space, yet it travels on day by day and
+year by year, held by invisible forces that the brain of man has
+discovered and measured.
+
+Of course, every planet gives a pull at every other planet too, but
+these pulls are so small compared with that of the sun that we need not
+at present notice them. Then we come to another point. We said that
+every body pulled every other body in proportion to their weights and
+their distance. Now, gravity acts much more strongly when things are
+near together than when they are far away from each other; so that if a
+smaller body is near to another somewhat larger than itself, it is
+pulled by it much more strongly than by a very much larger one at a
+considerably greater distance. We have an instance of this in the case
+of the earth and moon: as the earth responds to the pull of the sun, so
+the moon responds to the pull of the earth. The moon is so comparatively
+near to the earth that the earth-pull forces her to keep on going round
+and round, instead of leaving her free to circle round the sun by
+herself; and yet if you think of it the moon does go round the sun too.
+Recall that game we had when the sun was in the middle, and the two
+smaller girls, representing the earth and moon, went round it. The
+moon-child turned round the earth-child, but all the while the
+earth-child was going round the sun, so that in a year's time the moon
+had been all round the sun too, only not in a straight line. The moon is
+something like a dog who keeps on dancing round and round you when you
+go for a walk. He does go for the walk too, but he does much more than
+that in the same time. Thus we have further completed our idea of our
+world. We see it now hanging in space, with no visible support, held in
+its place by two mighty forces; spinning on year after year, attended by
+its satellite the moon, while we run, and walk, and cry, and laugh, and
+play about on its surface--little atoms who, except for the brain that
+God has given them, would never even have known that they are
+continually moving on through endless space.
+
+
+
+
+CHAPTER III
+
+THE SHINING MOON
+
+
+'Once upon a time,' long, long ago, the earth was not a compact, round,
+hard body such as she is now, but much larger and softer, and as she
+rotated a fragment broke off from her; it did not go right away from
+her, but still went on circling round with the motion it had inherited
+from her. As the ages passed on both the earth and this fragment, which
+had been very hot, cooled down, and in cooling became smaller, so that
+the distance between them was greater than it had been before they
+shrank. And there were other causes also that tended to thrust the two
+further from each other. Yet, compared with the other heavenly bodies,
+they are still near, and by looking up into the sky at night you can
+generally see this mighty fragment, which is a quarter the diameter of
+the earth--that is to say, a quarter the width of the earth measured
+from side to side through the middle. It is--as, of course, you have
+guessed--the moon. The moon is the nearest body to us in all space, and
+so vast is the distance that separates us from the stars that we speak
+as if she were not very far off, yet compared with the size of the earth
+the space lying between us and her is very great. If you went right
+round the world at the thickest part--that is to say, in the region of
+the Equator--and when you arrived at your starting-point went off once
+again, and so on until you had been round ten times, you would only then
+have travelled about as far as from the earth to the moon!
+
+The earth is not the only planet which has a moon, or as it is called, a
+satellite, in attendance. Some of the larger planets have several, but
+there is not one to compare with our moon. Which would you prefer if you
+had the choice, three or four small moons, some of them not much larger
+than a very big bright star, or an interesting large body like our own
+moon? I know which I should say.
+
+'You say that the moon broke off from the earth, so perhaps there may be
+some people living on her,' I hear someone exclaim.
+
+If there is one thing we have found out certainly about the moon, it is
+that no life, as we know it, could exist there, for there is neither air
+nor water. Whether she ever had any air or water, and if so, why they
+disappeared, are questions we cannot answer. We only know that now she
+is a dead world. Bright and beautiful as she is, shedding on us a pale,
+pure light, in vivid contrast with the fiery yellow rays of the sun, yet
+she is dead and lifeless and still. We can examine her surface with the
+telescope, and see it all very plainly. Even with a large opera-glass
+those markings which, to the naked eye, seem to be like a queer
+distorted face are changed, and show up as the shadows of great
+mountains. We can only see one side of the moon, because as I have said,
+she keeps always the same face turned to the earth; but as she sways
+slightly in her orbit, we catch a glimpse of sometimes a little more on
+one side and sometimes a little more on the other, and so we can judge
+that the unseen part is very much the same as that turned toward us.
+
+At first it is difficult to realize what it means to have no air.
+Besides supporting life in every breath that is drawn by living
+creatures, the air does numerous other kind offices for us--for
+instance, it carries sound. Supposing the most terrific volcano exploded
+in an airless world, it could not be heard. The air serves as a screen
+by day to keep off the burning heat of the sun's rays, and as a blanket
+by night to keep in the heat and not let it escape too quickly. If there
+were no air there could be no water, for all water would evaporate and
+vanish at once. Imagine the world deprived of air; then the sun's rays
+would fall with such fierceness that even the strongest tropical sun we
+know would be as nothing in comparison with it, and every green thing
+would shrivel up and die; this scorching sun would shine out of a black
+sky in which the stars would all be visible in the daytime, not hidden
+by the soft blue veil of air, as they are now. At night the instant the
+sun disappeared below the horizon black darkness would set in, for our
+lingering twilight is due to the reflection of the sun in the upper
+layers of air, and a bitterness of deathly cold would fall upon the
+earth--cold fiercer than that of the Arctic regions--and everything
+would be frozen solid. It would need but a short time to reduce the
+earth to the condition of the moon, where there is nothing to shrivel
+up, nothing to freeze. Her surface is made up of barren, arid rocks, and
+her scenery consists of icy black shadows and scorching white plains.
+
+[Illustration: _Paris Observatory._
+
+THE MOON.]
+
+The black shadows define the mountains, and tremendous mountains they
+are. Most of them have craters. A crater is like a cup, and generally
+has a little peak in the middle of it. This is the summit of a volcano,
+and when the volcano has burst up and vomited out floods of lava and
+debris, this has fallen down in a ring a little distance away from it,
+leaving a clear space next to the peak, so that, as the mountain ceases
+vomiting and the lava cools down, the ring hardens and forms a circular
+ridge. The craters on the moon are immense, not only in proportion to
+her size, but immense even according to our ideas on the earth. One of
+the largest craters in our own world is in Japan, and this measures
+seven miles across, while in the moon craters of fifty, sixty, and even
+a hundred miles are by no means uncommon, though there are also hundreds
+and thousands of smaller ones. We can see the surface of the moon very
+plainly with the magnificent telescopes that have now been made, and
+with the best of these anything the size of a large town would be
+plainly visible. Needless to say, no town ever has been or ever will be
+seen upon the moon!
+
+All these mountains and craters show that at one time the moon must have
+been convulsed with terrific disturbances, far worse than anything that
+we have any knowledge of on our earth; but this must have been ages
+ago, while the moon still probably had an atmosphere of its own. Now it
+has long been quiet. Nothing changes there; even the forces that are
+always at work on the earth--namely, damp and mould and water--altering
+the surface and breaking up the rocks, do not act there, where there is
+no moisture of any sort. So far as we can see, the purpose of the moon
+is to be the servant of the earth, to give her light by night and to
+raise the tides. Beautiful light it is, soft and mysterious--light that
+children do not often have a chance of seeing, for they are generally in
+bed before the moon rises when she is at the full.
+
+We know that the moon has no heat of her own--she parted with all that
+long ago; she cannot give us glowing light from brilliant flames, as the
+sun does; she shines only by the reflection of the sun on her surface,
+and this is the reason why she appears to change her shape so
+constantly. She does not really change; the whole round moon is always
+there, only part of it is in shadow. Sometimes you can see the dark part
+as well as the bright. When there is a crescent moon it looks as if it
+were encircling the rest; some people call it, 'seeing the old moon in
+the new moon's arms.' I don't know if you would guess why it is we can
+see the dark part then, or how it is lighted up. It is by reason of our
+own shining, for we give light to the moon, as she does to us. The sun's
+rays strike on the earth, and are reflected on to the moon, so that the
+moon is lighted by earthshine as we are lighted by moonshine, and it is
+these reflected earth-rays that light up the dark part of the moon and
+enable us to see it. What a journey these rays have had! They travel
+from the sun to the earth, and the earth to the moon, and then back to
+the earth again! From the moon the earth must appear a much bigger and
+more glorious spectacle than she does to us--four times wider across and
+probably brighter--for the sun's light strikes often on our clouds,
+which shine more brilliantly than her surface.
+
+Once again we must use an illustration to explain the subject. Set a
+lamp in the middle of a dark room, and let that be the sun, then take a
+small ball to represent the earth and a smaller one for the moon. Place
+the moon-ball between the lamp and the earth-ball. You will see that the
+side turned to the earth-ball is dark, but if you move the moon to one
+side of the earth, then from the earth half of it appears light and half
+dark; if you put it right away from the lamp, on the outer side of the
+earth, it is all gloriously lit up, unless it happens to be exactly
+behind the earth, when the earth's shadow will darken it. This is the
+full explanation of all the changes of the moon.
+
+[Illustration: AN ECLIPSE OF THE MOON.]
+
+Does it ever fall within the earth's shadow? Yes, it does; for as it
+passes round the earth it is not always at the same level, but sometimes
+a little higher and sometimes a little lower, and when it chances to
+pass exactly behind it enters the shadow and disappears. That is what we
+call an eclipse of the moon. It is nothing more than the earth's shadow
+thrown on to the moon, and as the shadow is round that is one of the
+proofs that the earth is round too. But there is another kind of
+eclipse--the eclipse of the sun; and this is caused by the moon herself.
+For when she is nearest to the sun, at new moon--that is to say, when
+her dark side is toward us, and she happens to get exactly between us
+and the sun--she shuts out the face of the sun from us; for though she
+is tiny compared with him, she is so much nearer to us that she appears
+almost the same size, and can blot him right out. Thus the eclipses of
+both sun and moon are not difficult to understand: that of the moon can
+only happen at full moon, when she is furthest from the sun, and it is
+caused by the earth's shadow falling upon the moon; and that of the sun
+at new moon, when she is nearest to him, and it is caused by the solid
+body of the moon coming between us and the sun.
+
+[Illustration: AN ECLIPSE OF THE SUN.]
+
+Besides giving us light by night, the moon serves other important
+purposes, and the most important of all is the raising of the tides.
+Without the rising of the sea twice in every day and night our coasts
+would become foul and unwholesome, for all the dead fish and rotting
+stuff lying on the beach would poison the air. The sea tides scour our
+coasts day by day with never-ceasing energy, and they send a great
+breath of freshness up our large rivers to delight many people far
+inland. The moon does most of this work, though she is a little helped
+by the sun. The reason of this is that the moon is so near to the earth
+that, though her pull is a comparatively small one, it is very strongly
+felt. She cannot displace the actual surface to any great extent, as it
+is so solid; but when it comes to the water she can and does displace
+that, so that the water rises up in answer to her pull, and as the earth
+turns round the raised-up water lags behind, reaching backward toward
+the moon, and is drawn up on the beach, and makes high tide. But it is
+stopped there, and meantime, by reason of the earth's movement, the moon
+is left far behind, and pulls the water to itself further on, when the
+first high tide relapses and falls down again. At length the moon gets
+round to quite the opposite side of the earth to that where she began,
+and there she makes a high tide too; but as she draws the water to
+herself she draws also the solid earth beneath the water to her in some
+degree, and so pulls it away from the place where the first high tide
+occurred, leaving the water there deeper than before, and so causing a
+secondary high tide.
+
+[Illustration: THE MOON RAISING THE TIDES.]
+
+The sun has some influence on the tides too, and when moon and sun are
+in the same line, as at full and new moon, then the tides are highest,
+and are called spring tides; but when they pull in different directions,
+as when it is half-moon, then the tides are lowest and are called neap
+tides.
+
+
+
+
+CHAPTER IV
+
+THE EARTH'S BROTHERS AND SISTER
+
+
+The earth is not the only world that, poised in space, swings around the
+sun. It is one of a family called the Solar System, which means the
+system controlled and governed by the sun. When we look up at the
+glorious sky, star-studded night by night, it might seem to us that the
+stars move only by reason of the earth's rotation; but when men first
+began to study the heavens attentively--and this is so long ago that the
+record of it is not to be found--they noticed that, while every shining
+object in the sky was apparently moving round us, there were a few which
+also had another movement, a proper motion of their own, like the moon.
+These curious stars, which appeared to wander about among the other
+stars, they called planets, or wanderers. And the reason, which was
+presently discovered, of our being able to see these movements was that
+these planets are very much nearer to us than any of the real stars,
+and in fact form part of our own solar system, while the stars are at
+immeasurable distances away. Of all the objects in the heavens the
+planets are the most intensely interesting to us; for though removed
+from us by millions of miles, the far-reaching telescope brings some of
+them within such range that we can see their surfaces and discover their
+movements in a way quite impossible with the stars. And here, if
+anywhere, might we expect to find traces of other living beings like
+ourselves; for, after all the earth is but a planet, not a very large
+nor a very small one, and in no very striking position compared with the
+other planets; and thus, arguing by what seems common-sense, we say, If
+this one planet has living beings on its surface, may not the other
+planets prove to be homes for living beings also? Counting our own
+earth, there are eight of these worlds in our solar system, and also a
+number of tiny planets, called asteroids; these likewise go round the
+sun, but are very much smaller than any of the first eight, and stand in
+a class by themselves, so that when the planets are mentioned it is
+generally the eight large well-known planets which are referred to.
+
+If we go back for a moment to the illustration of the large lamp
+representing our sun, we shall now be able to fill in the picture with
+much more detail. The orbits of the planets, as their paths round the
+sun are called, lie like great circles one outside another at various
+distances, and do not touch or cut each other. Where do you suppose our
+own place to be? Will it be the nearest to the sun or the furthest away
+from him? As a matter of fact, it is neither, we come third in order
+from the sun, for two smaller planets, one very small and the other
+nearly as large as the earth, circle round and round the sun in orbits
+lying inside ours. Now if we want to place objects around our lamp-sun
+which will represent these planets in size, and to put them in places
+corresponding to their real positions, we should find no room large
+enough to give us the space we ought to have. We must take the lamp out
+into a great open field, where we shall not be limited by walls. Then
+the smallest planet, named Mercury, which lies nearest of all to the
+sun, would have to be represented by a pea comparatively close to the
+sun; Venus, the next, would be a greengage plum, and would be about
+twice as far away; then would come the earth, a slightly larger plum,
+about half as far again as Venus. After this there would be a lesser
+planet, called Mars, like a marble. These are the first four, all
+comparatively small; beyond them there is a vast gap, in which we find
+the asteroids, and after this we come to four larger planets, mighty
+indeed as regards ourselves, for if our earth were a greengage plum,
+the first of these, Jupiter, would have to be the size of a football at
+least, and the next, Saturn, a smaller football, while Uranus and
+Neptune, the two furthest out, would be about the size of the toy
+balloons children play with. The outermost one, Neptune, would be thirty
+times as far from the sun as we are.
+
+[Illustration: COMPARATIVE SIZES OF THE PLANETS.]
+
+This is the solar system, and in it the only thing that shines by its
+own light is the sun; all the rest, the planets and their moons, shine
+only because the rays of light from the sun strike on their surfaces and
+are reflected off again. Our earth shines like that, and from the nearer
+planets must appear as a brilliant star. The little solar system is
+separated by distances beyond the realm of thought from the rest of the
+universe. Vast as are the intervals between ourselves and our planetary
+neighbours, they are as nothing to the space that separates us from the
+nearest of the steady shining fixed stars. Why, removed as far from us
+as the stars, the sun himself would have sunk to a point of light; and
+as for the planets, the largest of them, Jupiter, could not possibly be
+seen. Thus, when we look at those stars across the great gulf of space,
+we know that though we see them they cannot see us, and that to them our
+sun must seem only a star; consequently we argue that perhaps these
+stars themselves are suns with families of planets attached to them; and
+though there are reasons for thinking that this is not the case with
+all, it may be with some. Now if, after learning this, we look again at
+the sky, we do so with very different eyes, for we realize that some of
+these shining bodies are like ourselves in many things, and are shining
+only with a light borrowed from the sun, while others are mighty glowing
+suns themselves, shining by their own light, some greater and brighter,
+some less than our sun. The next thing to do is to learn which are stars
+and which are planets.
+
+Of the planets you will soon learn to pick out one or two, and will
+recognize them even if they do change their places--for instance, Venus
+is at times very conspicuous, shining as an evening star in the west
+soon after the sun goes down, or us a morning star before he gets up,
+though you are not so likely to see her then; anyway, she is never found
+very far from the sun. Jupiter is the only other planet that compares
+with her in brilliancy, and he shines most beautifully. He is, of
+course, much further away from us than Venus, but so much larger that he
+rivals her in brightness. Saturn can be quite easily seen as a
+conspicuous object, too, if you know where to look for him, and Mars is
+sometimes very bright with a reddish glow. The others you would not be
+able to distinguish.
+
+It is to our earth's family of these eight large planets going steadily
+round the same sun that we must give our attention first, before going
+on to the distant stars. Many of the planets are accompanied by
+satellites or moons, which circle round them. We may say that the sun is
+our parent--father, mother, what you will--and that the planets are the
+family of children, and that the moons are _their_ children. Our earth,
+you see, has only one child, but that a very fine one, of which she may
+well be proud.
+
+When I say that the planets go round the sun in circles I am only
+speaking generally; as a matter of fact, the orbits of the planets are
+not perfect circles, though some are more circular than others. Instead
+of this they are as a circle might look if it were pressed in from two
+sides, and this is called an ellipse. The path of our own earth round
+the sun is one of the most nearly circular of them all, and yet even in
+her orbit she is a good deal nearer to the sun at one time than another.
+Would you be surprised to hear that she is nearer in our winter and
+further away in our summer? Yet that is the case. And for the first
+moment it seems absurd; for what then makes the summer hotter than the
+winter? That is due to an altogether different cause; it depends on the
+position of the earth's axis. If that axis were quite straight up and
+down in reference to the earth's path round the sun we should have equal
+days and nights all the year round, but it is not; it leans over a
+little, so that at one time the North Pole points towards the sun and at
+another time away from it, while the South Pole is pointing first away
+from it and then toward it in exactly the reverse way. When the North
+Pole points to the sun we in the Northern Hemisphere have our summer. To
+understand this you must look at the picture, which will make it much
+clearer than any words of mine can do. The dark part is the night, and
+the light part the day. When we are having summer any particular spot on
+the Northern Hemisphere has quite a long way to travel in the light, and
+only a very short bit in the dark, and the further north you go the
+longer the day and shorter the night, until right up near the North
+Pole, within the Arctic Circle, it is daylight all the time. You have,
+perhaps, heard of the 'midnight sun' that people go to see in the North,
+and what the expression means is that at what should be midnight the
+sun is still there. He seems just to circle round the horizon, never
+very far above, but never dipping below it.
+
+When the sun is high overhead, his rays strike down with much more force
+than when he is low. It is, for instance, hotter at mid-day than in the
+evening. Now, when the North Pole is bowed toward the sun, the sun
+appears to us to be higher in the sky. In the British Isles he never
+climbs quite to the zenith, as we call the point straight above our
+heads; he always keeps on the southern side of that, so that our shadows
+are thrown northward at mid-day, but yet he gets nearer to it than he
+does in winter. Look at the picture of the earth as it is in winter.
+Then we have long nights and short days, and the sun never appears to
+climb very high, because we are turned away from him. During the short
+days we do not receive a great deal of heat, and during the long night
+the heat we have received has time to evaporate to a great extent. These
+two reasons--the greater or less height of the sun in the sky and the
+length of the days--are quite enough to account for the difference
+between our summer and winter. There is one rather interesting point to
+remember, and that is that in the Northern Hemisphere, whether it is
+winter or summer, the sun is south at mid-day, so that you can always
+find the north then, for your shadow will point northwards.
+
+[Illustration: THE ENGLISH SUMMER (LEFT) AND WINTER (RIGHT).]
+
+New Zealand and Australia and other countries placed in the Southern
+Hemisphere, as we are in the Northern, have their summer while we have
+winter, and winter while we have summer, and their summer is warmer than
+ours, because it comes when the earth in its journey is three million
+miles nearer to the sun than in our summer.
+
+All this seems to refer to the earth alone, and this chapter should be
+about the planets; but, after all, what applies to one planet applies to
+another in some degree, and we can turn to the others with much more
+interest now to see if their axes are bowed toward the sun as ours is.
+It is believed that in the case of Mercury, in regard to its path round
+the sun, the axis is straight up and down; if it is the changes of the
+seasons must depend on the nearness of Mercury to the sun and nothing
+else, and as he is a great deal nearer at one time than another, this
+might make a very considerable difference. Some of the planets are like
+the earth in regard to the position of their axes, but the two outermost
+ones, Uranus and Neptune, are very peculiar, for one pole is turned
+right toward the sun and the other right away from it, so that in one
+hemisphere there is continuous day all the summer, in the other there is
+continuous night, and then the process is reversed. But these little
+peculiarities we shall have to note more particularly in the account of
+the planets separately.
+
+There is a curious fact in regard to the distances of the planets from
+the sun. Each one, after the first, is, very roughly, about double the
+distance from the sun of the one inside it. This holds good for all the
+first four, then there is a great gap where we might expect to find
+another planet, after which follow the four large planets. Now, this gap
+puzzled astronomers greatly; for though there seemed to be no reason why
+the planets should be at regular distances one outside the other, yet
+there the fact was, and that the series should be broken by a missing
+planet was annoying. So very careful search was made, and a thrill of
+excitement went all through the scientific world when it was known that
+a tiny planet had been discovered in the right place. But this was not
+the end of it, for within a few years three or four more tiny planets
+were observed not far from the first one, and, as years rolled on, one
+after another was discovered until now the number amounts to over six
+hundred and others are perpetually being added to the list! Here was a
+new feature in the solar system, a band of tiny planets not one of which
+was to be compared in size with the least of those already known. The
+largest may be about as large as Europe, and others perhaps about the
+size of Wales, while there may be many that have only a few square miles
+of surface altogether, and are too small for us to see. To account for
+this strange discovery many theories were advanced.
+
+One was that there had been a planet--it might be about the size of
+Mars--which had burst up in a great explosion, and that these were the
+pieces--a very interesting and exciting idea, but one which proved to be
+impossible. The explanation now generally accepted is a little
+complicated, and to understand it we must go back for a bit.
+
+When we were talking of the earth and the moon we realized that once
+long ago the moon must have been a part of the earth, at a time when the
+earth was much larger and softer than she now is; to put it in the
+correct way, we should say when she was less dense. There is no need to
+explain the word 'dense,' for in its ordinary sense we use it every day,
+but in an astronomical sense it does not mean exactly the same thing.
+Everything is made up of minute particles or atoms, and when these
+atoms are not very close together the body they compose is loose in
+texture, while if they are closer together the body is firmer. For
+instance, air is less dense than water, and water than earth, and earth
+than steel. You see at once by this that the more density a thing has
+the heavier it is; for as a body is attracted to another body by every
+atom or particle in it, so if it has more particles it will be more
+strongly attracted. Thus on the earth the denser things are really
+heavier. But 'weight' is only a word we use in connection with the
+earth; it means the earth's pulling power toward any particular thing at
+the surface, and if we were right out in space away from the earth, the
+pulling power of the earth would be less, and so the weight would be
+less; and as it would be impossible always to state just how far away a
+thing was from the earth, astronomers talk about density, which means
+the number of particles a body contains in proportion to other bodies.
+Thus the planet Jupiter is very much larger than the earth, but his
+density is less. That does not mean to say that if Jupiter were in one
+scale and the earth in the other he would weigh less, because he is so
+very much bigger he would outweigh the earth still; his total _mass_
+would be greater than that of the earth, but it means that a piece of
+Jupiter the same size as a piece of the earth would weigh less under the
+same conditions.
+
+Now, before there were any planets at all or any sun, in the place of
+our solar system was a vast gaseous cloud called a nebula, which slowly
+rotated, and this rotation was the first impulse or force which God gave
+it. It was not at all dense, and as it rotated a part broke off, and
+inheriting the first impulse, went on rotating too. The impulse would
+have sent it off in a straight line, but the pull of gravity from the
+nebula held it in place, and it circled round; then the nebula, as it
+rotated, contracted a little, and occupied less space and grew denser,
+and presently a second piece was thrown off, to become in time another
+planet. The same process was repeated with Saturn, and then with the
+huge Jupiter. The nebula was always rotating and always contracting. And
+as it behaved, so did the planets in their turn; they spun round and
+cooled and contracted, and the moons were flung off from them, just as
+they--the planets--had been flung off from the parent nebula.
+
+Now, after the original nebula had parted with the mighty mass of
+Jupiter, it never again made an effort so great, and for a long time
+the fragments that were detached were so small as hardly to be worth
+calling planets; they were the asteroids, little lumps and fragments
+that the nebula left behind. But as it still contracted in time there
+came Mars; and having recovered a little, the nebula with more energy
+got rid of the earth, and next Venus, and lastly little Mercury, the
+smallest of the eight planets. Then it contracted further, and perhaps
+you can guess what the remainder of it is--the sun; and by spinning in a
+plastic state the sun, like the earth, has become a globe, round and
+comparatively smooth; and its density is now too great to allow of its
+losing any more fragments, so, as far as we can see, the solar system is
+complete.
+
+This theory of the origin of the planets is called the nebula theory. We
+cannot prove it, but there are so many facts that can only be explained
+by it, we have strong reason for believing that something of the kind
+must have happened. When we come to speak of the starry heavens we shall
+see that there are many masses of glowing gas which are nebulae of the
+same sort, and which form an object-lesson in our own history.
+
+We have spoken rather lightly of the nebula rotating and throwing off
+planets; but we must not think of all this as having happened in a
+short time. It is almost as impossible for the human mind to conceive
+the ages required for such slow changes as to grasp the great gulfs of
+space that separate us from the stars. We can only do it by comparison.
+You know what a second is, and how the seconds race past without ceasing
+day and night. It makes one giddy to picture the seconds there are in a
+year; yet if each one of those seconds was a year in itself, what then?
+That seems a stupendous time, but it is nothing compared with the time
+needed to form a nebula into a planetary system. If we had five thousand
+of such years, with every second in them a year, we should then only
+have counted one billion real years, and billions must have passed since
+the sun was a gaseous nebula filling the outermost bounds of our
+system!
+
+
+
+
+CHAPTER V
+
+FOUR SMALL WORLDS
+
+
+What must the sun appear to Mercury, who is so much nearer to him than
+we are? To understand that we should have to imagine our sun increased
+to eight or nine times his apparent size, and pouring out far greater
+heat and light than anything that we have here, even in the tropics. It
+was at first supposed that Mercury must have an extra thick covering of
+clouds to protect him from this tremendous glare; but recent
+observations tend to prove that, far from this, he is singularly free
+from cloud. As this is so, no life as we know it could possibly exist on
+Mercury.
+
+His year--the time he takes to go round the sun and come back to the
+same place again--is eighty-eight days, or about one-quarter of ours. As
+his orbit is much more like an ellipse than a circle, it follows that he
+is much nearer to the sun at one time than at another--in fact, when he
+is nearest, the size of the sun must seem three and a half times
+greater than when he is furthest away from it! Even at the best Mercury
+is very difficult to observe, and what we can learn about him is not
+much; but, as we have heard, his axis is supposed to be upright. If so
+his seasons cannot depend on the bend toward or away from the sun, but
+must be influenced solely by the changes in his distance from the sun,
+which are much greater than in our own ease. There is some reason to
+believe, too, that Mercury's day and year are the same length. This
+means that as the planet circles round the sun he turns once. If this is
+so the sun will shine on one half of the planet, producing an
+accumulated heat terrific to think of; while the other side is plunged
+in blackness. The side which faces the sun must be heated to a pitch
+inconceivable to us during the nearer half of the orbit--a pitch at
+which every substance must be at boiling-point, and which no life as we
+know it could possibly endure. Seen from our point of view, Mercury goes
+through all the phases of the moon, as he shines by the reflected light
+of the sun; but this point we shall consider more particularly in regard
+to Venus, as Venus is nearer to us and easier to study. For a long time
+astronomers had a fancy that there might be another planet even nearer
+to the sun than Mercury, perhaps hidden from us by the great glare of
+the sun. They even named this imaginary planet Vulcan, and some thought
+they had seen it, but it is tolerably certain that Vulcan existed only
+in imagination. Mercury is the nearest planet to the sun, and also the
+smallest, of course excepting the asteroids. It is about three thousand
+miles in diameter, and as our moon is two thousand miles, it is not so
+much bigger than that. So far as we are concerned, it is improbable we
+shall ever know very much more about this little planet.
+
+But next we come to Venus, our beautiful bright neighbour, who
+approaches nearer to us than any other heavenly body except the moon.
+Alas! when she is nearest, she like Mercury, turns her dark side toward
+us, coming in between us and the sun, so that we cannot observe her at
+all.
+
+Everyone must have noticed Venus, however carelessly they have looked at
+the sky; but it is likely that far more people have seen her as an
+evening than a morning star, for most people are in bed when the sun
+rises, and it is only before sunrise or after sunset we can see Venus
+well. She is at her best from our point of view when she seems to us to
+be furthest from the sun, for then we can study her best, and at these
+times she appears like a half or three-quarter moon, as we only see a
+part of the side from which the sunlight is reflected. She shines like a
+little silver lamp, excelling every other planet, even Jupiter, the
+largest of all. If we look at her even with the naked eye, we can see
+that she is elongated or drawn out, but her brilliance prevents us from
+seeing her shape exactly; to do this we must use a telescope.
+
+[Illustration: DIFFERENT PHASES OF VENUS.]
+
+It is a curious fact that some planets shine much more brightly than
+others, without regard to their size--that is to say, the surface on
+which the sun's rays strike is of greater reflecting power in some than
+in others. One of the brightest things in Nature that we can imagine is
+a bank of snow in sunlight; it is so dazzling that we have to look away
+or wink hard at the sight; and the reflective power of the surface of
+Venus is as dazzling as if she were made of snow. This is probably
+because the light strikes on the upper surface of the clouds which
+surround her. In great contrast to this is the surface of Mercury, which
+reflects as dully as a mass of lead. Our own moon has not a high
+reflecting power, as will be easily understood if we imagine what the
+world would be if condemned to perpetual moonlight only. It would,
+indeed, be a sad deprivation if the mournful cold light of the moon,
+welcome enough as a change from sunlight, were to take the place of
+sunlight in the daytime.
+
+For a very long time astronomers could not discover what time Venus took
+in rotating on her own axis--that is to say, what the length of her day
+was. She is difficult to observe, and in order to find out the rotation
+it is necessary to note some fixed object on the surface which turns
+round with the planet and comes back to the same place again, so that
+the time it takes in its journey can be measured. But the surface of
+Venus is always changing, so that it is impossible to judge at all
+certainly. Opinions differ greatly, some astronomers holding that
+Venus's day is not much longer than an earthly day, while others believe
+that the planet's day is equal to her year, just as in the case of
+Mercury. Venus's year is 225 days, or about seven and a half of our
+months, and if, indeed, her day and year are the same length, very
+peculiar effects would follow. For instance, terrible heat would be
+absorbed by the side of the planet facing the sun in the perpetual
+summer; and the cold which would be felt in the dreary winter's night
+would far exceed our bitterest Arctic climate. We cannot but fancy that
+any beings who might live on a planet of this kind must be different
+altogether from ourselves. Then, there is another point: even here on
+earth very strong winds are caused by the heating of the tropics; the
+hot air, being lighter than the cold air, rises, and the colder air from
+the poles rushes in to supply its place. This causes wind, but the winds
+which would be raised on Venus by the rush of air from the icy side of
+the planet to the hot one would be tornadoes such as we could but
+faintly dream of. It is, of course, useless to speculate when we know so
+little, but in a subject so intensely interesting we cannot help
+guessing a little.
+
+Venus is only slightly smaller than the earth, and her density is not
+very unlike ours; therefore the pull of gravity must be pretty much
+there what it is here--that is to say, things will weigh at her surface
+about the same as they do here. Her orbit is nearly a circle, so that
+her distance from the sun does not vary much, and the heat will not be
+much greater from this cause at one time of the year than another.
+
+As her orbit is tilted up a little she does not pass between us and the
+sun at each revolution, but occasionally she does so, and this passing
+is called a transit. Many important facts have been learned by watching
+these transits. Mercury also has transits across the sun, but as she is
+so much smaller than Venus they are not of such great importance. It was
+by the close observation of Venus during her transits that the distance
+from the earth to the sun was first measured. Not until the year 2004
+will another transit of Venus occur.
+
+It is not difficult to imagine that the earth must appear a splendid
+spectacle from Venus, whence she is seen to great advantage. When
+nearest to us she must see us like a little moon, with markings as the
+continents and seas rotate, and these will change as they are obscured
+by the clouds rolling over them. At the North and South Poles will be
+glittering ice-caps, growing larger and smaller as they turn toward or
+away from the sun. A brilliant spectacle!
+
+[Illustration: ORBITS OF MARS, THE EARTH, VENUS, AND MERCURY.]
+
+We might say with a sigh, 'If only we could see such a world!' Well, we
+can see a world--not indeed, so large as Venus, yet a world that comes
+almost as near to us as Venus does, and which, unlike her, is outside us
+in order from the sun, so that when it is nearest to us the full
+sunlight is on it. This is Mars, our neighbour on the other side, and of
+all the fascinating objects in the sky Mars is the most fascinating, for
+there, if anywhere, should we be likely to discover beings like
+ourselves!
+
+Mars takes rather more than half an hour longer to rotate than we do,
+and as he is so much smaller than the earth, this means that he moves
+round more slowly. His axis is bent at nearly the same angle as ours is.
+Mars is much smaller than the earth, his diameter is about twice that of
+the moon, and his density is about three-quarters that of the earth, so
+that altogether, with his smaller size and less density, anything
+weighing a hundred pounds here would only weigh some forty pounds on
+Mars; and if, by some miraculous agency, you were suddenly transported
+there, you would find yourself so light that you could jump enormous
+distances with little effort, and skip and hop as if you were on
+springs.
+
+[Illustration: _Memoirs of the British Astronomical Association._
+
+MAP OF MARS.]
+
+Look at the map of Mars, in which the surface appears to be cut up into
+land and water, continents and oceans. The men who first observed Mars
+with accuracy saw that some parts were of a reddish colour and others
+greenish, and arguing from our own world, they called the greenish parts
+seas and the reddish land. For a long while no one doubted that we
+actually looked on a world like our own, more especially as there was
+supposed to be a covering of atmosphere. The so-called land and water
+are much more cut up and mixed together than ours, it is true. Here and
+there is a large sea, like that marked 'Mare Australe,' but otherwise
+the water and the land are strangely intermingled. The red colour of the
+part they named land puzzled astronomers a good deal, for our land seen
+at the same distance would not appear so red, and they came at last to
+the conclusion that vegetation on Mars must be red instead of green! But
+after a while another disturbing fact turned up to upset their theories,
+and that was that they saw canals, or what they called canals, on Mars.
+These were long, straight, dark markings, such as you see on the map.
+It is true that some people never saw these markings at all, and
+disbelieved in their existence; but others saw them clearly, and watched
+them change--first go fainter and then darker again. And quite recently
+a photograph has been obtained which shows them plainly, so they must
+have an existence, and cannot be only in the eye of the observer, as the
+most sceptical people were wont to suggest. But further than this, one
+astronomer announced that some of these lines appeared to be double, yet
+when he looked at them again they had grown single. It was like a
+conjuring trick. Great excitement was aroused by this, for if the canals
+were altered so greatly it really did look as if there were intelligent
+beings on Mars capable of working at them. In any case, if these are
+really canals, to make them would be a stupendous feat, and if they are
+artificial--that is, made by beings and not natural--they show a very
+high power of engineering. Imagine anyone on earth making a canal many
+miles wide and two thousand miles long! It is inconceivable, but that is
+the feat attributed to the Martians. The supposed doubling of the
+canals, as I say, caused a great deal of talk, and very few people could
+see that they were double at all. Even now the fact is doubted, yet
+there seems every reason to believe it is true. They do not all appear
+to be double, and those that do are always the same ones, while others
+undoubtedly remain single all the time. But the canals do not exhaust
+the wonders of Mars. At each pole there is an ice-cap resembling those
+found at our own poles, and this tells us pretty plainly something about
+the climate of Mars, and that there is water there.
+
+This ice-cap melts when the pole which it surrounds is directed toward
+the sun, and sometimes in a hot summer it dwindles down almost to
+nothing, in a way that the ice-caps at the poles of the earth never do.
+A curious appearance has been noticed when it is melting: a dark shadow
+seems to grow underneath the edge of it and extends gradually, and as it
+extends the canals near it appear much darker and clearer than they did
+before, and then the canals further south undergo the same change. This
+looks as if the melting of the snow filled up the canals with water, and
+was a means of watering the planet by a system totally different from
+anything we know here, where our poles are surrounded by oceans, and the
+ice-caps do not in the least affect our water-supply. But, then, another
+strange fact had to be taken into consideration. These straight lines
+called canals ran out over the seas occasionally, and it was impossible
+to believe that if they were canals they could do that. Other things
+began to be discussed, such as the fact that the green parts of Mars did
+not always remain green. In what is the springtime of Mars they are so,
+but afterwards they become yellow, and still later in the season parts
+near the pole turn brown. Thus the idea that the greenish parts are seas
+had to be quite given up, though it appeared so attractive. The idea now
+generally believed is that the greenish parts are vegetation--trees and
+bushes and so on, and that the red parts are deserts of reddish sand,
+which require irrigation--that is to say, watering--before anything can
+be grown on them. The apparent doubling of the canals may be due to the
+green vegetation springing up along the banks. This might form two broad
+lines, while the canal itself would not be seen, and when the vegetation
+dies down, we should see only the trench of the canal, which would
+possibly appear faint and single. Therefore the arrangements on Mars
+appear to be a rich and a barren season on each hemisphere, the growth
+being caused by the melting of the polar ice-cap, which sends floods
+down even beyond the Equator. If we could imagine the same thing on
+earth we should have to think of pieces of land lying drear and dry and
+dead in winter between straight canal-like ditches of vast size. A
+little water might remain in these ditches possibly, but not enough to
+water the surrounding land. Then, as summer progressed, we should hear,
+'The floods are coming,' and each deep, huge canal would be filled up
+with a tide of water, penetrating further and further. The water drawn
+up into the air would fall in dew or rain. Vegetation would spring up,
+especially near the canal banks, and instead of dreary wastes rich
+growths would cover the land, gradually dying down again in the winter.
+So far Mars seems in some important respects very different from the
+earth. He is also less favourably placed than we are, for being so much
+further from the sun, he receives very much less heat and light. His
+years are 687 of our days, or one year and ten and a half months, and
+his atmosphere is not so dense as ours. With this greater distance from
+the sun and less air we might suppose the temperature would be very cold
+indeed, and that the surface would be frost-bound, not only at the
+poles, but far down towards the Equator. Instead of this being so, as we
+have seen, the polar caps melt more than those on the earth. We can
+only surmise there must be some compensation we do not know of that
+softens down the rigour of the seasons, and makes them milder than we
+should suppose possible.
+
+Of course, the one absorbing question is, Are there people on Mars? To
+this it is at present impossible to reply. We can only say the planet
+seems in every way fitted to support life, even if it is a little
+different from our earth. It is most certainly a living world, not a
+dead one like the moon, and as our knowledge increases we may some day
+be able to answer the question which so thrills us.
+
+Our opportunities for the observation of Mars vary very greatly, for as
+the earth's orbit lies inside that of Mars, we can best see him when we
+are between him and the sun. Of course, it must be remembered that the
+earth and the other planets are so infinitely small in regard to the
+space between them that there is no possibility of any one of them
+getting in such a position that it would throw a shadow on any other or
+eclipse it. The planets are like specks in space, and could not
+interfere with one another in this way. When Mars, therefore, is in a
+line with us and the sun we can see him best, but some of these times
+are better than others, for this reason--the earth's orbit is nearly a
+circle, and that of Mars more of an ellipse.
+
+[Illustration: ORBITS OF THE EARTH AND MARS.]
+
+Look at the illustration and remember that Mars' year is not quite two
+of ours--that is to say, every time we swing round our orbit we catch
+him up in a different place, for he will have progressed less than half
+his orbit while we go right round ours.
+
+Sometimes when we overtake him he may be at that part which is furthest
+away from us, or he may be at that part which is nearest to us, and if
+he is in the latter position we can see him best. Now at these, the most
+favourable times of all, he is still more than thirty-five millions of
+miles away--that is to say, one hundred and forty times as far as the
+moon, yet comparatively we can see him very well. He is coming nearer
+and nearer to us, and very soon will be nearer than he has been since
+1892, or fifteen years ago. Then many telescopes will be directed on
+him, and much may be learned about him.
+
+For a long time it was supposed that Mars had no moons, and when Dean
+Swift wrote 'Gulliver's Travels' he wanted to make the Laputans do
+something very clever, so he described their discovery of two moons
+attending Mars, and to make it quite absurd he said that when they
+observed these moons they found that one of them went round the planet
+in about ten hours. Now, as Mars takes more than twenty-four hours to
+rotate, this was considered ridiculous, for no moon known then took
+less time to go round its primary world than the primary world took to
+turn on its own axis. Our own moon, of course, takes thirty times as
+long--that is a month contains thirty days. Then one hundred and fifty
+years later this jest of Dean Swift's came true, for two moons were
+really discovered revolving round Mars, and one of them does actually
+take less time to complete its orbit than the planet does to
+rotate--namely, a little more than seven hours! So the absurdity in
+'Gulliver's Travels' was a kind of prophecy!
+
+These two moons are very small, the outer one perhaps five or six miles
+in diameter, and the inner one about seven; therefore from Mars the
+outer one, Deimos, cannot look much more than a brilliant star, and the
+inner one would be but a fifth part the apparent width of our own moon.
+So Mars is not very well off, after all. Still, there is great variety,
+for it must be odd to see the same moon appearing three times in the
+day, showing all the different phases as it goes from new to full, even
+though it is small!
+
+Such wonderful discoveries have already been made that it is not too
+much to say that perhaps some day we may be able to establish some sort
+of communication with Mars, and if it be inhabited by any intelligent
+beings, we may be able to signal to them; but it is almost impossible
+that any contrivance could bridge the gulf of airless space that
+separates us, and it is not likely that holiday trips to Mars will ever
+become fashionable!
+
+
+
+
+CHAPTER VI
+
+FOUR LARGE WORLDS
+
+
+I have told you about the four lesser worlds of which our earth is one,
+and you know that beyond Mars, the last of them, there lies a vast
+space, in which are found the asteroids, those strange small planets
+circling near to each other, like a swarm of bees. After this there
+comes Jupiter, who is so enormous, so superb in size compared with us,
+that he might well serve as the sun of a little system of his own. You
+remember that we represented him by a football, while the earth was only
+a greengage plum. But Jupiter himself is far less in comparison with the
+sun than we are in comparison with him. He differs from the planets we
+have heard about up to the present in that he seems to glow with some
+heat that he does not receive from the sun. The illumination which makes
+him appear as a star to us is, of course, merely reflected sunlight, and
+what we see is the external covering, his envelope of cloud.
+
+There is every reason to believe that the great bulk of Jupiter is
+still at a high temperature. We know that in the depths of the earth
+there is still plenty of heat, which every now and then makes its
+presence felt by bursting up through the vents we call volcanoes, the
+weak spots in the earth's crust; but our surface long ago cooled, for
+the outside of any body gets cool before the inside, as you may have
+found if ever you were trying to eat hot porridge, and circled round the
+edge of the plate with a spoon. A large body cools more slowly than a
+small one, and it is possible that Jupiter, being so much larger than we
+are, has taken longer to cool. One reason we have for thinking this is
+that he is so very light compared with his size--in other words, his
+density is so small that it is not possible he could be made of
+materials such as the earth is made of.
+
+As I said, when we study him through telescopes we see just the
+exterior, the outer envelope of cloud, and as we should expect, this
+changes continually, and appears as a series of belts, owing to the
+rotation of the planet. Jupiter's rotation is very rapid; though he is
+so much greater than the earth, he takes less than half the time the
+earth does to turn round--that is to say, only ten hours. His days and
+nights of five hours each seem short to us, accustomed to measure
+things by our own estimates. But we must remember that everything is
+relative; that is to say, there is really no such thing as fast or slow;
+it is all by comparison. A spider runs fast compared with a snail, but
+either is terribly slow compared with an express train; and the speed of
+an express train itself is nothing to the velocity of light.
+
+In the same way there is nothing absolutely great or small; it is all by
+comparison. We say how marvellous it is that a little insect has all the
+mechanism of life in its body when it is so tiny, but if we imagine that
+insect magnified by a powerful microscope until it appears quite large,
+the marvel ceases. Again, imagine a man walking on the surface of the
+earth as seen from a great distance through a telescope: he would seem
+less than an insect, and we might ask how could the mechanism of life be
+compressed into anything so small? Thus, when we say enormous or tiny we
+must always remember we are only speaking by the measurements of our own
+standards.
+
+There is nothing very striking about Jupiter's orbit. He takes between
+eleven and twelve of our years to get round the sun, so you see, though
+his day is shorter, his year is longer than ours. And this is not only
+because his path is much larger, but because by the law of gravity the
+more distant a planet is from the sun the more slowly it travels, so
+that while the earth speeds over eighteen miles Jupiter has only done
+eight. Of course, we must be careful to remember the difference between
+rotation and revolution. Jupiter rotates much quicker than the
+earth--that is to say, he turns round more quickly--but he actually gets
+over the ground more slowly. The sun appears much smaller to him than it
+does to us, and he receives considerably less light and heat. There are
+various spots on his surface, and one remarkable feature is a dark mark,
+which is called the 'great red spot.' If as we suppose what we see of
+the planet is merely the cloudy upper atmosphere, we should not expect
+to find anything permanent there, for the markings would change from day
+to day, and this they do with this exception--that this spot, dark red
+in colour, has been seen for many years, turning as the planet turned.
+It was first noticed in 1878, and was supposed to be some great mountain
+or excrescence peeping up through the clouds. It grew stronger and
+darker for several years, and then seemed to fade, and was not so easily
+seen, and though still remaining it is now pale. But, most startling
+to say, it has shifted its position a little--that is, it takes a few
+seconds longer to get round the planet than it did at first. A few
+seconds, you will say, but that is nothing! It does not seem much, but
+it shows how marvellously accurate astronomers are. Discoveries of vast
+importance have been made from observing a few seconds' discrepancy in
+the time the heavenly bodies take in their journeys, and the fact that
+this spot takes a little longer in its rotation than it did at first
+shows that it cannot be attached to the body of the planet. It is
+impossible for it to be the summit of a mountain or anything of that
+sort. What can it be? No one has yet answered that question.
+
+[Illustration: JUPITER AND ONE OF HIS MOONS]
+
+When we get to the chapter on the sun, we shall find curiosities
+respecting the spots there as well.
+
+Jupiter has seven moons, and four of these are comparatively large. They
+have the honour of having been the first heavenly bodies ever actually
+discovered, for the six large planets nearest the sun have been known so
+long that there is no record of their first discovery, and of course our
+own moon has always been known. Galileo, who invented the telescope,
+turned it on to the sky in 1610, when our King Charles I. was on the
+throne, and he saw these curious bodies which at first he could not
+believe to be moons. The four which he saw vary in size from two
+thousand one hundred miles in diameter to nearly three thousand six
+hundred. You remember our own moon is two thousand miles across, so even
+the smallest is larger than she. They go round at about the same level
+as the planet's Equator, and therefore they cross right in front of him,
+and go behind him once in every revolution. Since then the other three
+have been discovered in the band of Jupiter's satellites--one a small
+moon closer to him than any of the first set, and two others further
+out. It was by observation of the first four, however, that very
+interesting results were obtained. Mathematicians calculated the time
+that these satellites ought to disappear behind Jupiter and reappear
+again, but they found that this did not happen exactly at the time
+predicted; sometimes the moons disappeared sooner than they should have
+done, and sometimes later. Then this was discovered to have some
+relation to the distance of our earth from Jupiter. When he was at the
+far side of his immense orbit he was much more distant from us than when
+he was on the nearer side--in fact, the difference may amount to more
+than three hundred millions of miles. And it occurred to some clever man
+that the irregularities in time we noticed in the eclipses of the
+satellites corresponded with the distance of Jupiter from us. The
+further he drew away from us, the later were the eclipses, and as he
+came nearer they grew earlier. By a brilliant inspiration, this was
+attributed to the time light took to travel from them to us, and this
+was the first time anyone had been able to measure the velocity or speed
+of light. For all practical purposes, on the earth's surface we hold
+light to be instantaneous, and well we may, for light could travel more
+than eight times round the world in one second. It makes one's brain
+reel to think of such a thing. Then think how far Jupiter must be away
+from us at the furthest, when you hear that sometimes these eclipses
+were delayed seventeen minutes--minutes, not seconds--because it took
+that time for light to cross the gulf to us!
+
+[Illustration: JUPITER AND HIS PRINCIPAL MOONS.]
+
+Sound is very slow compared with light, and that is why, if you watch a
+man hammering at a distance, the stroke he gives the nail does not
+coincide with the bang that reaches you, for light gets to you
+practically at once, and the sound comes after it. No sound can travel
+without air, as we have heard, therefore no sound reaches us across
+space. If the moon were to blow up into a million pieces we should see
+the amazing spectacle, but should hear nothing of it. Light travels
+everywhere throughout the universe, and by the use of this universal
+carrier we have learnt all that we know about the stars and planets.
+When the time that light takes to travel had been ascertained by means
+of Jupiter's satellites, a still more important problem could be
+solved--that was our own distance from the sun, which before had only
+been known approximately, and this was calculated to be ninety-two
+millions seven hundred thousand miles, though sometimes we are a little
+nearer and sometimes a little further away.
+
+Jupiter is marvellous, but beyond him lies the most wonderful body in
+the whole solar system. We have found curiosities on our way out: we
+have studied the problem of the asteroids, of the little moon that goes
+round Mars in less time than Mars himself rotates; we have considered
+the 'great red spot' on Jupiter, which apparently moves independently
+of the planet; but nothing have we found as yet to compare with the
+rings of Saturn. May you see this amazing sight through a telescope one
+day!
+
+Look at the picture of this wonderful system, and think what it would be
+like if the earth were surrounded with similar rings! The first question
+which occurs to all of us is what must the sky look like from Saturn?
+What must it be to look up overhead and see several great hoops or
+arches extending from one horizon to another, reflecting light in
+different degrees of intensity? It would be as if we saw several immense
+rainbows, far larger than any earthly rainbow, and of pure light, not
+split into colours, extending permanently across the sky, and now and
+then broken by the black shadow of the planet itself as it came between
+them and the sun. However, we must begin at the beginning, and find out
+about Saturn himself before we puzzle ourselves over his rings. Saturn
+is not a very great deal less than Jupiter, though, so small are the
+other planets in comparison, that if Saturn and all the rest were rolled
+together, they would not make one mass so bulky as Jupiter! Saturn is
+so light--in other words, his density is so small--that he is actually
+lighter than water. He is the lightest, in comparison with his size, of
+any of the planets. Therefore he cannot be made largely of solid land,
+as our earth is, but must be to a great extent, composed of air and
+gaseous vapour, like his mighty neighbour. He approaches at times as
+near to Jupiter as Jupiter does to us, and on these occasions he must
+present a splendid spectacle to Jupiter. He takes no less than
+twenty-nine and a half of our years to complete his stately march around
+the sun, and his axis is a little more bent than ours; but, of course,
+at his great distance from the sun, this cannot have the same effect on
+the seasons that it does with us. Saturn turns fast on his axis, but not
+so fast as Jupiter, and in turning his face, or what we call his
+surface, presents much the same appearance to us that we might expect,
+for it changes very frequently and looks like cloud belts.
+
+The marvellous feature about Saturn is, of course, the rings. There are
+three of these, lying one within the other, and separated by a fine line
+from each other. The middle one is much the broadest, probably about ten
+thousand miles in width, and the inner one, which is the darkest, was
+not discovered until some time after the others. As the planet swings in
+his orbit the rings naturally appear very different to us at different
+times. Sometimes we can only see them edgewise, and then even in the
+largest telescope they are only like a streak of light, and this shows
+that they cannot be more than fifty or sixty miles in thickness. The one
+which is nearest to Saturn's surface does not approach him within ten
+thousand miles. Saturn has no less than ten satellites, in addition to
+the rings, so that his midnight sky must present a magnificent
+spectacle. The rings, which do not shine by their own light but by
+reflected sunlight, are solid enough to throw a shadow on the body of
+the planet, and themselves receive his shadow. Sometimes for days
+together a large part of Saturn must suffer eclipse beneath the
+encircling rings, but at other times, at night, when the rings are clear
+of the planet's body, so that the light is not cut off from them, they
+must appear as radiant arches of glory spanning the sky.
+
+The subject of these rings is so complicated by the variety of their
+changes that it is difficult for us even to think about it. It is one of
+the most marvellous of all the features of our planetary system. What
+are these rings? what are they made of? It has been positively proved
+that they cannot be made of continuous matter, either liquid or solid,
+for the force of gravity acting on them from the planet would tear them
+to pieces. What, then, can they be? It is now pretty generally believed
+that they are composed of multitudes of tiny bodies, each separate, and
+circling separately round the great planet, as the asteroids circle
+round the sun. As each one is detached from its neighbour and obeys its
+own impulses, there is none of the strain and wrench there would be were
+they all connected. According to the laws which govern planetary bodies,
+those which are nearest to the planet will travel more quickly than
+those which are further away. Of course, as we look at them from so
+great a distance, and as they are moving, they appear to us to be
+continuous. It is conjectured that the comparative darkness of the
+inside ring is caused by the fact that there are fewer of the bodies
+there to reflect the sunlight. Then, in addition to the rings, enough
+themselves to distinguish him from all other planets, there are the ten
+moons of richly-endowed Saturn to be considered. It is difficult to
+gather much about these moons, on account of our great distance from
+them. The largest is probably twice the diameter of our own moon. One of
+them seems to be much brighter--that is to say, of higher reflecting
+power--on one side than the other, and by distinguishing the sides
+and watching carefully, astronomers have come to the conclusion that it
+presents always the same face to Saturn in the same way as our own moon
+does to us; in fact, there is reason to think that all the moons of
+large planets do this.
+
+[Illustration: THE PLANET SATURN WITH TWO OF HIS MOONS.]
+
+All the moons lie outside the rings, and some at a very great distance
+from Saturn, so that they can only appear small as seen from him. Yet at
+the worst they must be brighter than ordinary stars, and add greatly to
+the variations in the sky scenery of this beautiful planet. In
+connection with Saturn's moons there is another of those astonishing
+facts that are continually cropping up to remind us that, however much
+we know, there is such a vast deal of which we are still ignorant. So
+far in dealing with all the planets and moons in the solar system we
+have made no remark on the way they rotate or revolve, because they all
+go in the same direction, and that direction is called
+counter-clockwise, which means that if you stand facing a clock and turn
+your hand slowly round the opposite direction to that in which the hands
+go, you will be turning it in the same way that the earth rotates on its
+axis and revolves in its orbit. It is, perhaps, just as well to give
+here a word of caution. Rotating of course means a planet's turning on
+its own axis, revolving means its course in its orbit round the sun.
+Mercury, Venus, Earth, Mars, Jupiter, and all their moons, as well as
+Saturn himself, rotate on their axes in this one
+direction--counter-clockwise--and revolve in the same direction as they
+rotate. Even the queer little moon of Mars, which runs round him quicker
+than he rotates, obeys this same rule. Nine of Saturn's moons follow
+this example, but one independent little one, which has been named
+Phoebe, and is far out from the planet, actually revolves in the
+opposite way. We cannot see how it rotates, but if, as we said just now,
+it turns the same face always to Saturn, then of course it rotates the
+wrong way too. A theory has been suggested to account for this curious
+fact, but it could not be made intelligible to anyone who has not
+studied rather high mathematics, so there we must just leave it, and put
+it in the cabinet of curiosities we have already collected on our way
+out to Saturn.
+
+For ages past men have known and watched the planets lying within the
+orbit of Saturn, and they had made up their minds that this was the
+limit of our system. But in 1781 a great astronomer named Herschel was
+watching the heavens through a telescope when he noticed one strange
+object that he was certain was no star. The vast distance of the stars
+prevents their having any definite outline, or what is called a disc.
+The rays dart out from them in all directions and there is no 'edge' to
+them, but in the case of the planets it is possible to see a disc with a
+telescope, and this object which attracted Herschel's attention had
+certainly a disc. He did not imagine he had discovered a new planet,
+because at that time the asteroids had not been found, and no one
+thought that there could be any more planets. Yet Herschel knew that
+this was not a star, so he called it a comet! He was actually the first
+who discovered it, for he knew it was not a fixed star, but it was after
+his announcement of this fact that some one else, observing it
+carefully, found it to be a real planet with an orbit lying outside that
+of Saturn, then the furthest boundary of the solar system. Herschel
+suggested calling it Georgius Sidus, in honour of George III., then
+King; but luckily this ponderous name was not adopted, and as the other
+planets had been called after the Olympian deities, and Uranus was the
+father of Saturn, it was called Uranus. It was subsequently found that
+this new planet had already been observed by other astronomers and
+catalogued as a star no less than seventeen times, but until Herschel's
+clear sight had detected the difference between it and the fixed stars
+no one had paid any attention to it. Uranus is very far away from the
+sun, and can only sometimes be seen as a small star by people who know
+exactly where to look for him. In fact, his distance from the sun is
+nineteen times that of the earth.
+
+Yet to show at all he must be of great size, and that size has actually
+been found out by the most delicate experiments. If we go back to our
+former comparison, we shall remember that if the earth were like a
+greengage plum, then Uranus would be in comparison about the size of one
+of those coloured balloons children play with; therefore he is much
+larger than the earth.
+
+In this far distant orbit the huge planet takes eighty-four of our years
+to complete one of his own. A man on the earth will have grown from
+babyhood to boyhood, from boyhood to the prime of life, and lived longer
+than most men, while Uranus has only once circled in his path.
+
+But in dealing with Uranus we come to another of those startling
+problems of which astronomy is full. So far we have dealt with planets
+which are more or less upright, which rotate with a rotation like that
+of a top. Now take a top and lay it on one side on the table, with one
+of its poles pointing toward the great lamp we used for the sun and the
+other pointing away. That is the way Uranus gets round his path, on his
+side! He rotates the wrong way round compared with the planets we have
+already spoken of, but he revolves the same way round the sun that all
+the others do. It seems wonderful that even so much can be found out
+about a body so far from us, but we know more: we have discovered that
+Uranus is made of lighter material than the earth; his density is less.
+How can that be known? Well, you remember every body attracts every
+other body in proportion to the atoms it contains. If, therefore, there
+were any bodies near to Uranus, it could be calculated by his influence
+on them what was his own mass, which, as you remember, is the word we
+use to express what would be weight were it at the earth's surface; and
+far away as Uranus is, the bodies from which such calculations may be
+made have been discovered, for he has no less than four satellites, or
+moons. Considering now the peculiar position of the planet, we might
+expect to find these moons revolving in a very different way from
+others, and this is indeed the case. They turn round the planet at
+about its Equator--that is to say, if you hold the top representing
+Uranus as was suggested just now, these moons would go above and below
+the planet in passing round it. Only we must remember there is really no
+such thing as above and below absolutely. We who are on one side of the
+world point up to the sky and down to the earth, while the people on the
+other side of the earth, say at New Zealand, also point up to the sky
+and down to the earth, but their pointings are directly the opposite of
+ours. So when we speak of moons going above and below that is only
+because, for the moment, we are representing Uranus as a top we hold in
+our hands, and so we speak of above and below as they are to us.
+
+It was Herschel who discovered these satellites, as well as the planet,
+and for these great achievements he occupies one of the grandest places
+in the role of names of which England is proud. But he did much more
+than this: his improvements in the construction of telescopes, and his
+devotion to astronomy in many other ways, would have caused him to be
+remembered without anything else.
+
+Of Uranus's satellites one, the nearest, goes round in about two and a
+half days, and the one that is furthest away takes about thirteen and a
+half days, so both have a shorter period than our moon.
+
+The discovery of Uranus filled the whole civilized world with wonder.
+The astronomers who had seen him, but missed finding out that he was a
+planet, must have felt bitterly mortified, and when he was discovered he
+was observed with the utmost accuracy and care. The calculations made to
+determine his path in the sky were the easier because he had been noted
+as a star in several catalogues previously, so that his position for
+some time past was known. Everybody who worked at astronomy began to
+observe him. From these facts mathematicians set to work, and, by
+abstruse calculations, worked out exactly the orbit in which he ought to
+move; then his movements were again watched, and behold he followed the
+path predicted for him; but there was a small difference here and there:
+he did not follow it exactly. Now, in the heavens there is a reason for
+everything, though we may not always be clever enough to find it out,
+and it was easily guessed that it was not by accident that Uranus did
+not precisely follow the path calculated for him. The planets all act
+and react on one another, as we know, according to their mass and their
+distance, and in the calculations the pull of Jupiter on Saturn and of
+Saturn on Uranus were known and allowed for. But Uranus was pulled by
+some unseen influence also.
+
+A young Englishman named Adams, by some abstruse and difficult
+mathematical work far beyond the power of ordinary brains, found out not
+only the fact that there must be another planet nearly as large as
+Uranus in an orbit outside his, but actually predicted where such a
+planet might be seen if anyone would look for it. He gave his results to
+a professor of astronomy at Cambridge. Now, it seems an easy thing to
+say to anyone, 'Look out for a planet in such and such a part of the
+sky,' but in reality, when the telescope is turned to that part of the
+sky, stars are seen in such numbers that, without very careful
+comparison with a star chart, it is impossible to say which are fixed
+stars and which, if any, is an intruder. There happened to be no star
+chart of this kind for the particular part of the sky wanted, and thus a
+long time elapsed and the planet was not identified. Meantime a young
+Frenchman named Leverrier had also taken up the same investigation, and,
+without knowing anything of Adams' work, had come to the same
+conclusion. He sent his results to the Berlin Observatory, where a star
+chart such as was wanted was actually just being made. By the use of
+this the Berlin astronomers at once identified this new member of our
+system, and announced to the astonished world that another large planet,
+making eight altogether, had been discovered. Then the English
+astronomers remembered that they too held in their hands the means for
+making this wonderful discovery, but, by having allowed so much time to
+elapse, they had let the honour go to France. However, the names of
+Adams and Leverrier will always be coupled together as the discoverers
+of the new planet, which was called Neptune. The marvel is that by pure
+reasoning the mind of man could have achieved such results.
+
+If the observation of Uranus is difficult, how much more that of
+Neptune, which is still further plunged in space! Yet by patience a few
+facts have been gleaned about him. He is not very different in size from
+Uranus. He also is of very slight density. His year includes one hundred
+and sixty-five of ours, so that since his discovery in 1846 he has only
+had time to get round less than a third of his path. His axis is even
+more tilted over than that of Uranus, so that if we compare Uranus to a
+top held horizontally, Neptune will be like a top with one end pointing
+downwards. He rotates in this extraordinary position, in the same manner
+as Uranus--namely, the other way over from all the other planets, but he
+revolves, as they all do, counter-clockwise.
+
+Seen from Neptune the sun can only appear about as large as Venus
+appears to us at her best, and the light and heat received are but one
+nine-hundreth part of what he sends us. Yet so brilliant is sunshine
+that even then the light that falls on Neptune must be very
+considerable, much more than that which we receive from Venus, for the
+sun itself glows, and from Venus the light is only reflected. The sun,
+small as it must appear, will shine with the radiance of a glowing
+electric light. To get some idea of the brilliance of sunlight, sit near
+a screen of leaves on some sunny day when the sun is high overhead, and
+note the intense radiance of even the tiny rays which shine through the
+small holes in the leaves. The scintillating light is more glorious than
+any diamond, shooting out coloured rays in all directions. A small sun
+the apparent size of Venus would, therefore, give enough light for
+practical purposes to such a world as Neptune, even though to us a world
+so illuminated would seem to be condemned to a perpetual twilight.
+
+
+
+
+CHAPTER VII
+
+THE SUN
+
+
+So far we have referred to the sun just so much as was necessary to show
+the planets rotating round him, and to acknowledge him as the source of
+all our light and heat; but we have not examined in detail this
+marvellous furnace that nourishes all the life on our planet and burns
+on with undiminished splendour from year to year, without thought or
+effort on our part. To sustain a fire on the earth much time and care
+and expense are necessary; fuel has to be constantly supplied, and men
+have to stoke the fire to keep it burning. Considering that the sun is
+not only vastly larger than all the fires on the earth put together, but
+also than the earth itself, the question very naturally occurs to us,
+Who supplies the fuel, and who does the stoking on the sun? Before we
+answer this we must try to get some idea of the size of this stupendous
+body. It is not the least use attempting to understand it by plain
+figures, for the figures would be too great to make any impression on
+us--they would be practically meaningless; we must turn to some other
+method. Suppose, for instance, that the sun were a hollow ball; then, if
+the earth were set at the centre, the moon could revolve round her at
+the same distance she is now, and there would be as great a distance
+between the moon and the shell of the sun as there is between the moon
+and the earth. This gives us a little idea of the size of the sun.
+Again, if we go back to that solar system in which we represented the
+planets by various objects from a pea to a football, and set a lamp in
+the centre to do duty for the sun, what size do you suppose that lamp
+would have to be really to represent the sun in proportion to the
+planets? Well, if our greengage plum which did duty for the earth were
+about three-quarters of an inch in diameter we should want a lamp with a
+flame as tall as the tallest man you know, and even then it would not
+give a correct idea unless you imagined that man extending his arms
+widely, and you drew round him a circle and filled in all the circle
+with flame! If this glorious flame burnt clear and fair and bright,
+radiating beams of light all around, the little greengage plum would not
+have to be too near, or it would be shrivelled up as in the blast of a
+furnace. To place it at anything resembling the distance it is from the
+sun in reality you would have to walk away from the flaming light for
+about three hundred steps, and set it down there; then, after having
+done all this, you would have some little idea of the relative sizes of
+the sun and the earth, and of the distance between them.
+
+Of course, all the other planets would have to be at corresponding
+distances. On this same scale, Neptune, the furthest out, would be three
+miles from our artificial sun! It seems preposterous to think that some
+specks so small as to be quite invisible, specks that crawl about on
+that plum, have dared to weigh and measure the gigantic sun; but yet
+they have done it, and they have even decided what he is made of. The
+result of the experiments is that we know the sun to be a ball of
+glowing gas at a temperature so high that nothing we have on earth could
+even compare with it. Of his radiating beams extending in all directions
+few indeed fall on our little plum, but those that do are the source of
+all life, whether animal or vegetable. If the sun's rays were cut off
+from us, we should die at once. Even the coal we use to keep us warm is
+but sun's heat stored up ages ago, when the luxuriant tropical
+vegetation sprang up in the warmth and then fell down and was buried in
+the earth. At night we are still enjoying the benefit of the sun's
+rays--that is, of those which are retained by our atmosphere; for if
+none remained even the very air itself would freeze, and by the next
+morning not one inhabitant would be left alive to tell the awful tale.
+Yet all this life and growth and heat we receive on the whole earth is
+but one part in two thousand two hundred millions of parts that go out
+in all directions into space. It has been calculated that the heat which
+falls on to all the planets together cannot be more than one part in one
+hundred millions and the other millions of parts seem to us to be simply
+wasted.
+
+For untold ages the sun has been pouring out this prodigal profusion of
+glory, and as we know that this cannot go on without some sort of
+compensation, we want to understand what keeps up the fires in the sun.
+It is true that the sun is so enormous that he might go on burning for a
+very long time without burning right away; but, then, even if he is
+huge, his expenditure is also huge. If he had been made of solid coal he
+would have been all used up in about six thousand years, burning at the
+pace he does. Now, we know that the ancient Egyptians kept careful note
+of the heavenly bodies, and if the sun were really burning away he must
+have been very much larger in their time; but we have no record of this;
+on the contrary, all records of the sun even to five thousand years ago
+show that he was much the same as at present. It is evident that we must
+search elsewhere for an explanation. It has been suggested that his
+furnace is supplied by the number of meteors that fall into him. Meteors
+are small bodies of the same materials as the planets, and may be
+likened to the dust of the solar system. It is not difficult to
+calculate the amount of matter he would require on this assumption to
+keep him going, and the amount required is so great as to make it
+practically impossible that this is the source of his supply. We have
+seen that all matter influences all other matter, and the quantity of
+meteoric stuff that would be required to support the sun's expenditure
+would be enough to have a serious effect on Mercury, an effect that
+would certainly have been noticed. There can, therefore, be no such mass
+of matter near the sun, and though there is no doubt a certain number of
+meteors do fall into his furnaces day by day, it is not nearly enough to
+account for his continuous radiation. It seems after this as if nothing
+else could be suggested; but yet an answer has been found, an answer so
+wonderful that it is more like a fairy tale than reality.
+
+To begin at the beginning, we must go back to the time when the sun was
+only a great gaseous nebula filling all the space included in the orbit
+of Neptune. This nebula was not in itself hot, but as it rotated it
+contracted. Now, heat is really only a form of energy, and energy and
+heat can be interchanged easily. This is a very startling thing when
+heard for the first time, but it is known as surely as we know anything
+and has been proved again and again. When a savage wants to make a fire
+he turns a piece of hard wood very very quickly between his
+palms--twiddles it, we should say expressively--into a hole in another
+piece of wood, until a spark bursts out. What is the spark? It is the
+energy of the savage's work turned to heat. When a horse strikes his
+iron-shod hoofs hard on the pavement you see sparks fly; that is caused
+by the energy of the horse's leg. When you pump hard at your bicycle you
+feel your pump getting quite hot, for part of the energy you are putting
+into your work is transformed into heat; and so on in numberless
+instances. No energetic action of any kind in this world takes place
+without some of the energy being turned into heat, though in many
+instances the amount is so small as to be unnoticeable. Nothing falls
+to the ground without some heat being generated. Now, when this great
+nebula first began its remarkable career, by the action of gravity all
+the particles in it were drawn toward the centre; little by little they
+fell in, and the nebula became smaller. We are not now concerned with
+the origin of the planets--we leave that aside; we are only
+contemplating the part of the nebula which remained to become the sun.
+Now these particles being drawn inward each generated some heat, so as
+the nebula contracted its temperature rose. Throughout the ages, over
+the space of millions and millions of miles, it contracted and grew
+hotter. It still remained gaseous, but at last it got to an immense
+temperature, and is the sun as we know it. What then keeps it shining?
+It is still contracting, but slowly, so slowly that it is quite
+imperceptible to our finest instruments. It has been calculated that if
+it contracts two hundred and fifty feet in diameter in a year, the
+energy thus gained and turned into heat is quite sufficient to account
+for its whole yearly output. This is indeed marvellous. In comparison
+with the sun's size two hundred and fifty feet is nothing. It would take
+nine thousand years at this rate before any diminution could be noticed
+by our finest instruments! Here is a source of heat which can continue
+for countless ages without exhaustion. Thus to all intents and purposes
+we may say the sun's shining is inexhaustible. Yet we must follow out
+the train of reasoning, and see what will happen in the end, in eras and
+eras of time, if nothing intervenes. Well, some gaseous bodies are far
+finer and more tenuous than others, and when a gaseous body contracts it
+is all the time getting denser; as it grows denser and denser it at last
+becomes liquid, and then solid, and then it ceases to contract, as of
+course the particles of a solid body cannot fall freely toward the
+centre, as those of a gaseous body can. Our earth has long ago reached
+this stage. When solid the action ceases, and the heat is no more kept
+up by this source of energy, therefore the body begins to cool--surface
+first, and lastly the interior; it cools more quickly the smaller it is.
+Our moon has parted with all her heat long ago, while the earth still
+retains some internally. In the sun, therefore, we have an object-lesson
+of the stages through which all the planets must have passed. They have
+all once been glowing hot, and some may be still hot even on the
+surface, as we have seen there is reason to believe is the case with
+Jupiter.
+
+By this marvellous arrangement for the continued heat of the sun we can
+see that the warmth of our planets is assured for untold ages. There is
+no need to fear that the sun will wear out by burning. His brightness
+will continue for ages beyond the thoughts of man.
+
+Besides this, a few other things have been discovered about him. He is,
+of course, exceptionally difficult to observe; for though he is so
+large, which should make it easy, he is so brilliant that anyone
+regarding him through a telescope without the precaution of prepared
+glasses to keep off a great part of the light would be blinded at once.
+One most remarkable fact about the sun is that his surface is flecked
+with spots, which appear sometimes in greater numbers and sometimes in
+less, and the reason and shape of these spots have greatly exercised
+men's minds. Sometimes they are large enough to be seen without a
+telescope at all, merely by looking through a piece of smoked or
+coloured glass, which cuts off the most overpowering rays. When they are
+visible like this they are enormous, large enough to swallow many earths
+in their depths. At other times they may be observed by the telescope,
+then they may be about five thousand miles across. Sometimes one spot
+can be followed by an astronomer as it passes all across the sun,
+disappears at the edge, and after a lapse of time comes back again round
+the other edge. This first showed men that the sun, like all the
+planets, rotated on his axis, and gave them the means of finding out how
+long he took in doing so. But the spots showed a most surprising result,
+for they took slightly different times in making their journey round the
+sun, times which differed according to their position. For instance, a
+spot near the equator of the sun took twenty-five days to make the
+circuit, while one higher up or lower down took twenty-six days, and one
+further out twenty-seven; so that if these spots are, as certainly
+believed, actually on the surface, the conclusion is that the sun does
+not rotate all in one piece, but that some parts go faster than others.
+No one can really explain how this could be, but it is certainly more
+easily understood in the case of a body of gas than of a solid body,
+when it would be simply impossible to conceive. The spots seem to keep
+principally a little north and a little south of the equator; there are
+very few actually at it, and none found near the poles, but no reason
+for this distribution has been discovered. It has been noted that about
+every eleven years the greatest number of spots appears, and that
+they become fewer again, mounting up in number to the next eleven years,
+and so on. All these curious facts show there is much yet to be solved
+about the sun. The spots were supposed for long to be eruptions bursting
+up above the surface, but now they are generally held to be deep
+depressions like saucers, probably caused by violent tempests, and it is
+thought that the inrush of cooler matter from above makes them look
+darker than the other parts of the sun's surface. But when we use the
+words 'cooler' and 'darker,' we mean only by comparison, for in reality
+the dark parts of the spots are brighter than electric light.
+
+[Illustration: _Royal Observatory, Greenwich._
+
+SUN-SPOTS.]
+
+The fact that the spots are in reality depressions or holes is shown by
+their change of appearance as they pass over the face of the sun toward
+the edge; for the change of shape is exactly that which would be caused
+by foreshortening.
+
+It sounds odd to say that the best time for observing the sun is during
+a total eclipse, for then the sun's body is hidden by the moon. But yet
+to a certain extent this is true, and the reason is that the sun's own
+brilliance is our greatest hindrance in observing him, his rays are so
+dazzling that they light up our own atmosphere, which prevents us seeing
+the edges. Now, during a total eclipse, when nearly all the rays are
+cut off, we can see marvellous things, which are invisible at other
+times. But total eclipses are few and far between, and so when one is
+approaching astronomers make great preparations beforehand.
+
+A total eclipse is not visible from all parts of the world, but only
+from that small part on which the shadow of the moon falls, and as the
+earth travels, this shadow, which is really a round spot, passes along,
+making a dark band. In this band astronomers choose the best
+observatories, and there they take up their stations. The dark body of
+the moon first appears to cut a little piece out of the side of the sun,
+and as it sails on, gradually blotting out more and more, eager
+telescopes follow it; at last it covers up the whole sun, and then a
+marvellous spectacle appears, for all round the edges of the black moon
+are seen glorious red streamers and arches and filaments of marvellous
+shapes, continually changing. These are thrown against a background of
+pale green light that surrounds the black moon and the hidden sun. In
+early days astronomers thought these wonderful coloured streamers
+belonged to the moon; but it was soon proved that they really are part
+of the sun, and are only invisible at ordinary times, because our
+atmosphere is too bright to allow them to be seen. An instrument has
+now been invented to cut off most of the light of the sun, and when this
+is attached to a telescope these prominences, as they are called, can be
+seen at any time, so that there is no need to wait for an eclipse.
+
+[Illustration: THE EARTH AS IT WOULD APPEAR IN COMPARISON WITH THE
+FLAMES SHOOTING OUT FROM THE SUN.]
+
+What are these marvellous streamers and filaments? They are what they
+seem, eruptions of fiery matter discharged from the ever-palpitating sun
+thousands of miles into surrounding space. They are for ever shooting
+out and bursting and falling back, fireworks on a scale too enormous for
+us to conceive. Some of these brilliant flames extend for three hundred
+thousand miles, so that in comparison with one of them the whole world
+would be but a tiny ball, and this is going on day and night without
+cessation. Look at the picture where the artist has made a little black
+ball to represent the earth as she would appear if she could be seen in
+the midst of the flames shooting out from the sun. Do not make a mistake
+and think the earth really could be in this position; she is only shown
+there so that you may see how tiny she is in comparison with the sun.
+All the time you have lived and your father, and grandfather, and right
+back to the beginnings of English history, and far, far further into the
+dim ages, this stupendous exhibition of energy and power has continued,
+and only of late years has anyone known anything about it; even now a
+mere handful of people do know, and the rest, who are warmed and fed and
+kept alive by the gracious beams of this great revolving glowing
+fireball, never give it a thought.
+
+I said just now a pale green halo surrounded the sun, extending far
+beyond the prominences; this is called the corona and can only be seen
+during an eclipse. It surrounds the sun in a kind of shell, and there is
+reason to believe that it too is made of luminous stuff ejected by the
+sun in its burning fury. It is composed of large streamers or filaments,
+which seem to shoot out in all directions; generally these are not much
+larger than the apparent width of the sun, but sometimes they extend
+much further. The puzzle is, this corona cannot be an atmosphere in any
+way resembling that of our earth; for the gravitational force of the
+sun, owing to its enormous size, is so great that it would make any such
+atmosphere cling to it much more densely near to the surface, while it
+would be thinner higher up, and the corona is not dense in any way, but
+thin and tenuous throughout. This makes it very difficult to explain; it
+is supposed that some kind of electrical force enters into the problem,
+but what it is exactly we are far from knowing yet.
+
+
+
+
+CHAPTER VIII
+
+SHINING VISITORS
+
+
+Our solar system is set by itself in the midst of a great space, and so
+far as we have learnt about it in this book everything in it seems
+orderly: the planets go round the sun and the satellites go round the
+planets, in orbits more or less regular; there seems no place for
+anything else. But when we have considered the planets and the
+satellites, we have not exhausted all the bodies which own allegiance to
+the sun. There is another class, made up of strange and weird members,
+which flash in and out of the system, coming and going in all directions
+and at all times--sometimes appearing without warning, sometimes
+returning with a certain regularity, sometimes retiring to infinite
+depths of space, where no human eye will ever see them more. These
+strange visitors are called comets, and are of all shapes and sizes and
+never twice alike. Even as we watch them they grow and change, and then
+diminish in splendour. Some are so vast that men see them as flaming
+signs in the sky, and regard them with awe and wonder; some cannot be
+seen at all without the help of the telescope. From the very earliest
+ages those that were large enough to be seen without glasses have been
+regarded with astonishment. Men used to think that they were signs from
+heaven foretelling great events in the world. Timid people predicted
+that the end of the world would come by collision with one of them.
+Others, again, fancifully likened them to fishes in that sea of space in
+which we swim--fishes gigantic and terrifying, endowed with sense and
+will.
+
+It is perhaps unnecessary to say that comets are no more alive than is
+our own earth, and as for causing the end of the world by collision,
+there is every reason to believe the earth has been more than once right
+through a comet's tail, and yet no one except scientific men even
+discovered it. These mysterious visitors from the outer regions of space
+were called comets from a Greek word signifying hair, for they often
+leave a long luminous trail behind, which resembles the filaments of a
+woman's hair. It is not often that one appears large and bright enough
+to be seen by the naked eye, and when it does it is not likely to be
+soon forgotten. In the year 1910 such a comet is expected, a comet
+which at its former appearance compelled universal attention by its
+brilliancy and strangeness. At the time of the Norman Conquest of
+England a comet believed to be the very same one was stretching its
+glorious tail half across the sky, and the Normans seeing it, took it as
+a good omen, fancying that it foretold their success. The history of the
+Norman Conquest was worked in tapestry--that is to say, in what we
+should call crewels on a strip of linen--and in this record the comet
+duly appears. Look at him in the picture as the Normans fancied him. He
+has a red head with blue flames starting from it, and several tails. The
+little group of men on the left are pointing and chattering about him.
+We can judge what an impression this comet must have made to be recorded
+in such an important piece of work.
+
+[Illustration: THE COMET IN THE BAYEUX TAPESTRY.]
+
+But we are getting on too fast. We have yet to learn how anyone can know
+that the comet which appeared at the time of the Norman Conquest is the
+same as that which has come back again at different times, and above
+all, how anyone can tell that it will come again in the year 1910. All
+this involves a long story.
+
+Before the invention of telescopes of course only those comets could be
+seen which were of great size and fine appearance. In those days men
+did not realize that our world was but one of a number and of no great
+importance except to ourselves, and they always took these blazing
+appearances in the heavens as a particular warning to the human race.
+But when astronomers, by the aid of the telescope, found that for one
+comet seen by the eye there were hundreds which no mortal eye unaided
+could see, this idea seemed, to say the least of it, unlikely. Yet even
+then comets were looked upon as capricious visitors from outer space;
+odd creatures drawn into our system by the attraction of the sun, who
+disappeared, never to return. It was Newton, the same genius who
+disclosed to us the laws of gravity, who first declared that comets
+moved in orbits, only that these orbits were far more erratic than any
+of those followed by the planets.
+
+So far we have supposed that the planets were all on what we should call
+a level--that is to say, we have regarded them as if they were floating
+in a sea of water around the sun; but this is only approximately
+correct, for the orbits of the planets are not all at one level. If you
+had a number of slender hoops or rings to represent the planetary
+orbits, you would have to tilt one a little this way and another a
+little that way, only never so far but that a line through the centre
+of the hoop from one side to another could pass through the sun. The way
+in which the planetary orbits are tilted is slight in comparison with
+that of the orbits of comets, for these are at all sorts of angles--some
+turned almost sideways, and others slanting, and all of them are
+ellipses long drawn out and much more irregular than the planetary
+orbits; but erratic as they are, in every case a line drawn through the
+sun and extended both ways would touch each side of the orbits.
+
+A great astronomer called Halley, who was born in the time of the
+Commonwealth, was lucky enough to see a very brilliant comet, and the
+sight interested him so much that he made all the calculations necessary
+to find out just in what direction it was travelling in the heavens. He
+found out that it followed an ellipse which brought it very near to the
+sun at one part of its journey, and carried it far beyond the orbit of
+the earth, right out to that of Neptune, at the other. Then he began to
+search the records for other comets which had been observed before his
+time. He found that two particularly bright ones had been carefully
+noted--one about seventy-five years before that which he had seen, and
+the other seventy-five years before that again. Both these comets had
+been watched so scientifically that the paths in which they had
+travelled could be computed. A brilliant inspiration came to Halley. He
+believed that instead of these three, his own and the other two, being
+different comets, they were the same one, which returned to the sun
+about every seventy-five years. This could be proved, for if this idea
+were correct, of course the comet would return again in another
+seventy-five years, unless something unforeseen occurred. But Halley was
+in the prime of life: he could not hope to live to see his forecast
+verified. The only thing he could do was to note down exact particulars,
+by means of which others who lived after him might recognize his comet.
+And so when the time came for its return, though Halley was in his
+grave, numbers of astronomers were watching eagerly to see the
+fulfilment of his prediction. The comet did indeed appear, and since
+then it has been seen once again, and now we expect it to come back in
+the year 1910, when you and I may see it for ourselves. When the
+identity of the comet was fully established men began to search further
+back still, to compare the records of other previous brilliant comets,
+and found that this one had been noticed many times before, and once as
+I said, at the time of the Norman Conquest. Halley's comet is peculiar
+in many ways. For instance, it is unusual that so large and interesting
+a comet should return within a comparatively limited time. It is the
+smaller comets, those that can only be seen telescopically, that usually
+run in small orbits. The smallest orbits take about three and a half
+years to traverse, and some of the largest orbits known require a period
+of one hundred and ten thousand years. Between these two limits lies
+every possible variety of period. One comet, seen about the time
+Napoleon was born, was calculated to take two thousand years to complete
+its journey, and another, a very brilliant one seen in 1882, must
+journey for eight hundred years before it again comes near to the sun.
+But we never know what might happen, for at any moment a comet which has
+traversed a long solitary pathway in outer darkness may flash suddenly
+into our ken, and be for the first time noted and recorded, before
+flying off at an angle which must take it for ever further and further
+from the sun.
+
+Everything connected with comets is mysterious and most fascinating.
+From out of the icy regions of space a body appears; what it is we know
+not, but it is seen at first as a hairy or softly-glowing star, and it
+was thus that Herschel mistook Uranus for a comet when he first
+discovered it. As it draws nearer the comet sends out some fan-like
+projections toward the sun, enclosing its nucleus in filmy wrappings
+like a cocoon of light, and it travels faster and faster. From its head
+shoots out a tail--it may be more than one--growing in splendour and
+width, and always pointing away from the sun. So enormous are some of
+these tails that when the comet's head is close to the sun the tail
+extends far beyond the orbit of the earth. Faster still and faster flies
+the comet, for as we have seen it is a consequence of the law of
+gravitation that the nearer planets are to the sun the faster they move
+in their orbits, and the same rule applies to comets too. As the comet
+dashes up to the sun his pace becomes something indescribable; it has
+been reckoned for some comets at three hundred miles a second! But
+behold, as the head flies round the sun the tail is always projected
+outwards. The nucleus or head may be so near to the sun that the heat it
+receives would be sufficient to reduce molten iron to vapour; but this
+does not seem to affect it: only the tail expands. Sometimes it becomes
+two or more tails, and as it sweeps round behind the head it has to
+cover a much greater space in the same time, and therefore it must
+travel even faster than the head. The pace is such that no calculations
+can account for it, if the tail is composed of matter in any sense as we
+know it. Then when the sun is passed the comet sinks away again, and as
+it goes the tail dies down and finally disappears. The comet itself
+dwindles to a hairy star once more and goes--whither? Into space so
+remote that we cannot even dream of it--far away into cold more
+appalling than anything we could measure, the cold of absolute space.
+More and more slowly it travels, always away and away, until the sun, a
+short time back a huge furnace covering all the sky, is now but a faint
+star. Thus on its lonely journey unseen and unknown the comet goes.
+
+This comet which we have taken as an illustration is a typical one, but
+all are not the same. Some have no tails at all, and never develop any;
+some change utterly even as they are watched. The same comet is so
+different at different times that the only possible way of identifying
+it is by knowing its path, and even this is not a certain method, for
+some comets appear to travel at intervals along the same path!
+
+Now we come to the question that must have been in the mind of everyone
+from the beginning of this chapter, What are comets? This question no
+one can answer definitely, for there are many things so puzzling about
+these strange appearances that it is difficult even to suggest an
+explanation. Yet a good deal is known. In the first place, we are
+certain that comets have very little density--that is to say, they are
+indescribably thin, thinner than the thinnest kind of gas; and air,
+which we always think so thin, would be almost like a blanket compared
+with the material of comets. This we judge because they exercise no sort
+of influence on any of the planetary bodies they draw near to, which
+they certainly would do if they were made of any kind of solid matter.
+They come sometimes very close to some of the planets. A comet was so
+near to Jupiter that it was actually in among his moons. The comet was
+violently agitated; he was pulled in fact right out of his old path, and
+has been going on a new one ever since; but he did not exercise the
+smallest effect on Jupiter, or even on the moons. And, as I said earlier
+in this chapter, we on the earth have been actually in the folds of a
+comet's tail. This astonishing fact happened in June, 1861. One evening
+after the sun had set a golden-yellow disc, surrounded with filmy
+wrappings, appeared in the sky. The sun's light, diffused throughout
+our atmosphere, had prevented its being seen sooner. This was apparently
+the comet's head. It is described as 'though a number of light, hazy
+clouds were floating around a miniature full moon.' From this a cone of
+light extended far up into the sky, and when the head disappeared below
+the horizon this tail was seen to reach to the zenith. But that was not
+all. Strange shafts of light seemed to hang right overhead, and could
+only be accounted for by supposing that they were caused by another tail
+hanging straight above us, so that we looked up at it foreshortened by
+perspective. The comet's head lay between the earth and the sun, and its
+tail, which extended over many millions of miles, stretched out behind
+in such a way that the earth must have gone right through it. The fact
+that the comet exercised no perceptible influence on the earth at all,
+and that there were not even any unaccountable magnetic storms or
+displays of electricity, may reassure us so that if ever we do again
+come in contact with one of these extremely fine, thin bodies, we need
+not be afraid.
+
+There is another way in which we can judge of the wonderful tenuity or
+thinness of comets--that is, that the smallest stars can be seen
+through their tails, even though those tails must be many thousands of
+miles in thickness. Now, if the tails were anything approaching the
+density of our own atmosphere, the stars when seen through them would
+appear to be moved out of their places. This sounds odd, and requires a
+word of explanation. The fact is that anything seen through any
+transparent medium like water or air is what is called refracted--that
+is to say, the rays coming from it look bent. Everyone is quite familiar
+with this in everyday life, though perhaps they may not have noticed it.
+You cannot thrust a stick into the water without seeing that it looks
+crooked. Air being less dense than water has not quite so strong a
+refracting power, but still it has some. We cannot prove it in just the
+same way, because we are all inside the atmosphere ourselves, and there
+is no possibility of thrusting a stick into it from the outside! The
+only way we know it is by looking at something which is 'outside'
+already, and we find plenty of objects in the sky. As a matter of fact,
+the stars are all a little pulled out of their places by being seen
+through the air, and though of course we do not notice this, astronomers
+know it and have to make allowance for it. The effect is most
+noticeable in the case of the sun when he is going down, for the
+atmosphere bends his rays up, and though we see him a great glowing red
+ball on the horizon, and watch him, as we think, drop gradually out of
+sight, we are really looking at him for the last moment or two when he
+has already gone, for the rays are bent up by the air and his image
+lingers when the real sun has disappeared.
+
+[Illustration: A STICK THRUST INTO THE WATER APPEARS CROOKED.]
+
+Therefore in looking through the luminous stuff that forms a comet's
+tail astronomers might well expect to see the stars displaced, but not a
+sign of this appears. It is difficult to imagine, therefore, what the
+tail can be made of. The idea is that the sun exercises a sort of
+repulsive effect on certain elements found in the comet's head--that is
+to say, it pushes them away, and that as the head approaches the sun,
+these elements are driven out of it away from the sun in vapour. This
+action may have something to do with electricity, which is yet little
+understood; anyway, the effect is that, instead of attracting the matter
+toward itself, in which case we should see the comet's tails stretching
+toward the sun, the sun drives it away! In the chapter on the sun we had
+to imagine something of the same kind to account for the corona, and the
+corona and the comet's tails may be really akin to each other, and
+could perhaps be explained in the same way. Now we come to a stranger
+fact still. Some comets go right through the sun's corona, and yet do
+not seem to be influenced by it in the smallest degree. This may not
+seem very wonderful at first perhaps, but if you remember that a dash
+through anything so dense as our atmosphere, at a pace much less than
+that at which a comet goes, is enough to heat iron to a white heat, and
+then make it fly off in vapour, we get a glimpse of the extreme fineness
+of the materials which make the corona.
+
+Here is Herschel's account of a comet that went very near the sun:
+
+'The comet's distance from the sun's centre was about the 160th part of
+our distance from it. All the heat we enjoy on this earth comes from the
+sun. Imagine the heat we should have to endure if the sun were to
+approach us, or we the sun, to one 160th part of its present distance.
+It would not be merely as if 160 suns were shining on us all at once,
+but, 160 times 160, according to a rule which is well known to all who
+are conversant with such matters. Now, that is 25,600. Only imagine a
+glare 25,600 times fiercer than that of the equatorial sunshine at noon
+day with the sun vertical. In such a heat there is no substance we know
+of which would not run like water, boil, and be converted into smoke or
+vapour. No wonder the comet gave evidence of violent excitement, coming
+from the cold region outside the planetary system torpid and ice-bound.
+Already when arrived even in our temperate region it began to show signs
+of internal activity; the head had begun to develop, and the tail to
+elongate, till the comet was for a time lost sight of--not for days
+afterwards was it seen; and its tail, whose direction was reversed, and
+which could not possibly be the same tail it had before, had already
+lengthened to an extent of about ninety millions of miles, so that it
+must have been shot out with immense force in a direction away from the
+sun.'
+
+We remember that comets have sometimes more than one tail, and a theory
+has been advanced to account for this too. It is supposed that perhaps
+different elements are thrust away by the sun at different angles, and
+one tail may be due to one element and another to another. But if the
+comet goes on tail-making to a large extent every time it returns to the
+sun, what happens eventually? Do the tails fall back again into the head
+when out of reach of the sun's action? Such an idea is inconceivable;
+but if not, then every time a comet approaches the sun he loses
+something, and that something is made up of the elements which were
+formerly in the head and have been violently ejected. If this be so we
+may well expect to see comets which have returned many times to the sun
+without tails at all, for all the tail-making stuff that was in the head
+will have been used up, and as this is exactly what we do see, the
+theory is probably true.
+
+Where do the comets come from? That also is a very large question. It
+used to be supposed they were merely wanderers in space who happened to
+have been attracted by our sun and drawn into his system, but there are
+facts which go very strongly against this, and astronomers now generally
+believe that comets really belong to the solar system, that their proper
+orbits are ellipses, and that in the case of those which fly off at such
+an angle that they can never return they must at some time have been
+pulled out of their original orbit by the influence of one of the
+planets.
+
+[Illustration: _Royal Observatory, Cape of Good Hope._
+
+A GREAT COMET.]
+
+To get a good idea of a really fine comet, until we have the opportunity
+of seeing one for ourselves, we cannot do better than look at this
+picture of a comet photographed in 1901 at the Cape of Good Hope. It is
+only comparatively recently that photography has been applied to comets.
+When Halley's comet appeared last time such a thing was not thought
+of, but when he comes again numbers of cameras, fitted up with all the
+latest scientific appliances, will be waiting to get good impressions of
+him.
+
+
+
+
+CHAPTER IX
+
+SHOOTING STARS AND FIERY BALLS
+
+
+All the substances which we are accustomed to see and handle in our
+daily lives belong to our world. There are vegetables which grow in the
+earth, minerals which are dug out of it, and elementary things, such as
+air and water, which have always made up a part of this planet since man
+knew it. These are obvious, but there are other things not quite so
+obvious which also help to form our world. Among these we may class all
+the elements known to chemists, many of which have difficult names, such
+as oxygen and hydrogen. These two are the elements which make up water,
+and oxygen is an important element in air, which has nitrogen in it too.
+There are numbers and numbers of other elements perfectly familiar to
+chemists, of which many people never even hear the names. We live in the
+midst of these things, and we take them for granted and pay little
+attention to them; but when we begin to learn about other worlds we at
+once want to know if these substances and elements which enter so
+largely into our daily lives are to be found elsewhere in the universe
+or are quite peculiar to our own world. This question might be answered
+in several ways, but one of the most practical tests would be if we
+could get hold of something which had not been always on the earth, but
+had fallen upon it from space. Then, if this body were made up of
+elements corresponding with those we find here, we might judge that
+these elements are very generally diffused throughout the bodies in the
+solar system.
+
+It sounds in the highest degree improbable that anything should come
+hurling through the air and alight on our little planet, which we know
+is a mere speck in a great ocean of space; but we must not forget that
+the power of gravity increases the chances greatly, for anything coming
+within a certain range of the earth, anything small enough, that is, and
+not travelling at too great a pace, is bound to fall on to it. And,
+however improbable it seems, it is undoubtedly true that masses of
+matter do crash down upon the earth from time to time, and these are
+called meteorites. When we think of the great expanse of the oceans, of
+the ice round the poles, and of the desert wastes, we know that for
+every one of such bodies seen to fall many more must have fallen unseen
+by any human being. Meteors large enough to reach the earth are not very
+frequent, which is perhaps as well, and as yet there is no record of
+anyone's having been killed by them. Most of them consist of masses of
+stone, and a few are of iron, while various substances resembling those
+that we know here have been found in them. Chemists in analyzing them
+have also come across certain elements so far unknown upon earth, though
+of course there is no saying that these may not exist at depths to which
+man has not penetrated.
+
+A really large meteor is a grand sight. If it is seen at night it
+appears as a red star, growing rapidly bigger and leaving a trail of
+luminous vapour behind as it passes across the sky. In the daytime this
+vapour looks like a cloud. As the meteor hurls itself along there may be
+a deep continuous roar, ending in one supreme explosion, or perhaps in
+several explosions, and finally the meteor may come to the earth in one
+mass, with a force so great that it buries itself some feet deep in the
+soil, or it may burst into numbers of tiny fragments, which are
+scattered over a large area. When a meteor is found soon after its fall
+it is very hot, and all its surface has 'run,' having been fused by
+heat. The heat is caused by the friction of our atmosphere. The meteor
+gets entangled in the atmosphere, and, being drawn by the attraction of
+the earth, dashes through it. Part of the energy of its motion is turned
+to heat, which grows greater and greater as the denser air nearer to the
+earth is encountered; so that in time all the surface of the meteor runs
+like liquid, and this liquid, rising to a still higher temperature, is
+blown off in vapour, leaving a new surface exposed. The vapour makes the
+trail of fire or cloud seen to follow the meteor. If the process went on
+for long the meteor would be all dissipated in vapour, and in any case
+it must reach the earth considerably reduced in size.
+
+Numbers and numbers of comparatively small ones disappear, and for every
+one that manages to come to earth there must be hundreds seen only as
+shooting stars, which vanish and 'leave not a wrack behind.' When a
+meteor is seen to fall it is traced, and, whenever possible, it is found
+and placed in a museum. Men have sometimes come across large masses of
+stone and iron with their surfaces fused with heat. These are in every
+way like the recognized meteorites, except that no eye has noted their
+advent. As there can be no reasonable doubt that they are of the same
+origin as the others, they too are collected and placed in museums, and
+in any large museum you would be able to see both kinds--those which
+have been seen to come to earth and those which have been found
+accidentally.
+
+The meteors which appear very brilliant in their course across the sky
+are sometimes called fire-balls, which is only another name for the same
+thing. Some of these are brighter than the full moon, so bright that
+they cause objects on earth to cast a shadow. In 1803 a fiery ball was
+noticed above a small town in Normandy; it burst and scattered stones
+far and wide, but luckily no one was hurt. The largest meteorites that
+have been found on the earth are a ton or more in weight; others are
+mere stones; and others again just dust that floats about in the
+atmosphere before gently settling. Of course, meteors of this last kind
+could not be seen to fall like the larger ones, yet they do fall in such
+numbers that calculations have been made showing that the earth must
+catch about a hundred millions of meteors daily, having altogether a
+total weight of about a hundred tons. This sounds enormous, but
+compared with the weight of the earth it is very small indeed.
+
+Now that we have arrived at the fact that strange bodies do come
+hurtling down upon us out of space, and that we can actually handle and
+examine them, the next question is, Where do they come from? At one time
+it was thought that they were fragments which had been flung off by the
+earth herself when she was subject to violent explosions, and that they
+had been thrown far enough to resist the impulse to drop down upon her
+again, and had been circling round the sun ever since, until the earth
+came in contact with them again and they had fallen back upon her. It is
+not difficult to imagine a force which would be powerful enough to
+achieve the feat of speeding something off at such a velocity that it
+passed beyond the earth's power to pull it back, but nothing that we
+have on earth would be nearly strong enough to achieve such a feat.
+Imaginative writers have pictured a projectile hurled from a cannon's
+mouth with such tremendous force that it not only passed beyond the
+range of the earth's power to pull it back, but so that it fell within
+the influence of the moon and was precipitated on to her surface! Such
+things must remain achievements in imagination only; it is not possible
+for them to be carried out. Other ideas as to the origin of meteors
+were that they had been expelled from the moon or from the sun. It would
+need a much less force to send a projectile away from the moon than from
+the earth on account of its smaller size and less density, but the
+distance from the earth to the moon is not very great, and any
+projectile hurled forth from the moon would cross it in a comparatively
+short time. Therefore if the meteorites come from the moon, the moon
+must be expelling them still, and we might expect to see some evidence
+of it; but we know that the moon is a dead world, so this explanation is
+not possible. The sun, for its part, is torn by such gigantic
+disturbances that, notwithstanding its vast size, there is no doubt
+sufficient force there to send meteors even so far as the earth, but the
+chances of their encountering the earth would be small. Both these
+theories are now discarded. It is believed that the meteors are merely
+lesser fragments of the same kind of materials as the planets, circling
+independently round the sun; and a proof of this is that far more
+meteorites fall on that part of the earth which is facing forward in its
+journey than on that behind, and this is what we should expect if the
+meteors were scattered independently through space and it was by reason
+of our movements that we came in contact with them. There is no need to
+explain this further. Everyone knows that in cycling or driving along a
+road where there is a good deal of traffic both ways the people we meet
+are more in number than those who overtake us, and the same result would
+follow with the meteors; that is to say, in travelling through space
+where they were fairly evenly distributed we should meet more than we
+should be overtaken by.
+
+You remember that it was suggested the sun's fuel might be obtained from
+meteors, and this was proved to be not possible, even though there are
+no doubt unknown millions of these strange bodies circling throughout
+the solar system.
+
+There are so many names for these flashing bodies that we may get a
+little confused: when they are seen in the sky they are meteors, or
+fire-balls; when they reach the earth they are called meteorites, and
+also aerolites. Then there is another class of the same bodies called
+shooting stars, and these are in reality only meteors on a smaller
+scale; but there ought to be no confusion in our thoughts, for all these
+objects are small bodies travelling round the sun, and caught by the
+earth's influence.
+
+When you watch the sky for some time on a clear night, you will seldom
+fail to see at least one star flash out suddenly in a path of thrilling
+light and disappear, and you cannot be certain whether that star had
+been shining in the sky a minute before, or if it had appeared suddenly
+only in order to go out. The last idea is right. We must get rid at once
+of the notion that it would be possible for any fixed star to behave in
+this manner. To begin with, the fixed stars are many of them actually
+travelling at a great velocity at present, yet so immeasurably distant
+are they that their movement makes no perceptible difference to us. For
+one of them to appear to dash across the heavens as a meteor does would
+mean a velocity entirely unknown to us, even comparing it with the speed
+of light. No, these shooting stars are not stars at all, though they
+were so named, long before the real motions of the fixed stars were even
+dimly guessed at. As we have seen, they belong to the same class as
+meteors.
+
+I remember being told by a clergyman, years ago, that one night in
+November he had gone up to bed very late, and as he pulled up his blind
+to look at the sky, to his amazement he saw a perfect hail of shooting
+stars, some appearing every minute, and all darting in vivid trails of
+light, longer or shorter, though all seemed to come from one point. So
+marvellous was the sight that he dashed across the village street,
+unlocked the church door, and himself pulled the bell with all his
+might. The people in that quiet country village had long been in bed,
+but they huddled on their clothes and ran out of their pretty thatched
+cottages, thinking there must be a great fire, and when they saw the
+wonder in the sky they were amazed and cried out that the world must be
+coming to an end. The clergyman knew better than that, and was able to
+reassure them, and tell them he had only taken the most effectual means
+of waking them so that they might not miss the display, for he was sure
+as long as they lived they would never see such another sight. A star
+shower of this kind is certainly well worth getting up to see, but
+though uncommon it is not unique. There are many records of such showers
+having occurred in times gone by, and when men put together and examined
+the records they found that the showers came at regular intervals. For
+instance, every year about the same time in November there is a star
+shower, not comparable, it is true, with the brilliant one the clergyman
+saw, but still noticeable, for more shooting stars are seen then than at
+other times, and once in every thirty-three years there is a specially
+fine one. It happened in fact to be one of these that the village people
+were wakened up to see.
+
+Not all at once, but gradually, the mystery of these shower displays was
+solved. It was realized that the meteors need not necessarily come from
+one fixed place in the sky because they seemed to us to do so, for that
+was only an effect of perspective. If you were looking down a long,
+perfectly straight avenue of tree-trunks, the avenue would seem to close
+in, to get narrower and narrower at the far end until it became a point;
+but it would not really do so, for you would know that the trees at the
+far end were just the same distance from each other as those between
+which you were standing. Now, two meteors starting from the same
+direction at a distance from each other, and keeping parallel, would
+seem to us to start from a point and to open out wider and wider as they
+approached, but they would not really do so; it would only be, as in the
+case of the avenue, an effect of perspective. If a great many meteors
+did the same thing, they would appear to us all to start from one point,
+whereas really they would be on parallel lines, only as they rushed to
+meet us or we rushed to meet them this effect would be produced.
+Therefore the first discovery was that these meteors were thousands and
+thousands of little bodies travelling in lines parallel to each other,
+like a swarm of little planets. To judge that their path was not a
+straight line but a circle or ellipse was the next step, and this was
+found to be the case. From taking exact measurements of their paths in
+the sky an astronomer computed they were really travelling round the sun
+in a lengthened orbit, an ellipse more like a comet's orbit than that of
+a planet. But next came the puzzling question, Why did the earth
+apparently hit them every year to some extent, and once in thirty-three
+years seem to run right into the middle of them? This also was answered.
+One has only to imagine a swarm of such meteors at first hastening
+busily along their orbit, a great cluster all together, then, by the
+near neighbourhood of some planet, or by some other disturbing causes,
+being drawn out, leaving stragglers lagging behind, until at last there
+might be some all round the path, but only thinly scattered, while the
+busy, important cluster that formed the nucleus was still much thicker
+than any other part. Now, if the orbit that the meteors followed cut the
+orbit or path of the earth at one point, then every time the earth came
+to what we may call the level crossing she must run into some of the
+stragglers, and if the chief part of the swarm took thirty-three years
+to get round, then once in about thirty-three years the earth must
+strike right into it. This would account for the wonderful display. So
+long drawn-out is the thickest part of the swarm that it takes a year to
+pass the points at the level crossing. If the earth strikes it near the
+front one year, she may come right round in time to strike into the rear
+part of the swarm next year, so that we may get fine displays two years
+running about every thirty-three years. The last time we passed through
+the swarm was in 1899, and then the show was very disappointing. Here in
+England thick clouds prevented our seeing much, and there will not be
+another chance for us to see it at its best until 1932.
+
+These November meteors are called Leonids, because they _seem_ to come
+from a group of stars named Leo, and though the most noticeable they are
+not the only ones. A shower of the same kind occurs in August too, but
+the August meteors, called Perseids, because they seem to come from
+Perseus, revolve in an orbit which takes a hundred and forty-two years
+to traverse! So that only every one hundred and forty-second year could
+we hope to see a good display. When all these facts had been gathered
+up, it seemed without doubt that certain groups of meteors travelled in
+company along an elliptical orbit. But there remained still something
+more--a bold and ingenious theory to be advanced. It was found that a
+comet, a small one, only to be seen with the telescope, revolved in
+exactly the same orbit as the November meteors, and another one, larger,
+in exactly the same orbit as the August ones; hence it could hardly be
+doubted that comets and meteors had some connection with each other,
+though what that connection is exactly no one knows. Anyway, we can have
+no shadow of doubt when we find the comet following a marked path, and
+the meteors pursuing the same path in his wake, that the two have some
+mysterious affinity. There are other smaller showers besides these of
+November and August, and a remarkable fact is known about one of them.
+This particular stream was found to be connected with a comet named
+Biela's Comet, that had been many times observed, and which returned
+about every seven years to the sun. After it had been seen several
+times, this astonishing comet split in two and appeared as two comets,
+both of which returned at the end of the next seven years. But on the
+next two occasions when they were expected they never came at all, and
+the third time there came instead a fine display of shooting stars, so
+it really seemed as if these meteors must be the fragments of the lost
+comet.
+
+It is very curious and interesting to notice that in these star showers
+there is no certain record of any large meteorite reaching the earth;
+they seem to be made up of such small bodies that they are all
+dissipated in vapour as they traverse our air.
+
+
+
+
+CHAPTER X
+
+THE GLITTERING HEAVENS
+
+
+On a clear moonless night the stars appear uncountable. You see them
+twinkling through the leafless trees, and covering all the sky from the
+zenith, the highest point above your head, down to the horizon. It seems
+as if someone had taken a gigantic pepper-pot and scattered them far and
+wide so that some had fallen in all directions. If you were asked to
+make a guess as to how many you can see at one time, no doubt you would
+answer 'Millions!' But you would be quite wrong, for the number of stars
+that can be seen at once without a telescope does not exceed two
+thousand, and this, after the large figures we have been dealing with,
+appears a mere trifle. With a telescope, even of small power, many more
+are revealed, and every increase in the size of the telescope shows more
+still; so that it might be supposed the universe is indeed illimitable,
+and that we are only prevented from seeing beyond a certain point by our
+limited resources. But in reality we know that this cannot be so. If
+the whole sky were one mass of stars, as it must be if the number of
+them were infinite, then, even though we could not distinguish the
+separate items, we should see it bright with a pervading and diffused
+light. As this is not so, we judge that the universe is not unending,
+though, with all our inventions, we may never be able to probe to the
+end of it. We need not, indeed, cry for infinity, for the distances of
+the fixed stars from us are so immeasurable that to atoms like ourselves
+they may well seem unlimited. Our solar system is set by itself, like a
+little island in space, and far, far away on all sides are other great
+light-giving suns resembling our own more or less, but dwindled to the
+size of tiny stars, by reason of the great void of space lying between
+us and them. Our sun is, indeed, just a star, and by no means large
+compared with the average of the stars either. But, then, he is our own;
+he is comparatively near to us, and so to us he appears magnificent and
+unique. Judging from the solar system, we might expect to find that
+these other great suns which we call stars have also planets circling
+round them, looking to them for light and heat as we do to our sun.
+There is no reason to doubt that in some instances the conjecture is
+right, and that there may be other suns with attendant planets. It is
+however a great mistake to suppose that because our particular family in
+the solar system is built on certain lines, all the other families must
+be made on the same pattern. Why, even in our own system we can see how
+very much the planets differ from each other: there are no two the same
+size; some have moons and some have not; Saturn's rings are quite
+peculiar to himself, and Uranus and Neptune indulge in strange vagaries.
+So why should we expect other systems to be less varied?
+
+As science has advanced, the idea that these faraway suns must have
+planetary attendants as our sun has been discarded. The more we know the
+more is disclosed to us the infinite variety of the universe. For
+instance, so much accustomed are we to a yellow sun that we never think
+of the possibility of there being one of another colour. What would you
+say then to a ruby sun, or a blue one; or to two suns of different
+colours, perhaps red and green, circling round each other; or to two
+such suns each going round a dark companion? For there are dark bodies
+as well as shining bodies in the sky. These are some of the marvels of
+the starry sky, marvels quite as absorbing as anything we have found in
+the solar system.
+
+It requires great care and patience and infinite labour before the very
+delicate observations which alone can reveal to us anything of the
+nature of the fixed stars can be accomplished. It is only since the
+improvement in large telescopes that this kind of work has become
+possible, and so it is but recently men have begun to study the stars
+intimately, and even now they are baffled by indescribable difficulties.
+One of these is our inability to tell the distance of a thing by merely
+looking at it unless we also know its size. On earth we are used to
+seeing things appear smaller the further they are from us, and by long
+habit can generally tell the real size; but when we turn to the stars,
+which appear so much alike, how are we to judge how far off they are?
+Two stars apparently the same size and close together in the sky may
+really be as far one from another as the earth is from the nearest; for
+if the further one were very much larger than the nearer, they would
+then appear the same size.
+
+At first it was natural enough to suppose that the big bright stars of
+what we call the first magnitude were the nearest to us, and the less
+bright the next nearest, and so on down to the tiny ones, only revealed
+by the telescope, which would be the furthest away of all; but research
+has shown that this is not correct. Some of the brightest stars may be
+comparatively near, and some of the smallest may be near also. The size
+is no test of distance. So far as we have been able to discover, the
+star which seems nearest _is_ a first magnitude one, but some of the
+others which outshine it must be among the infinitely distant ones. Thus
+we lie in the centre of a jewelled universe, and cannot tell even the
+size of the jewels which cover its radiant robe.
+
+I say 'lie,' but that is really not the correct word. So far as we have
+been able to find out, there is no such thing as absolute rest in the
+universe--in fact, it is impossible; for even supposing any body could
+be motionless at first, it would be drawn by the attraction of its
+nearest neighbours in space, and gradually gain a greater and greater
+velocity as it fell toward them. Even the stars we call 'fixed' are all
+hurrying along at a great pace, and though their distance prevents us
+from seeing any change in their positions, it can be measured by
+suitable instruments. Our sun is no exception to this universal rule.
+Like all his compeers, he is hurrying busily along somewhere in
+obedience to some impulse of which we do not know the nature; and as he
+goes he carries with him his whole cortege of planets and their
+satellites, and even the comets. Yes, we are racing through space with
+another motion, too, besides those of rotation and revolution, for our
+earth keeps up with its master attractor, the sun. It is difficult, no
+doubt, to follow this, but if you think for a moment you will remember
+that when you are in a railway-carriage everything in that carriage is
+really travelling along with it, though it does not appear to move. And
+the whole solar system may be looked at as if it were one block in
+movement. As in a carriage, the different bodies in it continue their
+own movements all the time, while sharing in the common movement. You
+can get up and change your seat in the train, and when you sit down
+again you have not only moved that little way of which you are
+conscious, but a great way of which you are not conscious unless you
+look out of the window. Now in the case of the earth's own motion we
+found it necessary to look for something which does not share in that
+motion for purposes of comparison, and we found that something in the
+sun, who shows us very clearly we are turning on our axis. But in the
+case of the motion of the solar system the sun is moving himself, so we
+have to look beyond him again and turn to the stars for confirmation.
+Then we find that the stars have motions of their own, so that it is
+very difficult to judge by them at all. It is as if you were bicycling
+swiftly towards a number of people all walking about in different
+directions on a wide lawn. They have their movements, but they all also
+have an apparent movement, really caused by you as you advance toward
+them; and what astronomers had to do was to separate the true movements
+of the stars from the false apparent movement made by the advance of the
+sun. This great problem was attacked and overcome, and it is now known
+with tolerable certainty that the sun is sweeping onward at a pace of
+about twelve miles a second toward a fixed point. It really matters very
+little to us where he is going, for the distances are so vast that
+hundreds of years must elapse before his movement makes the slightest
+difference in regard to the stars. But there is one thing which we can
+judge, and that is that though his course appears to be in a straight
+line, it is most probably only a part of a great curve so huge that the
+little bit we know seems straight.
+
+When we speak of the stars, we ought to keep quite clearly in our minds
+the fact that they lie at such an incredible distance from us that it is
+probable we shall never learn a great deal about them. Why, men have not
+even yet been able to communicate with the planet Mars, at its nearest
+only some thirty-five million miles from us, and this is a mere nothing
+in measuring the space between us and the stars. To express the
+distances of the stars in figures is really a waste of time, so
+astronomers have invented another way. You know that light can go round
+the world eight times in a second; that is a speed quite beyond our
+comprehension, but we just accept it. Then think what a distance it
+could travel in an hour, in a day; and what about a year? The distance
+that light can travel in a year is taken as a convenient measure by
+astronomers for sounding the depths of space. Measured in this way light
+takes four years and four months to reach us from the nearest star we
+know of, and there are others so much more distant that hundreds--nay,
+thousands--of years would have to be used to convey it. Light which has
+been travelling along with a velocity quite beyond thought, silently,
+unresting, from the time when the Britons lived and ran half naked on
+this island of ours, has only reached us now, and there is no limit to
+the time we may go back in our imaginings. We see the stars, not as they
+are, but as they were. If some gigantic conflagration had happened a
+hundred years ago in one of them situated a hundred light-years away
+from us, only now would that messenger, swifter than any messenger we
+know, have brought the news of it to us. To put the matter in figures,
+we are sure that no star can lie nearer to us than twenty-five billions
+of miles. A billion is a million millions, and is represented by a
+figure with twelve noughts behind it, so--1,000,000,000,000; and
+twenty-five such billions is the least distance within which any star
+can lie. How much farther away stars may be we know not, but it is
+something to have found out even that. On the same scale as that we took
+in our first example, we might express it thus: If the earth were a
+greengage plum at a distance of about three hundred of your steps from
+the sun, and Neptune were, on the same scale, about three miles away,
+the nearest fixed star could not be nearer than the distance measured
+round the whole earth at the Equator!
+
+All this must provoke the question, How can anyone find out these
+things? Well, for a long time the problem of the distances of the stars
+was thought to be too difficult for anyone to attempt to solve it, but
+at last an ingenious method was devised, a method which shows once more
+the triumph of man's mind over difficulties. In practice this method is
+extremely difficult to carry out, for it is complicated by so many other
+things which must be made allowance for; but in theory, roughly
+explained, it is not too hard for anyone to grasp. The way of it is
+this: If you hold up your finger so as to cover exactly some object a
+few feet distant from you, and shut first one eye and then the other,
+you will find that the finger has apparently shifted very considerably
+against the background. The finger has not really moved, but as seen
+from one eye or the other, it is thrown on a different part of the
+background, and so appears to jump; then if you draw two imaginary
+lines, one from each eye to the finger, and another between the two
+eyes, you will have made a triangle. Now, all of you who have done a
+little Euclid know that if you can ascertain the length of one side of a
+triangle, and the angles at each end of it, you can form the rest of the
+triangle; that is to say, you can tell the length of the other two
+sides. In this instance the base line, as it is called--that is to say
+the line lying between the two eyes--can easily be measured, and the
+angles at each end can be found by an instrument called a sextant, so
+that by simple calculation anyone could find out what distance the
+finger was from the eye. Now, some ingenious man decided to apply this
+method to the stars. He knew that it is only objects quite near to us
+that will appear to shift with so small a base line as that between the
+eyes, and that the further away anything is the longer must the base
+line be before it makes any difference. But this clever man thought that
+if he could only get a base line long enough he could easily compute the
+distance of the stars from the amount that they appeared to shift
+against their background. He knew that the longest base line he could
+get on earth would be about eight thousand miles, as that is the
+diameter of the earth from one side to the other; so he carefully
+observed a star from one end of this immense base line and then from the
+other, quite confident that this plan would answer. But what happened?
+After careful observations he discovered that no star moved at all with
+this base line, and that it must be ever so much longer in order to make
+any impression. Then indeed the case seemed hopeless, for here we are
+tied to the earth and we cannot get away into space. But the astronomer
+was nothing daunted. He knew that in its journey round the sun the earth
+travels in an orbit which measures about one hundred and eighty-five
+millions of miles across, so he resolved to take observations of the
+stars when the earth was at one side of this great circle, and again,
+six months later, when she had travelled to the other side. Then indeed
+he would have a magnificent base line, one of one hundred and
+eighty-five millions of miles in length. What was the result? Even with
+this mighty line the stars are found to be so distant that many do not
+move at all, not even when measured with the finest instruments, and
+others move, it may be, the breadth of a hair at a distance of several
+feet! But even this delicate measure, a hair's-breadth, tells its own
+tale; it lays down a limit of twenty-five billion miles within which no
+star can lie!
+
+This system which I have explained to you is called finding the star's
+parallax, and perhaps it is easier to understand when we put it the
+other way round and say that the hair's-breadth is what the whole orbit
+of the earth would appear to have shrunk to if it were seen from the
+distance of these stars!
+
+Many, many stars have now been examined, and of them all our nearest
+neighbour seems to be a bright star seen in the Southern Hemisphere. It
+is in the constellation or star group called Centaurus, and is the
+brightest star in it. In order to designate the stars when it is
+necessary to refer to them, astronomers have invented a system. To only
+the very brightest are proper names attached; others are noted according
+to the degree of their brightness, and called after the letters of the
+Greek alphabet: alpha, beta, gamma, delta, etc. Our own word 'alphabet'
+comes, you know, from the first two letters of this Greek series. As
+this particular star is the brightest in the constellation Centaurus, it
+is called Alpha Centauri; and if ever you travel into the Southern
+Hemisphere and see it, you may greet it as our nearest neighbour in the
+starry universe, so far as we know at present.
+
+
+
+
+CHAPTER XI
+
+THE CONSTELLATIONS
+
+
+From the very earliest times men have watched the stars, felt their
+mysterious influence, tried to discover what they were, and noted their
+rising and setting. They classified them into groups, called
+constellations, and gave such groups the names of figures and animals,
+according to the positions of the stars composing them. Some of these
+imaginary figures seem to us so wildly ridiculous that we cannot
+conceive how anyone could have gone so far out of their way as to invent
+them. But they have been long sanctioned by custom, so now, though we
+find it difficult to recognize in scattered groups of stars any likeness
+to a fish or a ram or a bear; we still call the constellations by their
+old names for convenience in referring to them.
+
+Supposing the axis of the earth were quite upright, straight up and down
+in regard to the plane at which the earth goes round the sun, then we
+should always see the same set of stars from the Northern and the same
+set of stars from the Southern Hemispheres all the year round. But as
+the axis is tilted slightly, we can, during our nights in the winter in
+the Northern Hemisphere, see more of the sky to the south than we can in
+the summer; and in the Southern Hemisphere just the reverse is the case,
+far more stars to the north can be seen in the winter than in the
+summer. But always, whether it is winter or summer, there is one fixed
+point in each hemisphere round which all the other stars seem to swing,
+and this is the point immediately over the North or the South Poles.
+There is, luckily, a bright star almost at the point at which the North
+Pole would seem to strike the sky were it infinitely lengthened. This is
+not one of the brightest stars in the sky, but quite bright enough to
+serve the purpose, and if we stand with our faces towards it, we can be
+sure we are looking due north. How can we discover this star for
+ourselves in the sky? Go out on any starlight night when the sky is
+clear, and see if you can find a very conspicuous set of seven stars
+called the Great Bear. I shall not describe the Great Bear, because
+every child ought to know it already, and if they don't, they can ask
+the first grown-up person they meet, and they will certainly be told.
+(See map.)
+
+[Illustration: CONSTELLATIONS NEAR THE POLE STAR.]
+
+Having found the Great Bear, you have only to draw an imaginary line
+between the two last stars forming the square on the side away from the
+tail, and carry it on about three times as far as the distance between
+those two stars, and you will come straight to the Pole Star. The two
+stars in the Great Bear which help one to find it are called the
+Pointers, because they point to it.
+
+The Great Bear is one of the constellations known from the oldest times;
+it is also sometimes called Charles's Wain, the Dipper, or the Plough.
+It is always easily seen in England, and seems to swing round the Pole
+Star as if held by an invisible rope tied to the Pointers. Besides the
+Great Bear there is, not far from it, the Little Bear, which is really
+very like it, only smaller and harder to find. The Pole Star is the last
+star in its tail; from it two small stars lead away parallel to the
+Great Bear, and they bring the eye to a small pair which form one side
+of a square just like that in the Great Bear. But the whole of the
+Little Bear is turned the opposite way from the Great Bear, and the tail
+points in the opposite direction. And when you come to think of it,
+it is very ridiculous to have called these groups Bears at all, or to
+talk about tails, for bears have no tails! So it would have been better
+to have called them foxes or dogs, or almost any other animal rather
+than bears.
+
+Now, if you look at the sky on the opposite side of the Pole Star from
+the Great Bear, you will see a clearly marked capital W made up of five
+or six bright stars. This is called Cassiopeia, or the Lady's Chair.
+
+In looking at Cassiopeia you cannot help noticing that there is a zone
+or broad band of very many stars, some exceedingly small, which
+apparently runs right across the sky like a ragged hoop, and Cassiopeia
+seems to be set in or on it. This band is called the Milky Way, and
+crosses not only our northern sky, but the southern sky too, thus making
+a broad girdle round the whole universe. It is very wonderful, and no
+one has yet been able to explain it. The belt is not uniform and even,
+but it is here and there broken up into streamers and chips, having the
+same appearance as a piece of ribbon which has been snipped about by
+scissors in pure mischief; or it may be compared to a great river broken
+up into many channels by rocks and obstacles in its course.
+
+The Milky Way is mainly made up of thousands and thousands of small
+stars, and many more are revealed by the telescope; but, as we see in
+Cassiopeia, there are large bright stars in it too, though, of course,
+these may be infinitely nearer to us, and may only appear to us to be in
+the Milky Way because they are between us and it.
+
+Now, besides the few constellations that I have mentioned, there are
+numbers of others, some of which are difficult to discover, as they
+contain no bright stars. But there are certain constellations which
+every one should know, because in them may be found some of the
+brightest stars, those of the first magnitude. Magnitude means size, and
+it is really absurd for us to say a star is of the first magnitude
+simply because it appears to us to be large, for, as I have explained
+already, a small star comparatively near to us might appear larger than
+a greater one further away. But the word 'magnitude' was used when men
+really thought stars were large or small according to their appearance,
+and so it is used to this day. They called the biggest and brightest
+first magnitude stars. Of these there are not many, only some twenty, in
+all the sky. The next brightest--about the brightness of the Pole Star
+and the stars in the Great Bear--are of the second magnitude, and so
+on, each magnitude containing stars less and less bright. When we come
+to stars of the sixth magnitude we have reached the limit of our sight,
+for seventh magnitude stars can only be seen with a telescope. Now that
+we understand what is meant by the magnitude, we can go back to the
+constellations and try to find some more.
+
+If you draw an imaginary line across the two stars forming the backbone
+of the Bear, starting from the end nearest the tail, and continue it
+onward for a good distance, you will come to a very bright star called
+Capella, which you will know, because near it are three little ones in a
+triangle. Now, Capella means a goat, so the small ones are called the
+kids. In winter Capella gets high up into the sky, and then there is to
+be seen below her a little cluster called the Pleiades. There is nothing
+else like this in the whole sky. It is formed of six stars, as it
+appears to persons of ordinary sight, and these stars are of the sixth
+magnitude, the lowest that can be seen by the naked eye. But though
+small, they are set so close together, and appear so brilliant,
+twinkling like diamonds, that they are one of the most noticeable
+objects in the heavens. A legend tells that there were once seven stars
+in the Pleiades clearly visible, and that one has now disappeared. This
+is sometimes spoken of as 'the lost Pleiad,' but there does not seem to
+be any foundation for the story. In old days people attached particular
+holiness or luck to the number seven, and possibly, when they found that
+there were only six stars in this wonderful group, they invented the
+story about the seventh.
+
+As the Pleiades rise, a beautiful reddish star of the first magnitude
+rises beneath them. It is called Aldebaran, and it, as well as the
+Pleiades, forms a part of the constellation of Taurus the bull. In
+England we can see in winter below Aldebaran the whole of the
+constellation of Orion, one of the finest of all the constellations,
+both for the number of the bright stars it contains and for the extent
+of the sky it covers. Four bright stars at wide distances enclose an
+irregular four-sided space in which are set three others close together
+and slanting downwards. Below these, again, are another three which seem
+to fall from them, but are not so bright. The figure of Orion as drawn
+in the old representations of the constellations is a very magnificent
+one. The three bright stars form his belt, and the three smaller ones
+the hilt of his sword hanging from it.
+
+[Illustration: ORION AND HIS NEIGHBOURS.]
+
+If you draw an imaginary line through the stars forming the belt and
+prolong it downwards slantingly, you will see, in the very height of
+winter, the brightest star in all the sky, either in the Northern or
+Southern Hemisphere. This is Sirius, who stands in a class quite by
+himself, for he is many times brighter than any other first magnitude
+star. He never rises very high above the horizon here, but on crisp,
+frosty nights may be seen gleaming like a big diamond between the
+leafless twigs and boughs of the rime-encrusted trees. Sirius is the Dog
+Star, and it is perhaps fortunate that, as he is placed, he can be seen
+sometimes in the southern and sometimes in the northern skies, so that
+many more people have a chance of looking at his wonderful brilliancy,
+than if he had been placed near the Pole star. In speaking of the
+supreme brightness of Sirius among the stars, we must remember that
+Venus and Jupiter, which outrival him, are not stars, but planets, and
+that they are much nearer to us. Sirius is so distant that the measures
+for parallax make hardly any impression on him, but, by repeated
+experiments, it has now been proved that light takes more than eight
+years to travel from him to us. So that, if you are eight years old, you
+are looking at Sirius as he was when you were a baby!
+
+Not far from the Pleiades, to the left as you face them, are to be
+found two bright stars nearly the same size; these are the Heavenly
+Twins, or Gemini.
+
+Returning now to the Great Bear, we find, if we draw a line through the
+middle and last stars of his tail, and carry it on for a little
+distance, we come fairly near to a cluster of stars in the form of a
+horseshoe; there is only one fairly bright one in it, and some of the
+others are quite small, but yet the horseshoe is distinct and very
+beautiful to look at. This is the Northern Crown. The very bright star
+not far from it is another first-class star called Arcturus.
+
+To the left of the Northern Crown lies Hercules, which is only mentioned
+because near it is the point to which the sun with all his system
+appears at present to be speeding.
+
+For other fascinating constellations, such as Leo or the Lion, Andromeda
+and Perseus, and the three bright stars by which we recognize Aquila the
+Eagle, you must wait awhile, unless you can get some one to point them
+out.
+
+Those which you have noted already are enough to lead you on to search
+for more.
+
+Perhaps some of you who live in towns and can see only a little strip of
+sky from the nursery or schoolroom windows have already found this
+chapter dull, and if so you may skip the rest of it and go on to the
+next. For the others, however, there is one more thing to know before
+leaving the subject, and that is the names of the string of
+constellations forming what is called the Zodiac. You may have heard the
+rhyme:
+
+  'The Ram, the Bull, the Heavenly Twins,
+  And next the Crab, the Lion shines,
+  The Virgin and the Scales;
+  The Scorpion, Archer, and He-goat,
+  The Man that holds the watering-pot,
+  The Fish with glittering tails.'
+
+This puts in a form easy to remember the signs of the constellations
+which lie in the Zodiac, an imaginary belt across the whole heavens. It
+is very difficult to explain the Zodiac, but I must try. Imagine for a
+moment the earth moving round its orbit with the sun in the middle. Now,
+as the earth moves the sun will be seen continually against a different
+background--that is to say, he will appear to us to move not only across
+our sky in a day by reason of our rotation, but also along the sky,
+changing his position among the stars by reason of our revolution. You
+will say at once that we cannot see the stars when the sun is there, and
+no more we can. But the stars are there all the same, and every month
+the sun seems to have moved on into a new constellation, according to
+astronomers' reckoning. If you count up the names of the constellations
+in the rhyme, you will find that there are just twelve, one for each
+month, and at the end of the year the sun has come round to the first
+one again. The first one is Aries the Ram, and the sun is seen projected
+or thrown against that part of the sky where Aries is, in April, when we
+begin spring; this is the first month to astronomers, and not January,
+as you might suppose. Perhaps you will learn to recognize all the
+constellations in the Zodiac one day; a few of them, such as the Bull
+and the Heavenly Twins, you know already if you have followed this
+chapter.
+
+
+
+
+CHAPTER XII
+
+WHAT THE STARS ARE MADE OF
+
+
+How can we possibly tell what the stars are made of? If we think of the
+vast oceans of space lying between them and us, and realize that we can
+never cross those oceans, for in them there is no air, it would seem to
+be a hopeless task to find out anything about the stars at all. But even
+though we cannot traverse space ourselves, there is a messenger that
+can, a messenger that needs no air to sustain him, that moves more
+swiftly than our feeble minds can comprehend, and this messenger brings
+us tidings of the stars--his name is Light. Light tells us many
+marvellous things, and not the least marvellous is the news he gives us
+of the workings of another force, the force of gravitation. In some ways
+gravitation is perhaps more wonderful than light, for though light
+speeds across airless space, it is stopped at once by any opaque
+substance--that is to say, any substance not transparent, as you know
+very well by your own shadows, which are caused by your bodies stopping
+the light of the sun. Light striking on one side of the earth does not
+penetrate through to the other, whereas gravitation does. You remember,
+of course, what the force of gravitation is, for we read about that very
+early in this book. It is a mysterious attraction existing between all
+matter. Every atom pulls every other atom towards itself, more or less
+strongly according to distance. Now, solid matter itself makes no
+difference to the force of gravitation, which acts through it as though
+it were not there. The sun is pulling the earth toward itself, and it
+pulls the atoms on the far side of the earth just as strongly as it
+would if there were nothing lying between it and them. Therefore, unlike
+light, gravitation takes no heed of obstacles in the way, but acts in
+spite of them. The gravitation of the earth holds you down just the
+same, though you are on the upper floor of a house, with many layers of
+wood and plaster between you and it. It cannot pull you down, for the
+floor holds you up, but it is gravitation that keeps your feet on the
+ground all the same. A clever man made up a story about some one who
+invented a kind of stuff which stopped the force of gravitation going
+through it, just as a solid body stops light; when this stuff was made,
+of course, it went right away off into space, carrying with it anyone
+who stood on it, as there was nothing to hold it to the earth! That was
+only a story, and it is not likely anyone could invent such stuff, but
+it serves to make clear the working of gravitation. These two tireless
+forces, light and gravitation, run throughout the whole universe, and
+carry messages of tremendous importance for those who have minds to
+grasp them. Without light we could know nothing of these distant worlds,
+and without understanding the laws of gravity we should not be able to
+interpret much that light tells us.
+
+To begin with light, what can we learn from it? We turn at once to our
+own great light-giver, the sun, to whom we owe not only all life, but
+also all the colour and beauty on earth. It is well known to men of
+science that colour lies in the light itself, and not in any particular
+object. That brilliant blue cloak of yours is not blue of itself, but
+because of the light that falls on it. If you cannot believe this, go
+into a room lighted only by gas, and hey, presto! the colour is changed
+as if it were a conjuring trick. You cannot tell now by looking at the
+cloak whether it is blue or green! Therefore you must admit that as the
+colour changes with the change of light it must be due to light, and not
+to any quality belonging to the material of the cloak. But, you may
+protest, if the colour is solely due to light, and light falls on
+everything alike, why are there so many colours? That is a very fair
+question. If the light that comes from the sun were of only one
+colour--say blue or red--then everything would be blue or red all the
+world over. Some doors in houses are made with a strip of red or blue
+glass running down the sides. If you have one in your house like that,
+go and look through it, and you will see an astonishing world made up of
+different tones of the same colour. Everything is red or blue, according
+to the colour of the glass, and the only difference in the appearance of
+objects lies in the different shades, whether things are light or dark.
+This is a world as it might appear if the sun's rays were only blue or
+only red. But the sun's light is not of one colour only, fortunately for
+us; it is of all the colours mixed together, which, seen in a mass, make
+the effect of white light. Now, objects on earth are only either seen by
+the reflected light of the sun or by some artificial light. They have no
+light of their own. Put them in the dark and they do not shine at all;
+you cannot see them. It is the sun's light striking on them that makes
+them visible. But all objects do not reflect the light equally, and this
+is because they have the power of absorbing some of the rays that strike
+on them and not giving them back at all, and only those rays that are
+given back show to the eye. A white thing gives back all the rays, and
+so looks white, for we have the whole of the sun's light returned to us
+again. But how about a blue thing? It absorbs all the rays except the
+blue, so that the blue rays are the only ones that come back or rebound
+from it again to meet our eyes, and this makes us see the object blue;
+and this is the case with all the other colours. A red object retains
+all rays except the red, which it sends back to us; a yellow object
+gives back only the yellow rays, and so on. What an extraordinary and
+mysterious fact! Imagine a brilliant flower-garden in autumn. Here we
+have tall yellow sunflowers with velvety brown centres, clustering pink
+and crimson hollyhocks, deep red and bright yellow peonies, slender
+fairy-like Japanese anemones, great bunches of mauve Michaelmas daisies,
+and countless others, and mingled with all these are many shades of
+green. Yet it is the light of the sun alone that falling on all these
+varied objects, makes that glorious blaze of colour; it seems
+incredible. It may be difficult to believe, but it is true beyond all
+doubt. Each delicate velvety petal has some quality in it which causes
+it to absorb certain of the sun's rays and send back the others, and its
+colour is determined by those it sends back.
+
+Well then how infinitely varied must be the colours hidden in the sun's
+light, colours which, mixed all together, make white light! Yes, this is
+so, for all colours that we know are to be found there. In fact, the
+colours that make up sunlight are the colours to be seen in the rainbow,
+and they run in the same order. Have you ever looked carefully at a
+rainbow? If not, do so at the next chance. You will see it begins by
+being dark blue at one end, and passes through all colours until it gets
+to red at the other.
+
+We cannot see a rainbow every day just when we want to, but we can see
+miniature rainbows which contain just the same colours as the real ones
+in a number of things any time the sun shines. For instance, in the
+cut-glass edge of an inkstand or a decanter, or in one of those
+old-fashioned hanging pieces of cut-glass that dangle from the
+chandelier or candle-brackets. Of course you have often seen these
+colours reflected on the wall, and tried to get them to shine upon your
+face. Or you have caught sight of a brilliant patch of colour on the
+wall and looked around to see what caused it, finally tracing it to some
+thick edge of shining glass standing in the sunlight. Now, the cut-glass
+edge shows these colours to you because it breaks up the light that
+falls upon it into the colours it is made of, and lets each one come out
+separately, so that they form a band of bright colours instead of just
+one ray of white light.
+
+This is perhaps a little difficult to understand, but I will try to
+explain. When a ray of white light falls on such a piece of glass, which
+is known as a prism, it goes in as white light at one side, but the
+three-cornered shape of the glass breaks it up into the colours it is
+made of, and each colour comes out separately at the other side--namely,
+from blue to red--like a little rainbow, and instead of one ray of white
+light, we have a broad band of all the colours that light is made of.
+
+Who would ever have thought a pretty plaything like this could have told
+us what we so much wanted to know--namely, what the sun and the stars
+are made of? It seems too marvellous to be true, yet true it is that for
+ages and ages light has been carrying its silent messages to our eyes,
+and only recently men have learnt to interpret them. It is as if some
+telegraph operator had been going steadily on, click, click, click, for
+years and years, and no one had noticed him until someone learnt the
+code of dot and dash in which he worked, and then all at once what he
+was saying became clear. The chief instrument in translating the message
+that the light brings is simply a prism, a three-cornered wedge of
+glass, just the same as those hanging lustres belonging to the
+chandeliers. When a piece of glass like this is fixed in a telescope in
+such a way that the sun's rays fall on it, then there is thrown on to a
+piece of paper or any other suitable background a broad coloured band of
+lovely light like a little rainbow, and this is called the sun's
+spectrum, and the instrument by which it is seen is called a
+spectroscope. But this in itself could tell us little; the message it
+brings lies in the fact that when it has passed through the telescope,
+so that it is magnified, it is crossed by hundreds of minute black
+lines, not placed evenly at all, but scattered up and down. There may be
+two so close together that they look like one, and then three far apart,
+and then some more at different distances. When this remarkable
+appearance was examined carefully it was found that in sunlight the
+lines that appeared were always exactly the same, in the same places,
+and this seemed so curious that men began to seek for an explanation.
+
+Someone thought of an experiment which might teach us something about
+the matter, and instead of letting sunlight fall on the prism, he made
+an artificial light by burning some stuff called sodium, and then
+allowed the band of coloured light to pass through the telescope; when
+he examined the spectrum that resulted, he found that, though numbers of
+lines to be found in the sun's spectrum were missing, there were a few
+lines here exactly matching a few of the lines in the sun's spectrum;
+and this could not be the result of chance only, for the lines are so
+mathematically exact, and are in themselves so peculiarly distributed,
+that it could only mean that they were due to the same cause. What could
+this signify, then, but that away up there in the sun, among other
+things, stuff called sodium, very well known to chemists on earth, is
+burning? After this many other substances were heated white-hot so as to
+give out light, in order to discover if the lines to be seen in their
+spectra were also to be found in the sun's spectrum. One of these was
+iron, and, astonishing to say, all the many little thread-like lines
+that appeared in its spectrum were reproduced to a hair's-breadth, among
+others, in the sun's spectrum. So we have found out beyond all
+possibility of doubt some of the materials of which the sun is made. We
+know that iron, sodium, hydrogen, and numerous other substances and
+elements, are all burning away there in a terrific furnace, to which any
+furnace we have on earth is but as the flicker of a match.
+
+It was not, of course, much use applying this method to the planets, for
+we know that the light which comes from them to us is only reflected
+sunlight, and this, indeed, was proved by means of the spectroscope. But
+the stars shine by their own light, and this opened up a wide field for
+inquiry. The difficulty was, of course, to get the light of one star
+separated from all the rest, because the light of one star is very faint
+and feeble to cast a spectrum at all. Yet by infinite patience
+difficulties were overcome. One star alone was allowed to throw its
+light into the telescope; the light passed through a prism, and showed a
+faint band of many colours, with the expected little black lines cutting
+across it more or less thickly. Examinations have thus been made of
+hundreds of stars. In the course of them some substances as yet unknown
+to us on earth have been encountered, and in some stars one
+element--hydrogen--is much stronger than in others; but, on the whole,
+speaking broadly, it has been satisfactorily shown that the stars are
+made on the same principles as our own sun, so that the reasoning of
+astronomers which had argued them to be suns was proved.
+
+[Illustration: THE SPECTRUM OF THE SUN AND SIRIUS.]
+
+We have here in the picture the spectrum of the sun and the spectrum of
+Arcturus. You can see that the lines which appear in the band of light
+belonging to Sirius are also in the band of light belonging to the sun,
+together with many others. This means that the substances flaming out
+and sending us light from the far away star are also giving out light
+from our own sun, and that the sun and Sirius both contain the same
+elements in their compositions.
+
+This, indeed, seems enough for the spectroscope to have accomplished; it
+has interpreted for us the message light brings from the stars, so that
+we know beyond all possibility of mistake that these glowing, twinkling
+points of light are brilliant suns in a state of intense heat, and that
+in them are burning elements with which we ourselves are quite familiar.
+But when the spectroscope had done that, its work was not finished, for
+it has not only told us what the stars are made of, but another thing
+which we could never have known without it--namely, if they are moving
+toward us or going away from us.
+
+
+
+
+CHAPTER XIII
+
+RESTLESS STARS
+
+
+You remember we have already remarked upon the difficulty of telling how
+far one star lies behind another, as we do not know their sizes. It is,
+to take another similar case, easy enough to tell if a star moves to one
+side or the other, but very difficult by ordinary observation to tell if
+it is advancing toward us or running away from us, for the only means we
+have of judging is if it gets larger or smaller, and at that enormous
+distance the fact whether it advances or recedes makes no difference in
+its size. Now, the spectroscope has changed all this, and we can tell
+quite as certainly if a star is coming toward us as we can if it moves
+to one side. I will try to explain this. You know, perhaps, that sound
+is caused by vibration in the air. The noise, whatever it is, jars the
+air and the vibrations strike on our ears. It is rather the same thing
+as the result of throwing a stone into a pond: from the centre of the
+splash little wavelets run out in ever-widening circles; so through the
+air run ever-widening vibrations from every sound. The more vibrations
+there are in a second the shriller is the note they make. In a high note
+the air-vibrations follow one another fast, pouring into one's ear at a
+terrific speed, so that the apparatus in the ear which receives them
+itself vibrates fiercely and records a high note, while a lower note
+brings fewer and slower vibrations in a second, and the ear is not so
+much disturbed. Have you ever noticed that if a railway engine is
+sweeping-toward you and screaming all the time, its note seems to get
+shriller and shriller? That is because the engine, in advancing, sends
+the vibrations out nearer to you, so more of them come in a second, and
+thus they are crowded up closer together, and are higher and higher.
+
+Now, light is also caused by waves, but they are not the same as sound
+waves. Light travels without air, whereas sound we know cannot travel
+without air, and is ever so much slower, and altogether a grosser,
+clumsier thing than light. But yet the waves or rays which make light
+correspond in some ways to the vibrations of sound. What corresponds to
+the treble on the piano is the blue end of the spectrum in light, and
+the bass is the red end. Now, when we are looking at the spectrum of
+any body which is advancing swiftly toward us, something of the same
+effect is observed as in the case of the shrieking engine. Take any star
+and imagine that that star is hastening toward us at a pace of three
+hundred miles a second, which is not at all an unusual rate for a star;
+then, if we examine the band of light, the spectrum, of such a star, we
+shall observe an extraordinary fact--all these little lines we have
+spoken of are shoved up toward the treble or blue end of the spectrum.
+They still remain just the same distances from each other, and are in
+twos and threes or single, so that the whole set of lines is unaltered
+as a set, but everyone of them is shifted a tiny fraction up toward the
+blue end of the spectrum, just a little displaced. Now if, instead of
+advancing toward us, this same star had been rushing away from us at a
+similar pace, all these lines would have been moved a tiny bit toward
+the red or bass end of the spectrum. This is known to be certainly true,
+so that by means of the spectroscope we can tell that some of these
+great sun-stars are advancing toward us and some receding from us,
+according to whether the multitudes of little lines in the spectrum are
+shifted slightly to the blue or the red end.
+
+You remember that it has been surmised that the pace the sun moves with
+his system is about twelve miles a second. This seems fast enough to us,
+who think that one mile a minute is good time for an express train, but
+it is slow compared with the pace of many of the stars. As I have said,
+some are travelling at a rate of between two hundred and three hundred
+miles a second; and it is due to the spectroscope that we know not only
+whether a star is advancing toward us or receding from us, but also
+whether the pace is great or not; it even tells us what the pace is, up
+to about half a mile a second, which is very marvellous. It is a curious
+fact that many of the small stars show greater movement than the large
+ones, which mayor may not mean that they are nearer to us.
+
+It may be taken as established that there is no such thing as absolute
+rest in the universe: everything, stars and nebulae alike, are moving
+somewhere; in an infinite variety of directions, with an infinite
+variety of speed they hasten this way and that. It would be impossible
+for any to remain still, for even supposing it had been so 'in the
+beginning,' the vast forces at work in the universe would not let it
+remain so. Out of space would come the persistent call of gravitation:
+atoms would cry silently to atoms. There could be no perfect equality
+of pull on all sides; from one side or another the pull would be the
+stronger. Slowly the inert mass would obey and begin falling toward it;
+it might be an inch at a time, but with rapid increase, until at last it
+also was hastening some whither in this universe which appears to us to
+be infinite.
+
+It must be remembered that these stars, even when moving at an enormous
+pace, do not change their places in the sky when regarded by ordinary
+observers. It would take thousands of years for any of the
+constellations to appear at all different from what they are now, even
+though the stars that compose them are moving in different directions
+with a great velocity, for a space of many millions of miles, at the
+distance of most of the stars, would be but as the breadth of a fine
+hair as seen by us on earth. So thousands of years ago men looked up at
+the Great Bear, and saw it apparently the same as we see it now; yet for
+all that length of time the stars composing it have been rushing in this
+direction and that at an enormous speed, but do not appear to us on the
+earth to alter their positions in regard to each other. I know of
+nothing that gives one a more overwhelming sense of the mightiness of
+the universe and the smallness of ourselves than this fact. From age to
+age men look on changeless heavens, yet this apparently stable universe
+is fuller of flux and reflux than is the restless ocean itself, and the
+very wavelets on the sea are not more numerous nor more restless than
+the stars that bestrew the sky.
+
+
+
+
+CHAPTER XIV
+
+THE COLOURS OF THE STARS
+
+
+Has it ever occurred to you that the stars are not all of the same
+colour? It is true that, just glancing at them casually, you might say
+they are all white; but if you examine them more carefully you cannot
+help seeing that some shine with a steely blue light, while others are
+reddish or yellowish. These colours are not easy to distinguish with the
+naked eye, and might not attract any attention at all unless they were
+pointed out; yet when attention is drawn to the fact, it is impossible
+to deny the redness of some, such as Aldebaran. But though we may admit
+this, we might add that the colours are so very faint and inconspicuous,
+that they might be, after all, only the result of imagination.
+
+To prove that the colours are constant and real we must use a telescope,
+and then we need have no further doubt of their reality, for instead of
+disappearing, the colours of some stars stand out quite vividly beyond
+the possibility of mistake. Red stars are a bright red, and they are the
+most easily seen of all, though the other colours, blue and yellow and
+green, are seen very decidedly by some people. The red stars have been
+described by various observers as resembling 'a drop of blood on a black
+field,' 'most magnificent copper-red,' 'most intense blood-red,' and
+'glowing like a live coal out of the darkness of space.' Some people see
+them as a shining red, like that of a glowing cloud at sunset. Therefore
+there can be no doubt that the colours are genuine enough, and are
+telling us some message. This message we are able to read, for we have
+begun to understand the language the stars speak to us by their light
+since the invention of the spectroscope. The spectroscope tells us that
+these colours indicate different stages in the development of the stars,
+or differences of constitution--that is to say, in the elements of which
+they are made. Our own sun is a yellow star, and other yellow stars are
+akin to him; while red and blue and green stars contain different
+elements, or elements in different proportions.
+
+Stars do not always remain the same colours for an indefinite time; one
+star may change slowly from yellow to white, and another from red to
+yellow; and there are instances of notable changes, such as that of the
+brilliant white Sirius, who was stated in old times by many different
+observers to be a red star. All this makes us think, and year by year
+thought leads us on to knowledge, and knowledge about these distant suns
+increases. But though we know a good deal now, there are still many
+questions we should like to ask which we cannot expect to have answered
+for a long time yet, if ever.
+
+The star colours have some meanings which we cannot even guess; we can
+only notice the facts regarding them. For instance, blue stars are never
+known to be solitary--they always have a companion, but why this should
+be so passes our comprehension. What is it in the constitution of a blue
+star which holds or attracts another? Whatever it may be, it is
+established by repeated instances that blue stars do not stand alone. In
+the constellation of Cygnus there are two stars, a blue and a yellow
+one, which are near enough to each other to be seen in the same
+telescope at the same time, and yet in reality are separated by an
+almost incredible number of billions of miles. But as we know that a
+blue star is never seen alone, and that it has often as its companion a
+yellowish or reddish star, it is probable that these two, situated at an
+enormous distance from one another, are yet in some mysterious way
+dependent on each other, and are not merely seen together because they
+happen to fall in the same field of view.
+
+Many double stars show most beautifully contrasted colours: among them
+are pairs of yellow and rose-red, golden and azure, orange and purple,
+orange and lilac, copper-colour and blue, apple-green and cherry-red,
+and so on. In the Southern Hemisphere there is a cluster containing so
+many stars of brilliant colours that Sir John Herschel named it 'the
+Jewelled Cluster.'
+
+I expect most of you have seen an advertisement of Pear's Soap, in which
+you are asked to stare at some red letters, and then look away to some
+white surface, such as a ceiling, when you will see the same letters in
+green. This is because green is the complementary or contrasting colour
+to red, and the same thing is the case with blue and yellow. When any
+one colour of either of these pairs is seen, it tends to make the other
+appear by reaction, and if the eye gazed hard at blue instead of red, it
+would next see yellow, and not green. Now, many people to whom this
+curious fact is known argue that perhaps the colours of the double
+stars are not real, but the effect of contrast only; for instance, they
+say a red star near a companion white one would tend to make the
+companion appear green, and so, of course, it would. But this does not
+account for the star colours, which are really inherent in the stars
+themselves, as may be proved by cutting off the light of one star, and
+looking only at the other, when its colour still appears unchanged.
+Another argument equally strong against the contrast theory is that the
+colours of stars in pairs are by no means always those which would
+appear if the effect was only due to complementary colours. It is not
+always blue and yellow or red and green pairs that we see, though these
+are frequent, but many others of various kinds, such as copper and blue,
+and ruddy and blue.
+
+We have therefore come to the conclusion that there are in this
+astonishing universe numbers of gloriously coloured suns, some of which
+apparently lie close together. What follows? Why, we want to know, of
+course, if these stars are really pairs connected with each other, or if
+they only appear so by being in the same line of sight, though one is
+infinitely more distant than the other. And that question also has been
+answered. There are now known thousands of cases in which stars,
+hitherto regarded as single, have been separated into two, or even more,
+by the use of a telescope. Of these thousands, some hundreds have been
+carefully investigated, and the result is that, though there are
+undoubtedly some in which the connexion is merely accidental, yet in by
+far the greater number of cases the two stars thus seen together have
+really some connexion which binds them to one another; they are
+dependent on one another. This has been made known to us by the working
+of the wonderful law of gravitation, which is obeyed throughout the
+whole universe. We know that by the operation of this law two mighty
+suns will be drawn toward each other with a certain pull, just as surely
+as we know that a stone let loose from the hand will fall upon the
+earth; so by noting the effect of two mighty suns upon each other many
+facts about them may be found out. By the most minute and careful
+measurements, by the use of the spectroscope, and by every resource
+known to science, astronomers have, indeed, actually found out with a
+near approach to exactness how far some of these great suns lie from
+each other, and how large they are in comparison with one another.
+
+The very first double star ever discovered was one which you have
+already seen, the middle one in the tail of the Great Bear. If you look
+at it you will be delighted to find that you can see a wee star close to
+it, and you will think you are looking at an example of a double star
+with your very own eyes; but you will be wrong, for that wee star is
+separated by untold distances from the large one to which it seems so
+near. In fact, any stars which can be seen to be separate by the naked
+eye must lie immeasurably far apart, however tiny seems the space
+between them. Such stars may possibly have some connexion with each
+other, but, at any rate in this case, such a connexion has not been
+proved. No, the larger star itself is made up of two others, which can
+only be seen apart in a telescope. Since this discovery double stars
+have been plentifully found in every part of the sky. The average space
+between such double stars as seen from our earth is--what do you think?
+It is the width of a single hair held up thirty-six feet from our eyes!
+This could not, of course, be seen without the use of a telescope or
+opera-glasses. It serves to give some impression of star distances when
+we think that the millions and millions of miles lying between those
+stars have shrunk to that hair's-breadth seen from our point of view.
+
+Twin stars circle together round a common centre of gravity, and are
+bound by the laws of gravitation just as the planets are. Our sun is a
+solitary star, with no companion, and therefore such a state of things
+seems to us to be incredible. Fancy two gigantic suns, one topaz-yellow
+and the other azure-blue, circling around in endless movement! Where in
+such a system would there be room for the planets? How could planets
+exist under the pull of two suns in opposite directions? Still more
+wonders are unfolded as the inquiry proceeds. Certain irregularities in
+the motions of some of these twin systems led astronomers to infer that
+they were acted upon by another body, though this other body was not
+discernible. In fact, though they could not see it, they knew it must be
+there, just as Adams and Leverrier knew of the existence of Neptune,
+before ever they had seen him, by the irregularities in the movements of
+Uranus. As the results showed, it was there, and was comparable in size
+to the twin suns it influenced, and yet they could not see it. So they
+concluded this third body must be dark, not light-giving like its
+companions. We are thus led to the strange conclusion that some of these
+systems are very complicated, and are formed not only of shining suns,
+but of huge dark bodies which cannot be called suns. What are they,
+then? Can they be immense planets? Is it possible that life may there
+exist? No fairy tale could stir the imagination so powerfully as the
+thought of such systems including a planetary body as large or larger
+than its sun or suns. If indeed life exists there, what a varied scene
+must be presented day by day! At one time both suns mingling their
+flashing rays may be together in the sky; at another time only one
+appears, a yellow or blue sun, as the case may be. The surface of such
+planets must undergo weird transformations, the foliage showing one day
+green, the next yellow, and the next blue; shadows of azure and orange
+will alternate! But fascinating as such thoughts are, we can get no
+further along that path.
+
+To turn from fancy to facts, we find that telescope and spectroscope
+have supplied us with quite enough matter for wonder without calling
+upon imagination. We have discovered that many of the stars which seem
+to shine with a pure single light are double, and many more consist not
+only of two stars, but of several, some of which may be dark bodies. The
+Pole Star was long known to be double, and is now discovered to have a
+third member in its system. These multiple systems vary from one
+another in almost every case. Some are made up of a mighty star and a
+comparatively small one; others are composed of stars equal in
+light-giving power--twin suns. Some progress swiftly round their orbits,
+some go slowly; indeed, so slowly that during the century they have been
+under observation only the very faintest sign of movement has been
+detected; and in other systems, which we are bound to suppose double,
+the stars are so slow in their movements that no progress seems to have
+been made at all.
+
+The star we know as the nearest to us in the heavens, Alpha Centauri, is
+composed of two very bright partners, which take about eighty-seven
+years to traverse their orbit. They sometimes come as near to each other
+as Saturn is to the sun. In the case of Sirius astronomers found out
+that he had a companion by reason of his irregularities of movement
+before they discovered that companion, which is apparently a very small
+star, only to be discerned with good telescopes. But here, again, it
+would be unwise to judge only by what we see. Though the star appears
+small, we know by the influence it exercises on Sirius that it is very
+nearly the same size as he is. Thus we judge that it is poor in
+light-giving property; in fact, its shining power is much less than that
+of its companion, though its size is so nearly equal. This is not
+wonderful, for Sirius's marvellous light-giving power is one of the
+wonders of the universe; he shines as brilliantly as twenty-nine or
+thirty of our suns!
+
+In some cases the dark body which we cannot see may even be larger than
+the shining one, through which alone we can know anything of it. Here we
+have a new idea, a hint that in some of these systems there may be a
+mighty earth with a smaller sun going round it, as men imagined our sun
+went around the earth before the real truth was found out.
+
+So we see that, when we speak of the stars as suns comparable with our
+sun, we cannot think of them all as being exactly on the same model.
+There are endless varieties in the systems; there are solitary suns like
+ours which may have a number of small planets going round them, as in
+the solar system; but there are also double suns going round each other,
+suns with mighty dark bodies revolving round them which may be planets,
+and huge dark bodies with small suns too. Every increase of knowledge
+opens up new wonders, and the world in which we live is but one kind of
+world amid an infinite number.
+
+In this chapter we have learnt an altogether new fact--the fact that the
+hosts of heaven comprise not only those shining stars we are accustomed
+to see, but also dark bodies equally massive, and probably equally
+numerous, which we cannot see. In fact, the regions of space may be
+strewn with such dark bodies, and we could have no possible means of
+discovering them unless they were near enough to some shining body to
+exert an influence upon it. It is not with his eyes alone, or with his
+senses, man knows of the existence of these great worlds, but often
+solely by the use of the powers of his mind.
+
+
+
+
+CHAPTER XV
+
+TEMPORARY AND VARIABLE STARS
+
+
+It is a clear night, nearly all the world is asleep, when an astronomer
+crosses his lawn on his way to his observatory to spend the dark hours
+in making investigations into profound space. His brilliant mind,
+following the rays of light which shoot from the furthest star, will
+traverse immeasurable distances, while the body is forgotten. Just
+before entering the observatory he pauses and looks up; his eye catches
+sight of something that arrests him, and he stops involuntarily. Yet any
+stranger standing beside him, and gazing where he gazes, would see
+nothing unusual. There is no fiery comet with its tail stretching across
+from zenith to horizon, no flaming meteor dashing across the darkened
+sky. But that there is something unusual to be seen is evident, for the
+astronomer breathes quickly, and after another earnest scrutiny of the
+object which has attracted him, he rushes into the observatory, searches
+for a star-chart, and examines attentively that part of the sky at
+which he has been gazing. He runs his finger over the chart: here and
+there are the well-known stars that mark that constellation, but here?
+In that part there is no star marked, yet he knows, for his own eyes
+have told him but a few moments ago, that here there is actually blazing
+a star, not large, perhaps, but clear enough to be seen without a
+telescope--a star, maybe, which no eye but his has yet observed!
+
+He hurries to his telescope, and adjusts it so as to bring the stranger
+into the field of view. A new star! Whence has it come? What does it
+mean?
+
+By the next day at the latest the news has flown over the wires, and all
+the scientific world is aware that a new star has been detected where no
+star ever was seen before. Hundreds of telescopes are turned on to it;
+its spectrum is noted, and it stands revealed as being in a state of
+conflagration, having blazed up from obscurity to conspicuousness. Night
+after night its brilliance grows, until it ranks with the brightest
+stars in heaven, and then it dies down and grows dim, gradually
+sinking--sinking into the obscurity from whence it emerged so briefly,
+and its place in the sky knows it no more. It may be there still, but
+so infinitely faint and far away that no power at our command can reveal
+it to us. And the amazing part of it is that this huge disaster, this
+mighty conflagration, is not actually happening as it is seen, but has
+happened many hundreds of years ago, though the message brought by the
+light carrier has but reached us now.
+
+There have not been a great many such outbursts recorded, though many
+may have taken place unrecorded, for even in these days, when trained
+observers are ceaselessly watching the sky, 'new' stars are not always
+noticed at once. In 1892 a new star appeared, and shone for two months
+before anyone noticed it. This particular one never rose to any very
+brilliant size. I twas situated in the constellation of Auriga, and was
+noticed on February 1. It remained fairly bright until March 6, when it
+began to die down; but it has now sunk so low that it can only be seen
+in the very largest telescopes.
+
+Photography has been most useful in recording these stars, for when one
+is noticed it has sometimes been found that it has been recorded on a
+photographic plate taken some time previously, and this shows us how
+long it has been visible. More and more photography becomes the useful
+handmaid of astronomers, for the photographic prepared plate is more
+sensitive to rays of light than the human eye, and, what is more useful
+still, such plates retain the rays that fall upon them, and fix the
+impression. Also on a plate these rays are cumulative--that is to say,
+if a very faint star shines continuously on a plate, the longer the
+plate is exposed, within certain limits, the clearer will the image of
+that star become, for the light rays fall one on the top of the other,
+and tend to enforce each other, and so emphasize the impression, whereas
+with our eyes it is not the same thing at all, for if we do not see an
+object clearly because it is too faint, we do not see it any better,
+however much we may stare at the place where it ought to be. This is
+because each light ray that reaches our eye makes its own impression,
+and passes on; they do not become heaped on each other, as they do on a
+photographic plate.
+
+One variable star in Perseus, discovered in 1901, rose to such
+brilliancy that for one night it was queen of the Northern Hemisphere,
+outshining all the other first-class stars.
+
+It rose into prominence with wonderful quickness, and sank equally fast.
+At its height it outshone our sun eight thousand times! This star was
+so far from us that it was reckoned its light must take about three
+hundred years to reach us, consequently the great conflagration, or
+whatever caused the outburst, must have taken place in the reign of
+James I., though, as it was only seen here in 1901, it was called the
+new star of the new century.
+
+When these new stars die down they sometimes continue to shine faintly
+for a long time, so that they are visible with a telescope, but in other
+cases they may die out altogether. We know very little about them, and
+have but small opportunity for observing them, and so it is not safe to
+hazard any theories to account for their peculiarities. At first men
+supposed that the great flame was made by a violent collision between
+two bodies coming together with great velocity so that both flared up,
+but this speculation has been shown by the spectroscope to be
+improbable, and now it is supposed by some people that two stars
+journeying through space may pass through a nebulous region, and thus
+may flare up, and such a theory is backed up by the fact that a very
+great number of such stars do seem to be mixed up in some strange way
+with a nebulous haze.
+
+All these new stars that we have been discussing so far have only
+blazed up once and then died down, but there is another class of stars
+quite as peculiar, and even more difficult to explain, and these are
+called variable stars. They get brighter and brighter up to a certain
+point, and then die down, only to become bright once more, and these
+changes occur with the utmost regularity, so that they are known and can
+be predicted beforehand. This is even more unaccountable than a sudden
+and unrepeated outburst, for one can understand a great flare-up, but
+that a star should flare and die down with regularity is almost beyond
+comprehension. Clearly we must look further than before for an
+explanation. Let us first examine the facts we know. Variable stars
+differ greatly from each other. Some are generally of a low magnitude,
+and only become bright for a short time, while others are bright most of
+the time and die down only for a short time. Others become very bright,
+then sink a little bit, but not so low as at first; then they become
+bright again, and, lastly, go right down to the lowest point, and they
+keep on always through this regular cycle of changes. Some go through
+the whole of these changes in three days, and others take much longer.
+The periods, as the intervals between the complete round of changes are
+called, vary, in fact, between three days and six hundred! It may seem
+impossible that changes covering so long as six hundred days could be
+known and followed, but there is nothing that the patience of
+astronomers will not compass.
+
+One very well-known variable star you can see for yourselves, and as an
+ounce of observation is worth a pound of hearsay, you might take a
+little trouble to find it. Go out on any clear starlight night and look.
+Not very far from Cassiopeia (W.), to the left as you face it, are three
+bright stars running down in a great curve. These are in the
+constellation called Perseus, and a little to the right of the middle
+and lowest one is the only variable star we can see in the sky without a
+telescope.
+
+This is Algol. For the greater part of three days he is a bright star of
+about the second magnitude, then he begins to fade, and for four and a
+half hours grows steadily dimmer. At the dimmest he remains for about
+twenty minutes, and then rises again to his ordinary brightness in three
+and a half hours. How can we explain this? You may possibly be able to
+suggest a reason. What do you say to a dark body revolving round Algol,
+or, rather, revolving with him round a common centre of gravity? If
+such a thing were indeed true, and if such a body happened to pass
+between us and Algol at each revolution, the light of Algol would be cut
+off or eclipsed in proportion to the size of such a body. If the dark
+body were the full size of Algol and passed right between him and us, it
+would cut off all the light, but if it were not quite the same size, a
+little would still be seen. And this is really the explanation of the
+strange changes in the brightness of Algol, for such a dark body as we
+are imagining does in reality exist. It is a large dark body, very
+nearly as large as Algol himself, and if, as we may conjecture, it is a
+mighty planet, we have the extraordinary example of a planet and its sun
+being nearly the same size. We have seen that the eclipse happens every
+three days, and this means, of course, that the planetary body must go
+round its sun in that time, so as to return again to its position
+between us and him, but the thing is difficult to believe. Why, the
+nearest of all our planets to the sun, the wee Mercury, takes
+eighty-seven days to complete its orbit, and here is a mighty body
+hastening round its sun in three! To do this in the time the large dark
+planet must be very near to Algol; indeed, astronomers have calculated
+that the surfaces of the two bodies are not more than about two million
+miles apart, and this is a trifle when we consider that we ourselves are
+more than forty-six times as far as that from the sun. At this distance
+Algol, as observed from the planet, will fill half the sky, and the heat
+he gives out must be something stupendous. Also the effects of
+gravitation must be queer indeed, acting on two such huge bodies so
+close together. If any beings live in such a strange world, the pull
+which draws them to their mighty sun must be very nearly equal to the
+pull which holds them to their own globe; the two together may
+counteract each other, but the effect must be strange!
+
+From irregularities in the movements of Algol it has been judged that
+there may be also in the same system another dark body, but of it
+nothing has been definitely ascertained.
+
+But all variable stars need not necessarily be due to the light being
+intercepted by a dark body. There are cases where two bright stars in
+revolving round each other produce the same effect; for when seen side
+by side the two stars give twice as much light as when one is hidden
+behind the other, and as they are seen alternately side by side and in
+line, they seem to alter regularly in lustre.
+
+
+
+
+CHAPTER XVI
+
+STAR CLUSTERS AND NEBULAE
+
+
+Could you point out any star cluster in the sky? You could if you would
+only think for a minute, for one has been mentioned already. This is the
+cluster known as the Pleiades, and it is so peculiar and so different
+from anything else, that many people recognize the group and know where
+to look for it even before they know the Great Bear, the favourite
+constellation in the northern sky, itself. The Pleiades is a real star
+cluster, and the chief stars in it are at such enormous distances from
+one another that they can be seen separately by the eye unaided, whereas
+in most clusters the stars appear to be so close together that without a
+telescope they make a mere blur of brightness. For a long time it was
+supposed that the stars composing the Pleiades could not really be
+connected because of the great distances between them; for, as you know,
+even a hair's-breadth apparently between stars signifies in reality many
+millions of miles.
+
+Light travelling from the Pleiades to us, at that incomprehensible pace
+of which you already know, takes a hundred and ninety years to reach us!
+At this incredibly remote distance lies the main part of the cluster
+from us; but it is more marvellous still that we have every reason to
+believe that the outlying stars of this cluster are as far from the
+central ones as the nearest star we know of, Alpha Centauri, is from us!
+Little wonder was it, then, that men hesitated to ascribe to the
+Pleiades any real connection with each other, and supposed them to be
+merely an assemblage of stars which seemed to us to lie together.
+
+With the unaided eye we see comparatively few stars in the Pleiades. Six
+is the usual number to be counted, though people with very good sight
+have made out fourteen. Viewed through the telescope, however, the scene
+changes: into this part of space stars are crowded in astonishing
+profusion; it is impossible to count them, and with every increase in
+the power of the telescope still more are revealed. Well over a thousand
+in this small space seems no exaggerated estimate. Now, it is impossible
+to say how many of these really belong to the group, and how many are
+seen there accidentally, but observations of the most prominent ones
+have shown that they are all moving in exactly the same direction at
+the same pace. It would be against probability to conceive that such a
+thing could be the result of mere chance, considering the infinite
+variety of star movements in general, and so we are bound to believe
+that this wonderful collection of stars is a real group, and not only an
+apparent one.
+
+So splendid are the great suns that illuminate this mighty system, that
+at least fifty or sixty of them far surpass our own sun in brilliancy.
+Therefore when we look at that tiny sparkling group we must in
+imagination picture it as a vast cluster of mighty stars, all controlled
+and swayed by some dominant impulse, though separated by spaces enough
+to make the brain reel in thinking of them. If these suns possess also
+attendant planets, what a galaxy of worlds, what a universe within a
+universe is here!
+
+Other star clusters there are, not so conspicuous as the Pleiades, and
+most of these can only be seen through a telescope, so we may be
+thankful that we have one example so splendid within our own vision.
+There are some clusters so far and faintly shining that they were at
+first thought to be nebulae, and not stars at all; but the telescope
+gradually revealed the fact that many of these are made up of stars,
+and so people began to think that all faint shining patches of nebulous
+light were really star clusters, which would be resolved into stars if
+only we had better telescopes. Since the invention of the spectroscope,
+however, fresh light has been thrown on the matter, for the spectrum
+which is shown by some of the nebulous patches is not the same as that
+shown by stars, and we know that many of these strange appearances are
+not made up of infinitely distant stars.
+
+We are talking here quite freely about nebulae because we have met one
+long ago when we discussed the gradual evolution of our own system, and
+we know quite well that a nebula is composed of luminous faintly-glowing
+gas of extreme fineness and thinness. We see in the sky at the present
+time what we may take to be object-lessons in our own history, for we
+see nebulae of all sorts and sizes, and in some stars are mixed up, and
+in others stars are but dimly seen, so that it does not require a great
+stretch of the imagination to picture these stars as being born,
+emerging from the swaddling bands of filmy webs that have enwrapped
+them; and other nebulae seem to be gas only, thin and glowing, with no
+stars at all to be found in it. We still know very little about these
+mysterious appearances, but the work of classifying and resolving them
+is going on apace. Nebulae are divided into several classes, but the
+easiest distinction to remember is that between white nebulae and green
+nebulae. This is not to say that we can see some coloured green, but that
+green appears in the spectrum of some of the nebulae, while the spectrum
+of a white nebula is more like that of a star.
+
+It is fortunate for us that in the sky we can see without a telescope
+one instance of each of the several objects of interest that we have
+referred to.
+
+We have been able to see one very vivid example of a variable star; we
+have seen one very beautiful example of a star cluster; and it remains
+to look for one very good example of a white nebula.
+
+Just as in finding Algol you were doing a little bit of practical work,
+proving something of which you had read, so by seeing this nebula you
+will remember more about nebulae in general than by reading many chapters
+on the subject. This particular nebula is in Andromeda, and is not far
+from Algol; and it is not difficult to find. It is the only one that can
+be well seen without a telescope, and was known to the ancients; it is
+believed to have been mentioned in a book of the tenth century!
+
+If you take an imaginary line down from the two left-hand stars of
+Cassiopeia, and follow it carefully, you will come before long to a
+rather faint star, and close to it is the nebula.
+
+When you catch sight of it you will, perhaps, at first be disappointed,
+for all you will see is a soft blur of white, as if someone had laid a
+dab of luminous paint on the sky with a finger; but as you gaze at it
+night after night and realize its unchangeableness, realize also that it
+is a mass of glowing gas, an island in space, infinitely distant,
+unsupported and inexplicable, something of the wonder of it will creep
+over you.
+
+[Illustration: _Dr. Max Wolf._
+
+THE GREAT NEBULA IN ANDROMEDA.]
+
+Thousands of telescopic nebulae are now known, and have been examined,
+and they are of all shapes. Roughly, they have been divided up into
+several classes--those that seem to us to be round and those that are
+long ovals, like this one in Andromeda; but these may, of course, be
+only round ones seen edgewise by us; others are very irregular, and
+spread over an enormous part of the sky. The most remarkable of these is
+that in Orion, and if you look very hard at the middle star in the
+sword-hilt of Orion, you may be able to make out a faint mistiness.
+This, when seen through a telescope, becomes a wonderful and
+far-spreading nebula, with brighter and darker parts like gulfs in
+it, and dark channels. It has been sometimes called the Fish-mouth
+Nebula, from a fanciful idea as to its shape. Indeed, so extraordinarily
+varied are these curious structures, that they have been compared with
+numbers of different objects. We have some like brushes, others
+resembling fans, rings, spindles, keyholes; others like animals--a fish,
+a crab, an owl, and so on; but these suggestions are imaginative, and
+have nothing to do with the real problem. In _The System of the Stars_
+Miss Clerke says: 'In regarding these singular structures we seem to see
+surges and spray-flakes of a nebulous ocean, bewitched into sudden
+immobility; or a rack of tempest-driven clouds hanging in the sky,
+momentarily awaiting the transforming violence of a fresh onset.
+Sometimes continents of pale light are separated by narrow straits of
+comparative darkness; elsewhere obscure spaces are hemmed in by luminous
+inlets and channels.'
+
+One curious point about the Orion Nebula is that the star which seems to
+be in the midst of it resolves itself under the telescope into not one
+but six, of various sizes.
+
+Nebulae are in most cases too enormously remote from the earth for us to
+have any possible means of computing the distance; but we may take it
+that light must journey at least a thousand years to reach us from them,
+and in many cases much more. Therefore, if at the time of the Norman
+Conquest a nebula had begun to grow dim and fade away, it would, for all
+intents and purposes, still be there for us, and for those that come
+after us for several generations, though all that existed of it in
+reality would be its pale image fleeting onward through space in all
+directions in ever-widening circles.
+
+That nebulae do sometimes change we have evidence: there are cases in
+which some have grown indisputably brighter during the years they have
+been under observation, and some nebulae that have been recorded by
+careful observers seem to have vanished. When we consider that these
+strange bodies fill many, many times the area of our whole solar system
+to the outermost bounds of Neptune's orbit, it is difficult to imagine
+what force it is that acts on them to revive or quench their light. That
+that light is not the direct result of heat has long been known; it is
+probably some form of electric excitement causing luminosity, very much
+as it is caused in the comets. Indeed, many people have been tempted to
+think of the nebulae as the comets of the universe, and in some points
+there are, no doubt, strong resemblances between the two. Both shine in
+the same way, both are so faint and thin that stars can be seen through
+them; but the spectroscope shows us that to carry the idea too far would
+be wrong, as there are many differences in constitution.
+
+We have seen that there are dark stars as well as light stars; if so,
+may there not be dark nebulae as well as light ones? It may very well be
+so. We have seen that there are reasons for supposing our own system to
+have been at first a cool dark nebula rotating slowly. The heavens may
+be full of such bodies, but we could not discern them. Their thinness
+would prevent their hiding any stars that happened to be behind them. No
+evidence of their existence could possibly be brought to us by any
+channel that we know.
+
+It is true that, besides the dark rifts in the bright nebulae, which may
+themselves be caused by a darker and non-luminous gas, there are also
+strange rifts in the Milky Way, which at one time were conjectured to be
+due to a dark body intervening between us and the starry background.
+This idea is now quite discarded; whatever may cause them, it is not
+that. One of the most startling of these rifts is that called the
+Coal-Sack, in the Southern Hemisphere, and it occurs in a part of the
+sky otherwise so bright that it is the more noticeable. No possible
+explanation has yet been suggested to account for it.
+
+Thus it may be seen that, though much has been discovered, much remains
+to be discovered. By the patient work of generations of astronomers we
+have gained a clear idea of our own position in the universe. Here are
+we on a small globe, swinging round a far mightier and a self-luminous
+globe, in company with seven other planets, many of which, including
+ourselves, are attended by satellites or moons. Between the orbits of
+these planets is a ring or zone of tiny bodies, also going round the
+sun. Into this system flash every now and then strange luminous
+bodies--some coming but once, never to return; others returning again
+and again.
+
+Far out in space lies this island of a system, and beyond the gulfs of
+space are other suns, with other systems: some may be akin to ours and
+some quite different. Strewn about at infinite distances are star
+clusters, nebulae, and other mysterious objects.
+
+The whole implies design, creation, and the working of a mighty
+intelligence; and yet there are small, weak creatures here on this
+little globe who refuse to believe in a God, or who, while acknowledging
+Him, would believe themselves to know better than He.
+
+THE END
+
+BILLING AND SONS, LTD., PRINTERS, GUILDFORD
+
+
+
+
+
+End of Project Gutenberg's The Children's Book of Stars, by G.E. Mitton
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