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diff --git a/old/51284-0.txt b/old/51284-0.txt deleted file mode 100644 index 8b1e847..0000000 --- a/old/51284-0.txt +++ /dev/null @@ -1,6132 +0,0 @@ -Project Gutenberg's The Ways of the Planets, by Martha Evans Martin - -This eBook is for the use of anyone anywhere in the United States and most -other parts of the world 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. If you are not located in the United States, you'll have -to check the laws of the country where you are located before using this ebook. - -Title: The Ways of the Planets - -Author: Martha Evans Martin - -Release Date: February 22, 2016 [EBook #51284] - -Language: English - -Character set encoding: UTF-8 - -*** START OF THIS PROJECT GUTENBERG EBOOK THE WAYS OF THE PLANETS *** - - - - -Produced by Shaun Pinder, Thiers Halliwell and the Online -Distributed Proofreading Team at http://www.pgdp.net (This -file was produced from images generously made available -by The Internet Archive) - - - - - -Transcriber’s notes: - -The text of this book has been preserved as in the original, apart from -a few obvious misspellings. - -Corrected misspellings and redundancies include the following: - - comparsion —> comparison - dining —> during - clamly —> calmly - atronomer —> astronomer - oi —> of - the —> (deleted) - a —> (deleted) - -In this digital version, paired underscores denote _italicised text_. -Footnotes have been positioned below the relevant paragraphs. - -The text contains symbols that will not necessarily display correctly -with all viewing devices, and one symbol (for the Full Moon) cannot be -replicated digitally. It is represented in this text by an open circle. -For best viewing, the device’s character encoding should be set to -Unicode (UTF-8), and a Unicode font selected such as Arial Unicode MS, -DejaVu, Segoe UI Symbol or FreeSerif. - - - - -[Illustration: A WHIRLING SPIRAL NEBULA, TYPICAL OF THAT FROM WHICH -THE SUN AND PLANETS WERE PROBABLY EVOLVED - -In the process of evolution the dense center becomes the controlling -sun and the smaller spots of condensation form the planets. This -particular nebula lies just under the end of the handle of the Big -Dipper. It was photographed at Mt. Wilson Observatory.] - - - - - THE WAYS OF THE PLANETS - - - BY - MARTHA EVANS MARTIN, A.M. - AUTHOR OF - “THE FRIENDLY STARS” - - - [Illustration] - - - NEW YORK AND LONDON - HARPER & BROTHERS PUBLISHERS - MCMXII - - - - - COPYRIGHT, 1912, BY HARPER & BROTHERS - - PRINTED IN THE UNITED STATES OF AMERICA - PUBLISHED OCTOBER, 1912 - - - - -CONTENTS - - - CHAP. PAGE - - I. ON MAKING ACQUAINTANCE WITH THE PLANETS 1 - - II. OUR RELATION TO THE PLANETS 11 - - III. WHAT THE PLANETS ARE, AND WHAT THEY APPEAR TO BE 17 - - IV. THE ORIGIN OF THE PLANETS 26 - - V. THE SEVEN GREAT PLANETS 38 - - VI. THE MOVEMENTS OF THE PLANETS 46 - - VII. HOW THE INFERIOR PLANETS SEEM TO MOVE 56 - - VIII. HOW THE SUPERIOR PLANETS SEEM TO MOVE 65 - - IX. THE PATH OF THE PLANETS 71 - - X. MERCURY--WHEN AND WHERE TO FIND MERCURY--DISTANCE AND - BRIGHTNESS--MERCURY’S SIZE AND THE CONSEQUENCES OF IT-- - WHAT THE SUN DOES FOR MERCURY--TRANSITS 93 - - XI. VENUS--WHEN AND WHERE TO SEE VENUS--DISTANCE AND - BRILLIANCY--VENUS’S LIKENESS TO THE EARTH--ATMOSPHERE, - DAY AND NIGHT, AND SEASONS--TRANSITS 122 - - XII. MARS--HOW TO IDENTIFY MARS--WHEN AND WHERE MARS MAY BE - SEEN--SIZE, ATMOSPHERE, AND TEMPERATURE--DISTANCE AND - BRILLIANCY--DAY AND NIGHT, AND SEASONS--SURFACE - ASPECTS--SATELLITES 151 - - XIII. JUPITER--PLACE IN THE SKY--DISTANCE, LIGHT, AND HEAT-- - DAY AND NIGHT, SEASONS, AND ATMOSPHERE--SURFACE - FEATURES--SYSTEM OF SATELLITES 183 - - XIV. SATURN--AROUND ONE CIRCUIT OF THE SKIES WITH SATURN-- - DISTANCE AND SIZE--SURFACE ASPECTS AND CONSTITUTION-- - DAY AND NIGHT--THE RINGS AND MOONS OF SATURN--SEASONS 206 - - XV. URANUS 225 - - XVI. NEPTUNE 234 - - XVII. THE LITTLE PLANETS, OR THE ASTEROIDS 244 - - XVIII. CONCLUSION 258 - - SYMBOLS USED IN ALMANACS 267 - - INDEX 269 - - - - -ILLUSTRATIONS - - - A WHIRLING SPIRAL NEBULA, TYPICAL OF THAT FROM WHICH - THE SUN AND PLANETS WERE PROBABLY EVOLVED _Frontispiece_ - - MAP SHOWING THE CONSTELLATIONS OF THE ZODIAC AND THE - LINE OF THE ECLIPTIC RUNNING THROUGH THEM _Facing p._ 76 - - THE LOVELY CRESCENT THAT VENUS SHOWS WHEN TO OUR VIEW - SHE IS AT HER GREATEST BRILLIANCY " 136 - - RELATIVE APPARENT SIZE OF VENUS AT DIFFERENT PHASES - OF ILLUMINATION _Page_ 137 - - THE TWO PHASES OF MARS _Facing p._ 152 - - MARS: DIFFERENCE IN ITS APPARENT SIZE AT ITS NEAREST, - MIDDLE, AND FARTHEST DISTANCE FROM THE EARTH _Page_ 169 - - JUPITER, THE MAMMOTH MEMBER OF THE SOLAR FAMILY-- - LARGER THAN ALL THE OTHER PLANETS PUT TOGETHER _Facing p._ 184 - - SATURN AND ITS RINGS " 220 - - - - -THE - -WAYS OF THE PLANETS - - - - -I - -ON MAKING ACQUAINTANCE WITH THE PLANETS - - -It is sought in the following pages to give a simple account of what -may now be said to be known of the character of the planets, and to -describe with as little technicality as possible their movements and -aspects and relations. An endeavor is made to impart concerning each -one of them not, surely, profound learning, but just a good, every-day, -practical notion, so that the mere name will call up a definite object, -with its own attributes, appearance, and behavior, entirely distinct -from any other planet or from any other object in the skies. - -An endeavor is made also to so simplify and direct the observation -that any one, after a little practice, will know almost without -hesitation, on seeing a planet in the sky, that it is a planet, and -not a fixed star, and exactly what planet it is. The situation and -aspect of it will then as quickly and clearly pronounce it to be -the individual planet that it is as the sight of a person of one’s -acquaintance proclaims him to be that person, and no other. The very -name of Venus, for example, and still more the sight of Venus, will -call up a conception of Venus, with the particular atmosphere and light -and movements and wanderings which make her what she is. On looking -at her the observer will at once know why she occupies the special -position in the sky in which he sees her, why she is not so bright as -she was when she was last in view, or is so much brighter than she was -then, about how long she is likely to remain where she is, and when she -goes what will become of her. - -For far off and truly mysterious as the planets are, it still is with -them as with most other objects in nature: a very little knowledge -of their aspects and their ways begets a sense about them that makes -the most casual observation of them interesting and, as far as it -goes, intelligent. The slightest glance at them betrays some shape, or -position, or light, or other quality, which at once makes recognition -of them unmistakable. They disclose themselves oftentimes, one can -scarcely say how, just as persons with whom we are intimate do by some -half-caught outline, motion, or posture; or just as the trees do to an -observer who knows, for example, an oak-tree from an elm, whether they -are covered with their own peculiar verdure, or whether they stand with -bare branches stretched out and colored in their own peculiar way. - -This instant recognition of the planets is, of course, not to be had -by simply reading about them. Such practical familiarity with them is -attained only by seeking them out over and over again and looking at -them with attention, with eagerness, and with all one’s faculty. With -them, as with other natural objects, it requires observation truly to -know them. But then, observation, when one gets a little started in it, -is a great deal more interesting, a great deal more absorbing, than any -reading about them can ever be. It is also a very easy thing to begin, -for, after all, it is not much more than looking and then looking -again. In doing this one can hardly tell just when an object ceases to -be strange, and then becomes familiar, and finally is so much a part -of every-day knowledge that one knows it at a glance. But this is what -happens in the case of any natural object when we observe it often and -with true attention. - -In the case of the planets, if one is interested at all, every stage -in the cultivation of such an acquaintance is full of pleasure. Even -to one who regards them only as a part of the general aspect of the -sky, they are the most beautiful objects in it and always the first to -attract special attention. Nine times out of ten, when any one asks -what a certain star is, it proves to be one of the planets. When one -of them is visible a person can hardly glance at the heavens without -noticing it, even if he does not stop to think about it. But if he -does stop to think about it and notices that it is far larger than any -star he has noted before, that it hangs low in the western sky early -in the evening, and shines with a brilliant silvery light, and if he -then learns that it is Venus, will he not always have a pleasant thrill -of recognition when he again sees such a star in such a position and -knows it as Venus, among the planets as surpassing in beauty as the -goddess of that name was among the immortals? Or, if in the east, at -the same time in the evening, he sees a brilliant, solid-looking, -unblinking star shining with a white light, but pinkish white, not -silvery, and finds it to be Jupiter, will not such a star in such a -situation be to him ever after a pleasant acquaintance that he can call -by name? Not that Jupiter and Venus are always in these positions, or -shine in just this way at all times. These are their places and aspects -at certain times, frequently recurring, and at such times always -unmistakably distinguish them. - -It is, then, merely the matter of a little more and yet a little more -observation, in order to come to know any one of the visible planets in -all its varying aspects and situations. Of course, at the start some -guidance is necessary, but only a little; and that little, if it is of -the right sort, should not be irksome. To provide such guidance is one -of the aims of this book. That is, indeed, its main aim. - -But in addition to what, as a help in observation, it may find to -say regarding the appearance and movements of the planets, it will -endeavor to give also ample information concerning their character and -constitution. - -It is hoped that this may be done without weighting the narrative -with figures, though some of the peculiarities of the planets must be -expressed by means of numbers. Certainly no mathematical problems will -be presented. But it will be profitable to remember that every one of -the intimate things we know about the planets has come to us through -the long and laborious mathematical work of astronomers. To them we owe -the extinguishable debt that we owe to all special workers who put us -in possession of the facts that interpret life to us. - -For the astrology and poetry and romance of the planets one must -go elsewhere. Nearly every book on the subject of the planets--and -there are many of them--has some information about these things; and -properly, too, for every genuine emotion and every real fancy has its -value. But neither curious lore of the planets nor the sentiment and -emotion they have produced in others is what the author of this book -is striving to set forth. It is something much more vital than this. -What we wish to contemplate here are the plain facts, the knowledge of -which enlivens and enriches one’s mind and nature. If the contemplation -of them kindles one’s fancy or excites one’s emotions, these results -at least will not be second-hand. If the bare facts, simply and -plainly told, and the view of the planets themselves as they wander -through their courses in the sky, do not awaken one’s understanding -and imagination, no amount of poetry or romance that other people have -built up around the planets will arouse anything more than a factitious -interest in them. It is when our own faculties are at work and our -own fancy plays over a subject that we become genuinely and lastingly -interested in it. - -The facts themselves are in the main quite simple, and will not be -given here as anything else than that. They have been fairly wrested -from that mysterious thing called space by the mighty power of mind -and unceasing labor. Our knowledge of them is due to long nights of -watching and long days of calculating; to long and careful testing -and considering of theories, only to find that something else must be -tried; to courage to begin all over again, to sudden inspirations, and -sometimes to those lucky discoveries that seem almost like miracles. - -The subject of the planets has in some respects a greater interest even -than that of the stars, because we know, after all, more about them. -We sometimes have a feeling, though, that we know more of the stars, -although the stars are so much farther off. Why we have this feeling -it is easy to explain. Knowing them to be so far removed from us, we -really approach the stars with a different expectation. The few things -that we have learned about them have in themselves such a magnitude -that it makes them seem a greater body of knowledge than they truly -are. The stars are indeed so far away, and what we know of them has -to be expressed in such large terms, that the mind does not demand in -that information the minute exactness that it seeks for in the case of -nearer objects. - -In the case of the stars, we seek mainly to know their distances, the -direction of their motions, the speed with which they travel, and -their probable connection with each other. The fact that in computing -the distance of a single star, many trillions of miles away, the -result may be a little less than exact does not keep us from learning -what ones are sufficiently near for their distances to be measured -at all and what ones are immeasurably remote. Whether they travel at -the rate of exactly three or three hundred miles a second, we can -learn that some are traveling at somewhat the same rate of speed as -our sun travels, and some incredibly faster; that certain groups are -going in one direction and certain groups in another; that some are -approaching us and some are receding from us. And thus we can classify -them and learn the significance of these facts, and, little by little, -gain a definite understanding of the construction and meaning of the -entire universe. Their very remoteness gives a certain compactness to -the information we have about the stars, by making it necessary to -generalize more than we would if they were near enough to yield more -details; and we are in a way satisfied with this more general sort of -knowledge of them. - -But the very fact of our knowing so much about the planets extends -our curiosity concerning them and makes us feel that we ought to -know more. The mind is provoked into more minute speculations about -them, and we demand more exactness of information and knowledge of -a more specific or intimate sort than would satisfy us in regard to -the stars. Atmosphere, habitability, exact size, seasons, and day and -night, are the kind of things we most seek to know in reference to the -planets. These are such definite things that conclusions concerning -them are subject to close criticism, and differences of opinion in -regard to them thus sometimes occur which tend to give one a more or -less confused notion of what is really known. As a matter of fact, our -information about the planets is much fuller than our knowledge of the -stars, as we would naturally expect it to be. Much of what we seek to -know about the stars has long been common knowledge about the planets. - - - - -II - -OUR RELATION TO THE PLANETS - - -To know about the planets is to know about ourselves. The earth is one -of them. Whatever their origin, the earth’s is the same. It and they -are formed from the same nebula, controlled by the same central body, -subject to the same laws, and destined for the same fate in the end. In -this, the stars and the planets are not alike. They all shine upon us -with the same sweet friendliness, and commonly we make no difference -between them in our feeling for them. But the stars are bright and -beautiful acquaintances living far away in their own domain. The -planets are members of our own family, bone of our bone and flesh of -our flesh, living comparatively near to us, within the domain of our -common source of life, the sun. - -One evening last autumn I was coming up Broadway, New York, with a -friend, when we encountered at Union Square a man with a six-inch -telescope directed toward the eastern sky. He was soliciting those -who passed to stop and look at Mars and Saturn. Both of these planets -were then very bright. They were also fairly near together, and so low -in the east that one could scarcely help seeing them. But the people -passed back and forth with hardly so much as a glance at the man and -his telescope, and for the most part never even raised their eyes to -the sky with a passing curiosity to see what it might be that he wanted -to show them. My friend and I stopped and took each a view first of -Mars and then of Saturn. While we were looking at the planets, a few -of the passers-by began to loiter about, half smiling at us for so -playing in public, slightly curious to see how we were faring at it, -but for the most part apparently indifferent to what we were seeing. We -had a fine view of Saturn lightly resting in his nest of rings, and an -equally good view of the comical “eye” of Mars. - -After we had finished, one or two others, evidently prompted by our -example, followed us at the telescope. One or two inquired of us what -the stars were that had so interested us, and one, pointing to Mars, -wanted to know if it was Venus. As the crowd grew larger a few more -ventured to take a look, much as they might venture to take their -chance at hitting the bull’s-eye in some shooting-gallery. With the -telescope pointed at Saturn, the man droningly chanted: “This planet -is 887,000,000 miles from the sun. The ring you see is 170,000 miles -in diameter,” and so on. These, to be sure, were the facts--and most -marvelous facts, too--but without much meaning to one who knows nothing -much about the planets; and the manner of their recital certainly did -not make them alluring. I could not myself help feeling that the people -there were missing a valuable opportunity, and that it would be only -fair to them for some one fairly to cry out: “Come here and look at -this planet. It is different from anything else you have ever seen or -ever will see. It was at one time a part of the same nebulous mass -that we were a part of. It is in the same system of worlds with us. It -was formed in the same way that this world was formed. It is in itself -the most wonderful thing you ever saw, and it is bound, as we are, to -the sun by the ever-drawing tie of gravitation. The very position of -our own world in space is more or less influenced by it. If anything -should happen to it, it might be a serious matter to us.” - -For it is true that we are thus closely bound to the planets. The -family tie among us is of far more force and significance than in any -ordinary case of common origin. Human family ties wear, as we know, -often into the merest threads, or even become no ties at all. But that -between the earth and the planets remains apparently as close and -strong as ever it was. The law of gravity, under which the earth draws -toward its center every atom of matter surrounding it, and thus holds -together all the atoms composing it, is not solely terrestrial in its -application. It is probably universal. It certainly applies to every -part of our little family of worlds. Every particle in the solar system -attracts toward it every other particle in that system with a force -determined by its mass and its distance. The sun, by reason of its -immense size, compels the earth and all the other planets forever to -circle around it. But the planets themselves have just as much power of -attraction as the sun, atom for atom. - -Thus, while the sun controls the motions of all of them, each pulls at -the other, and, according to its power, determines how much the path -of each shall vary from the course around the sun it otherwise would -make. In the case of the smaller planets, this gravitational influence -is, of course, very slight, and so subtle that we here on earth are not -even conscious of it. But it is, nevertheless, real and continuous. It -is greatest between the two largest planets, Jupiter and Saturn; but -it was enough in the case of Uranus and Neptune to lead, by its mere -manifestation on the earth, to the discovery of Neptune, the farthest -planet. - -Being thus of the same origin with the planets, having the same life -history, being bound to them in space by a tie that is perhaps eternal, -how can we fail to have the most intimate interest in their nature and -all that concerns them? - -But in addition to their close relationship to us there is, to make -them of peculiar interest, the fact that, after the sun and the moon, -they are for our eyes the most splendid objects in all the brilliant -panorama of the sky. Such of them as we can see at all with the naked -eye are most of the time much brighter than any first-magnitude star. -As they wander from constellation to constellation the soft light of -their placid faces gives a beauty and variety to the spectacle that -endears them to us, and at the same time enhances by contrast their own -charm and that of the glittering, unchanging stars. - -There is nothing that gives one such a sense of sweet familiarity with -the heavens as a really recognizing acquaintance with the planets. They -are not, like the stars, associated with particular seasons. They come -sometimes with the gay company of stars that dance their way across the -cold winter skies, and sometimes with those that shine during the soft -summer nights. Often in the spring and autumn we see some one of them -before the sun is fairly down, and, before the light of an ordinary -star can yet be seen, hanging in lone brilliancy as the evening star; -and often an early riser has the reward of seeing one as a morning star -glowing almost in the rays of the rising sun. Thus they are, one and -another, with us at all times and seasons, and it accords with the fact -of the relation being a family one that we have in their coming and -going a sense of frequency and informality which we cannot have in the -more regular and seasonal coming and going of the stars. - - - - -III - -WHAT THE PLANETS ARE, AND WHAT THEY APPEAR TO BE - - -The planets are dark, opaque bodies which revolve at varying distances -and at varying rates of speed in orbits more or less circular around -the sun as a center. They have no light of their own, as the stars -have, but shine wholly by reflected light received from the sun, which -itself is a star. The amount of light they show to us depends upon -their size, their distance, and their power of reflecting the light -they receive. - -In comparison with the stars, the planets are very near to us. Our -sun, which is in constitution a star, but very widely separated from -any other star in the universe, holds all his family of planets by -the tether of gravitation, and so keeps them circling about him in a -very small space, as astronomical space is measured. To all of the -planets except Mercury, we ourselves are nearer than the sun is. To -be sure, this distance between us and the planets, as measured by any -terrestrial measure, is not exactly small. It is only by comparison -that we can be said to have anything like a cozy relation to them. For -merely earthly affairs we use terrestrial measures. In solar affairs we -measure by an astronomical unit, which is the sun’s distance from the -earth, ninety-three millions of miles. When we say a planet’s distance -from the sun is thirty astronomical units, we mean it is thirty times -farther than the earth is from the sun. - -For matters outside of the solar system, the unit of measure is the -number of miles that light travels in a year. The speed of light is a -little more than 186,000 miles in a second. This is equal to about six -trillions of miles in a year, or about sixty-three thousand times the -distance of the sun from the earth, our family measuring-stick. From -the nearest star it takes light more than four years to come to us. -From the nearest planet light comes in less than three minutes, and -from the farthest one it makes the journey in a little more than four -hours. - -As compared with other heavenly bodies, therefore, the sun and the -planets are very near together, occupying a very small space in the -immensity of the universe, immeasurably isolated from all the other -systems and, so far as we know, immeasurably smaller as a system than -most of them. - -The whole body of the planets is divided according to size into two -classes, the major and the minor planets. When we refer generally to -the planets, the major planets only are meant. The minor planets are -usually called the asteroids, or planetoids. There are many hundreds -of them, and only one--and that barely--can be seen with the naked -eye. The other planets are eight in number, including the earth, which -is, after all, nothing but one of the smaller of the major planets. -They are, in the order of their distances from the sun: Mercury, the -nearest, Venus, the Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. -Of these only five--Mercury, Venus, Mars, Jupiter, and Saturn--can be -seen from the earth without optical aid. Occasionally, when Uranus is -very favorably situated, a person with an exceptionally good eye, who -knows exactly where to look for the planet, can see it. Neptune is -about equal to an eighth-magnitude star in brightness, and can never be -seen without the aid of a telescope. Mercury, while quite bright enough -to be seen, is not often situated favorably for observation. It is very -near the sun, and is generally obscured either by the light of the sun -when the sun and the planet are above the horizon, or by the haziness -of the atmosphere when the sun is below the horizon and the planet a -little above it. In regions of considerable altitude with a clear, rare -atmosphere, Mercury is more often seen; but never for very long at a -time. - -The only planets, therefore, that are a part of our evening spectacle -in the skies are Venus, Mars, Jupiter, and Saturn. These four happen -to be not only the ones we oftenest see, but also the most interesting -of all the planets from various points of view. Venus and Mars are -the nearest to the earth, and most resemble it, and hence are the -most inviting for speculations which have a human interest, such as -habitability, the presence of life, and kindred ideas. Jupiter and -Saturn are interesting above all the others in their splendor and -size, and in their importance as the centers of systems of their own. - -As seen by us, the planets are similar to the stars, but with very -distinct differences in appearance, which, when once familiar, mark -them so unmistakably as planets, and not fixed stars, that we need -never get the two confused. The first and easiest distinguishing mark -to notice is that they do not twinkle, as the stars do, but shine with -a steady light similar to that of the moon. This is an invariable -difference between stars and planets, and one needs only to stop and -truly look at them in order to detect it. And once it has become -familiar, it discloses itself at a glance. - -This difference between stars and planets is due almost solely to -difference of distance, though the twinkling is caused by our own -atmosphere. The stars are too far away to send us anything but a mere -point of light, and the unequal density of the waves of air sweeping -over this point of light keeps it dancing before our eyes, causing -the phenomenon that we call twinkling. But the planets, being nearer -to us, show a disc, from every point of which comes a line of light, -making the total light of some volume; and these inequalities of the -air are too small to interfere with it to any extent. Sometimes, when -the atmosphere is particularly unsteady, it happens that the light of -a planet is somewhat affected by it when the planet is just rising or -setting and is, consequently, near the horizon, and that it then seems -to twinkle a little. But this departure from the rule is always slight -and of short duration, in the case of the four planets most seen. -Mercury, never being seen anywhere except near the horizon, often seems -to twinkle; but then he is seldom seen at all, and, when visible, is in -other ways so well marked that one cannot fail to recognize him. - -So the steady light may justly be said to be invariable, because the -unusual conditions are easily detected. When the atmosphere is such as -to cause even the planets to blink a little, it has an effect also on -the stars. At such a time they will appear to be fairly dancing. This -effect is apt to occur on the clear nights of winter, the atmosphere -being more unsteady then. Such nights, because of the extreme -liveliness and brilliancy that they lend to the stars, are attractive -times for amateur observations. For the astronomer, however, they are -not so favorable. For the seeing of small details such as he seeks, the -steadiest atmosphere is necessary. - -Though the planets are near enough to show a disc, they are not -sufficiently near to show to the naked eye as sharp an outline as the -moon’s. Usually the edge is more or less rayed like that of a fixed -star, which adds somewhat to the difficulty of distinguishing them -from the stars until their aspect has become familiar to us. The fact -that we are looking at a disc is plainly shown when an occultation by -the moon occurs. When the moon occults a fixed star, it passes between -us and the star. At such times the star disappears behind the edge of -the moon instantly, as a mere point naturally would. When a planet is -occulted by the moon, it disappears gradually as the moon covers more -and more of its disc, thus showing unmistakably the nature of it. - -After steadiness of shining, the next most obvious mark of difference -between a planet and a star, from our point of view, is the movement -of the planets. A star remains always in one place with relation -to the other stars, while the planets move about from constellation -to constellation, seeming to travel sometimes toward the east and -sometimes toward the west. - -This difference also is due solely to a difference of distance. The -stars as well as the planets are constantly in motion. Most of them, in -truth, move at a rate which would make the rate of motion of a planet -a mere snail’s pace in comparison. Arcturus, for instance, is supposed -to be moving at the rate of two or three hundred miles a second, and -there are other fixed stars with an equally rapid motion. The swiftest -moving of the planets does not achieve much more than twenty-nine miles -a second, while the slowest swings along at a rate of but little more -than three miles in the same length of time. - -These are the real rates of speed of the stars and planets; but they -are not at all what they seem to us. The difference in distance is so -great that for centuries and centuries the flying stars have seemed -to men to remain in the same place in the skies, and so we call them -fixed. The planets, so slow-journeying as they are in comparison, seem -to us to be moving among the constellations at rates varying from more -than a degree a day in the swiftest to between two and three degrees a -year in the slowest. - -Hence, if through lack of practice in observation a person is not at -once able to distinguish the difference between the stars and the -planets in the character of their light--that is, whether they twinkle -or shine steadily--he can, by taking a little longer time, at most only -a few days, determine whether the object he sees is a star or a planet -by noticing whether it has any motion among the other stars. Venus and -Mars will show some movement in one evening. Jupiter and Saturn may -require a little more time to disclose their motion. - - - - -IV - -THE ORIGIN OF THE PLANETS - - -Different as the planets are as individuals, they have too many -characteristics in common to admit any question of their common origin. -They are not simply stars of one sort and another that happen to lie -nearer to us than the great body of stars that spangle the heavens, but -are, without doubt, all of one family with us in their origin, as well -as in their situation. How they originated, and exactly what has been -their course of evolution, has long been an engrossing problem among -philosophers; and it is not yet solved. - -In the sense that the human race is all of one family, the planets are -but a part of the great universe that lies about us and is in part -visible to us. The forms in which we know matter as existing in the -universe, outside of the solar system and of the minor forms in our own -world, are those of stars and nebulæ. It seems as if either of these -could, and in fact does, form out of the other. We do not at all know -how in the beginning matter took the form of either, or which came -first. But it is believed that a star is formed by the condensation -of a nebula, and that a nebula is often formed by the collision or -near approach of two stars and the consequent disintegration of their -particles. - -The sun is a star not very different from most of the other stars, as -we believe them to be, except that it is smaller than most of them. -It is the center around which we and all the planets revolve, and it -is believed that we were all once a part of the very body of it. For -astronomers are substantially agreed that the whole solar family, -including the sun and all the planets, has been evolved from a great -nebula which, in one form or another, at one time filled practically -the whole of the immense space from the sun to the outermost planet of -the system. While this cannot be said to have been exactly proved, yet -it accords with all the known facts of the solar system. As to how this -nebula originated, and what its shape was, and in just what way the -planets were formed from it, there is more diversity of opinion. - -Up to the middle of the eighteenth century no really scientific theory -of the evolution of the solar system was formulated, and it was not -until the very last years of that century that any theory of the origin -of the planets was published which received anything like universal -acceptance. - -This was the case, however, with the famous nebular hypothesis of -Laplace, which was published in 1796, and for a time seemed so nearly -to account for the various phenomena of the motions and relations of -the planets that it was not only accepted in the scientific world, but -became almost as much a part of universal knowledge as that the earth -is round. But even this theory has not completely stood the test of -time, which inevitably brings that close scientific investigation that -any theory must undergo when it is used as a working basis to which all -facts and secondary theories must be correlated. - -The original nebular hypothesis supposed this vast nebula to be -in rotation on its axis. As it condensed, the falling-in of the -particles caused its rotation to become more rapid, until finally, -under the strain of this, a ring of matter was “thrown off” from the -outer edge. Or, as was sometimes said, the inner part condensed and -left a detached ring of matter. This ring, continuing to rotate in -the direction given it by the rotation of the central mass, finally -condensed into a planet, rotating on its axis and revolving about -the central sun in the same direction as the ring had revolved. The -satellites of the planets were thought to have been formed by the same -process from the planets while these were still in a plastic state. -Saturn, with its wonderful system of rings and satellites, was thought -to be a minute object-lesson of a planet in course of evolution, and -this we have often heard said. - -I am sorry it is not so. I had much enthusiasm in my youth over this -beautiful and orderly arrangement of things: first, the splendid -hypothesis, the achievement of a noble mind; then the little model -showing the work in its progress; and, finally, the beautifully -finished system, the rings all rolled up into planets, traveling -unceasingly in paths which eternally marked the size of the central -body, or sun, at the time of the separation. - -But it is now pretty certain that this cannot be the way it all -happened. Closer investigation shows that there are mechanical -difficulties which were not at first fully recognized. A series of -rings could not have been left off by a body so wholly gaseous. The -particles composing them would not be sufficiently coherent to permit -of separation in any such compact, uniform, and decisive manner. Then, -even if such a ring were thrown off, it is not at all certain that -it could condense into a planet. Its tendency, indeed, would be to -disintegrate rather than to condense. In a body so tenuous the mutual -gravitation of its particles would be too feeble to complete the work. -Besides, in conflict with the theory is the fact that a few of the -satellites of the planets revolve in a direction contrary to that of -the planet. And there are other minor, but still important, details in -the mechanism of the solar system which cannot be accounted for by the -ring theory. - -And so, while astronomers are still agreed that the whole solar system, -which includes the planets, was evolved from a primeval nebula, the -theory of leaving off rings which condensed into planets is not found -tenable, and the search for some more acceptable theory or some -modification of the Laplace theory is now occupying a number of eminent -astronomers and philosophers. - -The result of all this is that no theory of the manner of the evolution -of the planets is definitely accepted by the body of astronomers. Much -hard labor and ingenious reasoning have been expended in endeavoring to -formulate some hypothesis by means of which we may account for observed -phenomena. The astronomers with whom these theories have originated -are, naturally, more or less ardent in setting them forth. Thus one -occasionally sees a decisive and authoritative statement of a theory -of the evolution of the planets that seems at first view to account -for everything. But no one of these has yet been entirely accepted by -astronomers, who are as a class cautious and conservative, and are -necessarily critical of any theory, because the value of much of their -future work depends upon its accuracy and sufficiency for all details. - -The theory which at present seems more nearly than any other to offer -a reasonable explanation of most planetary phenomena is based upon -the supposition that the nebula from which the sun and planets were -evolved was in the shape of a spiral, and not the gaseous mass that -the original nebular hypothesis supposed. The fact that among the -many thousands of nebulæ that have been discovered and observed a -very large proportion of them are in this form, aside from any other -consideration, suggests a great probability that the one from which the -solar system was evolved was a spiral. - -The spiral nebulæ seem to be of a somewhat different constitution from -the other nebulæ, and show on observation spots of condensation here -and there, which at least suggest the formation of systems of planets. -This indicates that ours may be only one of many such systems in -process of evolution; but it is certainly among the smallest of them, -for most of the spiral nebulæ are immensely greater in size than the -one required to form our little system. Its few trillions of miles of -diameter, though it seems so vast to us, is quite insignificant in -comparison with a large proportion of the spiral nebulæ in the universe. - -A spiral nebula is in the form of a disc somewhat resembling that -familiar form of fireworks known as a pinwheel. The typical form of it -has two arms projecting from opposite sides of the whirling figure. It -is much denser toward the center, where the spiral would naturally be -more tightly wound, and has smaller spots of condensation scattered -like knots here and there along the fiery arms. In the process of -evolution the denser center becomes the controlling sun, and the -smaller spots of condensation form the planets, which are eventually -detached from the revolving mass, but continue to revolve about the -center as they were doing from the beginning. According to the mass it -has in the beginning, the planet gathers up by gravitative attraction -all the material in its region, gaseous or more or less condensed, and -grows by this accretion. If the nucleus happened to be a large one -before it separated from the parent body, it will have sufficient force -of gravitation to gather in large quantities of material and greatly -increase its size, and thus become a large planet. If it is only a -small nucleus, it has less power of attraction, and gathers in less -material. - -When these condensations of matter which are the nuclei of the planets -break away from the parent body, they sometimes carry with them still -smaller nuclei, which, if they are not too near the original center, or -sun, are destined to remain under the control of the planets and become -their satellites. The number and size of the satellites a planet -has depends upon the size, and hence the controlling force, of the -nucleus which is its foundation, and also upon the number of spots of -condensation that chanced to be formed in its neighborhood sufficiently -near to come under the gravitational control of the planet. If by any -chance the nucleus which was to form the largest satellite of Jupiter -had been in the situation of Mercury, for instance, it might well have -given its allegiance to the sun, instead of to Jupiter, and thus have -become a planet. - -Under the ring theory the outermost planet, Neptune, would be the -oldest of the planet family, and the one nearest the sun, Mercury, -would be the latest born and youngest. But the physical development -of these planets seems to indicate, in truth, exactly the opposite -of this, as we shall see later on. Under the spiral-nebula theory -the planets may be nearly of the same age, their different states -of development being due mainly to difference in size and to some -peculiarities of situation. If the nucleus happened to be near the -outer edge of the spiral, it would be formed from the lighter matter -composing the outer part of the nebula, and this seems to be the case -with the outer planets. If it were near the dense center of the nebula, -it would be composed of denser material, and this seems to be so in the -case of the inner planets. - - * * * * * - -A nebula, it is thought, is formed by the collision or the near -approach of two of the many stars, or suns, that we know are traveling -about at high velocities as vagrants here and there through space. If -the two bodies come together centrally, the force of the impact will -generate heat sufficient to convert them into a nebula; but this will -not necessarily be spiral in form. If they come together obliquely, the -chances are that they will form into a rapidly rotating spiral disc. - -But in order to form a spiral, it is not necessary that there should -be an actual collision. Because of the force of gravitation the near -approach of two stars would subject them to an enormous strain from -their pull upon each other, and there is a limit within which they -cannot approach without being literally torn to pieces from the -effect of this tidal force. Even if they do not approach within this -fatal limit, which is a little less than two and one-half times the -radius of the body, they may come so near as to change their character -entirely, and, through their tidal influence on each other, form into a -rotating spiral nebula with two arms projecting from opposite sides of -the spiral. - -It now seems probable that it was after this manner that the sun and -its family of planets were formed. The matter which is contained in -them may have been in the form of a dark, solid body pursuing some -sort of course in space. In its journeying it came near another body -and was awakened into a life of activity in the form of a flat, spiral -nebula which was left spinning around in a pyrotechnic manner, the -matter composing it much diffused at the outer edges and densest in -the center. Scattered through it were the more or less condensed spots -which were the embryonic forms destined to come forth from the parent -body as the individual planets. - -When the separation was completed, each planet fed and grew upon all -the matter that it had the force to draw to it, and it swept clean the -space that lay within the limits of its power. If the particles thus -gathered in were small and slow of motion, they became a part of the -body of the planet. If they were large and swift, they became members -of the planet’s family as satellites. In whatever area of the nebula -each planet came into a separate existence, it fed upon the matter -which that area afforded. In the case of Neptune, at the outer edge of -the system, it was very diffuse matter; in Mercury’s region, nearer the -center, it was more dense. - -Thus in our family of planets, though its members were born of the same -parent and developed under the same guiding laws, each has a distinct -individuality arising from its inherent qualities and its environment -during the early stages of its existence. The spiral-nebula theory -seems to offer a better explanation of these individual qualities than -any other that has been advanced thus far, and in its main features -it is pretty generally accepted. But one must keep in mind that the -details of any theory of the beginning and growth of the planets are -more or less speculative, or, at least, have not yet been proved with -finality. - - - - -V - -THE SEVEN GREAT PLANETS - - -So far as we know, five of the planets--Mercury, Venus, Mars, Jupiter, -and Saturn--have been known from time immemorial. There are existing -records of them made thousands of years ago. There is no reason why -they should not have been thus known, since they have always been as -they are now, visible to the naked eye, and all of them save Mercury -are as easily seen as the sun or the moon. They do not, of course, -exact the instant attention that those great luminaries do, because, -being smaller, they are less isolated from the great body of the stars; -but they are in their seasons plainly visible, and can then always be -seen if one looks at them. - -In ancient times, when people lived more out-of-doors than is the habit -now, they did look at them. The same primitive shepherds that, while -tending their flocks at night on the hills, named the constellations -according to the fanciful shapes that the unchanging stars seemed to -outline, watched also the five wandering stars, more wonderful to them -than any of the others. They observed how mysteriously these stars came -at certain seasons and silently threaded their way across the shining -heavens, and then as mysteriously disappeared. They saw them not only -differing from the other stars in glory, but changing in their own -brilliancy from one time to another, until, in some cases, they failed -to recognize them as the same stars under varying aspects. Venus, for -instance, they called Phosphorus, or Lucifer, when they saw her as a -morning star, and Hesperus, or Vesper, when she shone in the evening. - -The sun and the moon, they noted, also moved from place to place among -the fixed stars, and they called all these errant bodies planets, which -means “wanderers.” These are the “seven planets” referred to in the -earlier literatures and in all early books on astronomy or astrology. -This is sometimes a little confusing, because, though the sun and the -moon are no longer called planets, we still (omitting the earth) have -seven. But Neptune and Uranus, not being visible to the naked eye, -were not known to the ancients. They were discovered by means of the -telescope, and that only within the last century and a half. So, owing -to these comparatively new-found members of the solar family, we have -yet the magic number of planets, seven. - -These seven are the major planets and the ones with which mainly it -will be our endeavor here to promote and strengthen an acquaintance. -With Uranus and Neptune the acquaintance will necessarily be less -intimate than with the others, because we cannot see them in the same -free way; but they are not on this account much less interesting than -the others, and a little knowledge of them is pleasant family history. -They simply do not live within sight. - -The planets that are nearer to the sun than we are, and hence lie -between us and the sun, are called the inferior, or sometimes interior, -planets. Those that lie outside the orbit of the earth are called the -superior, or the exterior, planets. In so grouping them the earth -is the dividing-point, and is not itself in either class. Mercury -and Venus are the inferior planets. The superior planets are Mars, -Jupiter, Saturn, Uranus, and Neptune. The distinction has importance, -especially when we are discussing the planets with relation to their -movements, as seen from the earth, because the planets with orbits -between us and the sun (the inferior planets) have very different -phases and apparent motions from those whose orbits are beyond us from -the sun (the superior planets). - -When considered in regard to size, constitution, development, and -their likeness to each other, the planets are sometimes distinguished -as the terrestrial planets and the major planets. This need occasion -no confusion with the general division of them into major and minor -planets, because, as has been said, when simply “the planets” are -mentioned, these seven large planets are always the ones that are -meant, the others being usually called asteroids, or planetoids. The -terrestrial planets are Mercury, Venus, Earth, and Mars. As the name -implies, they are so called because they are in some respects similar -to the earth. The major planets are Jupiter, Saturn, Uranus, and -Neptune. They are all larger than the terrestrial planets, and, in -addition, have some other characteristics in common which the planets -of the other group do not have. The two classes represent different -stages of evolution. - -The four planets forming the terrestrial group are sometimes called the -inner planets, and the four major planets are then known as the outer -planets. The point of division in mind then is the space between Mars -and Jupiter. This is so vast in comparison with the spaces between the -other planets from the sun out to Mars that it becomes a convenient -dividing-line, particularly as the groups divided by it are in some -respects essentially different from each other. - -Of the four planets which have an especial interest to us because of -their being the ones most easily seen, two are terrestrial, or inner, -planets, Mars and Venus, and two are major, or outer, planets, Jupiter -and Saturn. The differences between the two classes are solely matters -of constitution and situation, and have nothing to do with their -appearance to us. Venus, the brightest of them all, belongs to one -group; Jupiter, the second in brilliancy, belongs to the other. - -That there is at least one other planet beyond the present boundary of -our system (which is the orbit of Neptune) seems to be quite probable. -Some astronomers think there may be several others. There are certain -perturbations, or irregularities, in the movements of Neptune which the -influence of Uranus does not account for, and they seem to indicate -that there is some disturbing body even beyond the orbit of that -farthest known planet. - -Several astronomers are working on the problem of locating this -undiscovered body. At various times it has been announced that such -a planet would probably be found in a certain position in the skies -at a specified date; but as yet no one has been able to get a view -of it. Recently the orbit of a far-off hypothetical planet has been -calculated, and its place predicted for 1914. Perhaps it may be found -then. Of course it could never be seen through any but the most -powerful telescopes. Its calculated distance from the sun is one -hundred and five times that of the earth. This would be more than nine -billions of miles, or more than three times farther than Neptune is -from the sun. It would require fourteen hours for light to pass from -the sun to a planet at that distance, and the sun would appear to it -smaller than Saturn or an ordinary first-magnitude star does to us. - -A further reason for suspecting the existence of such a planet is -suggested by the orbits of certain comets. These erratic bodies, -when they chance to come within the bounds of the solar system, are -sometimes forced to remain because of the powerful influence of one of -the planets near which their path has taken them. Jupiter holds as many -as thirty of them in this way, Saturn and Uranus have two or three, and -Neptune has captured as many as six. But there are still others that -return to us in regular periods, but which go sufficiently far beyond -Neptune to escape entirely if there were not some still more distant -watch-dog to turn them back. So there seems good reason to believe that -Neptune is not really the outermost of the planets. - -There has also been much said about the possibility of a planet nearer -to the sun than Mercury. When Mercury is at perihelion, or nearest to -the sun, there are certain irregularities in his movements which might -be explained by the presence of another planet between Mercury and the -sun. In 1859 it was thought that such a planet had been observed. Its -time of revolution and its distance from the sun were estimated, and -it was named Vulcan. In some of the books of astronomy published about -that time, and even in some published as many as fifteen years later, -Vulcan is mentioned as a reality. But now it is believed that the -observation was a mistake, and no such body is known to exist. - -In 1878 it was again thought that two bodies nearer to the sun than -Mercury had been discovered during an eclipse. These observations -have never been explained or confirmed; but it is thought that the -objects seen were probably stars which were mistaken for planets by the -observers. If a body so situated does exist, it is so near the sun that -it probably can never be seen except during an eclipse, and the time of -observation is then so short and mistakes are so easily made that it -is difficult to verify the observation. The continued search for the -cause of the perturbations of Mercury may finally lead to the discovery -of something between it and the sun. But if it is a single body, this -seems a much less promising task than the search for a planet, or -planets, on the outer edge of the solar system. - - - - -VI - -THE MOVEMENTS OF THE PLANETS - - -In considering the movements of the planets, we have to regard their -actual motion in space and that motion as it appears to us. They all -have two principal motions in space. They revolve about the sun in -their orbits, and they rotate on their axes. The manner in which they -accomplish the rotation on their axes determines the length of their -days and nights, or whether, indeed, they shall have any such grateful -alternations of light and darkness. Those planets which, like the -earth, turn on their axes in less time than they make their journey -around the sun have one day and one night every time they make a -complete rotation. Those that turn on their axes in the same time that -they revolve around the sun, of which sort there seems to be at least -one, face always toward the sun, and have no alternations of day and -night. On one side it is always day; on the other it is always night. -The number of days a planet has during each revolution around the sun -depends upon how much time it requires to make a revolution, and how -fast it spins on its axis. In one year here on the earth we have three -hundred and sixty-five days and nights. Saturn, in its year, has more -than twenty-three thousand days and nights. - -The manner in which the revolution of the planets in their orbits takes -place determines the length and character of their year; the nearer a -planet is to the sun, the shorter its orbit is, and the faster the rate -of speed at which the sun compels it to move, and hence the shorter -its year. The nearest of the planets, Mercury, makes more than five -hundred revolutions around the sun, while the farthest, Neptune, makes -one. Three times in a year--that is, a terrestrial year--the nearest -planet speeds around its orbit and back to the starting-place with -seventeen days to spare. One hundred and sixty-five terrestrial years -are necessary for the farthest planet to make one circuit of its orbit. -The first goes at the average rate of nearly thirty miles a second over -a path more than two hundred million miles long. The second travels a -path more than seventeen billion miles in length, at the average rate -of three and four-tenths miles a second. Between these two extremes -the other planets have orbits and rates of speed varying with their -distances from the sun. The farther they are from the sun, the larger -the orbit and the slower the speed. - -To get something like a picture of the sun and the planets as they -actually lie and as they move in space, one should have in mind an -immense flat, circular disc five and a half billions of miles in -diameter passing through the sun, which is in the center of it. Around -the edge of the disc is the orbit through which Neptune moves. At -varying distances inside of it are the orbits of the other planets, -each growing smaller and smaller as one comes nearer and nearer to the -sun, until the orbit of Mercury, the planet nearest to the sun, is -reached. - -Since it is not a hard metal disc that we are considering, but only -an imaginary one in space, there may be a little latitude allowed -for the orbits to tip somewhat out of the exact plane of the disc -without materially altering the figure in mind. And this they do, -very slightly--most of them to the extent only of from one to two -degrees, though one of them falls outside of the common plane about -seven degrees. In these orbits all the planets, as seen from the sun, -are going around from west to east. At the same time they are turning -on their axes in the same direction, some standing almost erect, as it -were, in their orbits and whirling like a dancing dervish as they skim -along, and others more or less inclined like a traveling top. - -The time a planet requires to make one circuit of its orbit -constitutes, as with the earth, its year. But we who are on the earth -have, in our study of another planet, to regard it as having in a sense -two years. First, there is the time it takes, starting from a given -point in its orbit, to circle around the sun and return to that point. -This is known as its sidereal period, or year, and is so called from -_sidus_, meaning a star, because the only way to mark any point in -space is by a fixed star, and, as viewed from the sun, one revolution -of a planet would be from a given star back again to that star. - -Then there is the time a planet takes, starting when it is in a -straight line with the earth and the sun in space, to return to the -place where the three bodies will be again in the same relative -position. This is known as its synodic period, or year. Synodic is from -our word synod, meaning a meeting or assembly, and the synodic year is -the time between two successive and similar meetings of these three -bodies. The sidereal year concerns the planet in its relation to the -sun; the synodic year, in its relation to the earth. The synodic year -is the only one that much concerns us while regarding the planets as -a part of the spectacle of the sky. It is the one that we know from -observation, while the sidereal year is mathematically computed. - -The two periods, or years, are not of the same length, because the -sun with reference to the planet is always stationary, and the motion -resulting in the sidereal year is that of the planet only, while the -synodic year is the result of the movements of both the earth and the -planet, each, in its own orbit, being always in motion. - -An inferior planet, situated as it is nearer to the sun than the earth -is, and so having a shorter orbit than the earth’s, will, when it -finishes its sidereal year and comes around to the point from which -it started, find the earth advanced from that position and will, -therefore, have to travel farther on in order to overtake it and come -into the same relative position from which they started, which makes -the time of its circuit with reference to the earth obviously longer -than with reference to the sun. - -With the superior planets the case is just reversed. The earth is -the inside planet, or the one nearest the sun, and it must overtake -_them_. With one exception, they are all so far away from the sun and -move so slowly that it takes us but little more than one of our years -to overtake them and bring them into the same relative position with -us that they had when we started, while it requires many of our years -for any one of them to make a single circuit of the sun. Hence their -circuit with reference to the earth is shorter than with reference to -the sun. - -With Mars, the exception referred to, we have a more hardly fought -race. That planet is not so far from us as are the other superior -planets. It makes its revolution around the sun in a little less than -two of our years. We travel eighteen miles a second, and it travels -fifteen miles in the same length of time. If we are in line with it at -the beginning of our journey, we glide off swiftly, and easily leave -it far behind. When, however, we come back to the starting-point, it -has not loitered, and is many millions of miles ahead of us, and it -remains ahead until more than seven weeks after we have returned to the -starting-point a second time. Fifty days after we have begun to make -our third round we overtake it, and are again in a direct line with the -planet and the sun. This makes its period with reference to the earth -ninety-three days longer than its own year, and fifty days longer than -two of ours. This is the longest synodic period among the planets. - -The orbits in which the planets move all have the form of an -ellipse--that is, of a circle more or less flattened. This flattening, -or the extent to which an orbit departs from the form of a true circle, -is called its eccentricity. The sun is never at the exact center of an -orbit, but is always situated a little to one side of the center--that -is, it is at one of the foci of the ellipse. Consequently, the planet, -as it travels in its orbit, is not always at the same distance from -the sun, the amount of the variation in distance depending upon the -eccentricity of the orbit. The point in the orbit where the planet -is nearest to the sun is its perihelion, and the point at which it is -farthest is its aphelion. It is necessary to keep these elementary -facts in mind in order fully to understand the changes in the motions -and brightness of the planets. - -The influence of one body over another that is circling around it is -to make it move faster or more slowly according to its distance from -the central body. Since a planet varies in its distance from the sun in -the different parts of its orbit, it is forced to move fastest when it -is in that part of the orbit which is nearest to the sun, and slowest -when it is in the part farthest away. In other words, the motion of a -planet is more rapid at perihelion than at aphelion. The earth is in -perihelion, or nearest to the sun, in winter--that is, winter in the -northern latitudes--and in consequence it moves faster in winter than -in summer, and the northern winters are, for this reason, a little -shorter than the summers. - -These two simple movements of the planets--that around the sun and -that on their axes--are their principal real movements, and are such -as they would show to be if seen from the sun, which is the center -of them. There are also certain minor real movements arising from -various causes, one being the influence that the planets exercise on -one another; but for the ordinary observer these have no particular -significance. Then, the planets all share the one grand movement which -the sun itself is known to be making through limitless space to a -destination of which we are in utter ignorance, over even a path which -we know nothing of save that it leads toward the bright star Vega, in -the constellation of the Lyre. As the sun moves on in that direction -at the rate of eleven miles a second he takes with him all his family -of planets and planetoids, with their satellites, and whatever other -bodies have their abode in his domain. Thus they travel as a body, -each individual spinning on its axis, from the sun itself down to the -smallest planetoid, the satellites circling around the planets, and the -planets in their turn around the sun. And in all these movements the -earth takes part as one of the planets. The sun itself is following -a comparatively straight line in space, and, so far as we know, in -allegiance to no other body. It is, though, just possible that this -comparatively straight line may be the arc of a circle so vast that -we have not yet had time to discover its curvature, and that the sun -itself may be pursuing its own circuit around some still more powerful -body. - - - - -VII - -HOW THE INFERIOR PLANETS SEEM TO MOVE - - -Of the real movements of the planets, as described in the last chapter, -we get here on the earth only a very fragmentary view. Without the aid -of the telescope none of them is visible to us except the movements -in their orbits, and these, to our view, are somewhat different from -the simple, circling course apparent to an observer on the sun. The -difference is due to the fact that the earth itself is always in -movement in just the same way that the other planets are, and we, being -never at any time at the center of the orbits, do not see the movements -of the planets as they truly take place, but only as they are outlined -against the sky. So the appearances and disappearances and visible -travels among the stars by which we know the planets are only as we see -them. Some knowledge of the real movements is necessary to a proper -understanding of the apparent movements; but it is only with the latter -that, for ordinary observation, we need to be particularly acquainted. - -The rotation of the earth on its axis, as we know, causes the familiar -daily apparent rising, passing, and setting of all the heavenly bodies. -In this apparent motion the planets share as well as the sun, moon, and -stars. But it is their movement _among_ the fixed stars, and not _with_ -them, that distinguishes them as planets, and this it is necessary to -know in order to keep track of them and be able to recognize them in -their varying places and guises. For they sometimes shine in their -greatest glory in one season, and sometimes in another, and at the -recurrence of the same season they are sometimes in one part of the sky -and sometimes in another, so that their ways of coming and going border -almost on the mysterious, until one learns the manner of this apparent -vagrancy. Happily, this knowledge is easily attained, and then the -matter is simple enough. - -The apparent motions of the inferior planets, Mercury and Venus, always -take place near the sun. Venus never wanders more than forty-eight -degrees from it, and Mercury never more than twenty-eight. Most of -the time they are much nearer than this. Since we cannot see either -of them except when the sun is below the horizon, the consequence of -their being always thus near to him is that they are in view for only -a short time after the sun has set or before he has risen. If they are -in the evening sky, and hence east of the sun, they soon follow him -when he sinks below the western horizon. If they are west of the sun, -and, consequently, are the first to rise in the morning, it is not long -before his brilliant rays flood with light the eastern sky and blot the -planets from our view. Venus can be seen sometimes for three hours at -a time, Mercury for never more than one. Within this limited region of -the sky they appear to journey evening by evening away from the sun, -somewhat obliquely, but toward the zenith, until they have reached -the end of their tether. Then they journey back and pass to the other -side of the sun. There they climb their path toward the zenith, moving -westward and, as we see them, obliquely upward. Morning by morning -they get farther from the sun until their westward limit of freedom -is reached, when they again draw in toward the sun, pass it, appear in -the evening sky, and pull off up the sky toward the east again. Thus -they swing from east to west of the sun, and back again, in unceasing -repetition. - -As they pass the sun going from east to west--that is, from the evening -to the morning sky--the inferior planets go between us and the sun; -and when they swing back from west to east, or from the morning to the -evening sky, they pass on the side of the sun farthest away from us. -When they are in a direct line with the earth and the sun they are said -to be in conjunction. If at this point they are between us and the sun, -it is inferior conjunction. If they are on the other side of the sun, -they are said to be in superior conjunction. When the planet, as seen -in the evening, has traveled toward the east as far from the sun as -it will go during that particular revolution, it is said to be at its -greatest eastern elongation. Elongation means simply apparent distance -from the sun; hence, greatest eastern elongation is the greatest -distance possible east of the sun from our point of view. Greatest -western elongation, which we see in the morning before dawn, occurs -when the planet is at its greatest apparent distance west of the sun. - -While apparently drawing near and then away from the sun, traveling -obliquely up and down the evening and the morning sky, the planet has -all the time been moving in one direction around the sun; but we could -see the motion only as it appeared on the background of the sky. The -planet is in reality just as far from the sun when it is in conjunction -as at elongation. The difference is that we see it at a different -angle, or from a different point of view. But it has not been at all -times equally near to the earth. - -When an inferior planet is at greatest eastern elongation, it is, of -course, east of the sun, and can be seen above the sun in the evening -after sunset, and is an evening star. As it moves westward nearer and -nearer to the sun, it is above the horizon a proportionately shorter -time each evening, and is more and more obscured by the sun’s rays -until it reaches inferior conjunction, when it is exactly between us -and the sun, and hence at the point nearest to us. Here it becomes -invisible, largely because it has its dark side toward us, but partly -because the dazzling light of the sun entirely obscures it. Once in a -while our relative positions are such that we see it pass like a black -dot directly over the bright face of the sun. This is called a transit. -But a transit does not occur at every inferior conjunction. It would so -occur if the planet’s orbit and the earth’s were in exactly the same -plane. But the small tilt that they have is sufficient to throw the -planet, when it is passing the sun, into such an angle that it does not -pass directly between the disc of the sun and us, but a little above or -below. Thus transits are rather rare, though they occur periodically in -the case of both Venus and Mercury, and will be spoken of elsewhere. - -When the planet has passed inferior conjunction, it is then west of -the sun, and rises in the morning before the sun is up, and is a -morning star. For a few days it can be seen either not at all or with -difficulty. Then, as it works its way out of the rays of the sun and -on toward the west, it rises earlier each morning until it reaches its -farthest point west. - -As it starts back east again its distance from the earth increases -daily until it reaches its greatest distance from us at superior -conjunction. It is then the whole diameter of its orbit farther from us -than when it was at inferior conjunction, and it is again invisible. -The illuminated side of it is toward us; but it is at its smallest, -because it is at its greatest distance from us, and even when it is not -directly behind the sun the light of that luminary is too great for -successful competition. After it has passed superior conjunction it is -again in the evening sky, apparently moving farther from the sun each -day. It is at the same time actually coming nearer to us each day, and -these two facts cause a daily increase in its brightness. - -But an inferior planet is not, like the superior planets and the stars, -brightest when it is nearest to us. It is, in fact, darkest when it is -nearest--that is, when it is at inferior conjunction--and we cannot -see it at all. This is because an inferior planet passes through -phases, like the moon, changing gradually during its rounds from full -to crescent, and back again. Its full face is toward us when it is on -the opposite side of the sun and farthest from us. The proportion of -the face that is illuminated grows smaller as the planet approaches -its eastern elongation. But the planet grows brighter because it is -coming nearer to us and is getting out of the dazzling rays of the -sun. One-half of its surface is illuminated when it is at greatest -elongation; but it is brightest a few days later, when less than half -of its face is illuminated, because it is enough nearer to compensate -for the slight diminution in the proportion of light on its disc. It is -brightest in the morning a short time before its western elongation, -for the same reason. - -This in a general way describes the motion of an inferior planet, and -this is all that we need to know in order to understand its ordinary -visible movements. If we watch it carefully, however, we may detect -that shortly before inferior conjunction it pauses in its onward sweep -and seems for a time to be stationary, and then to retrace its way -among the stars until a short time after inferior conjunction, when it -again pauses and appears stationary, and finally starts off again in -its original direction on its way toward greatest western elongation. -During this capricious sort of progress the planet usually describes -more or less of a loop, sometimes almost a flourish, in its path. The -appearance is wholly due to the planet’s overtaking and passing us -in our journey around the sun. For a time it travels behind us, then -beside us, and then beyond us; and, since we are both in motion, the -effect is much the same as when one train passes another while they -are both traveling in the same direction. The orbits of the earth and -the planet are not exactly in the same plane, and, both bodies being -in motion, we are not in a position to see the planet at the same -angle more than once as it seems to pass back and forth, and so we get -the effect of its making a flourish or loop. But this effect, while -interesting, takes place only when the planet is so near the sun that -to the ordinary observer it itself does not count for much. We can see -but little of the inferior planets at that time, anyway, though it is -important for us to know where they are, in order to keep track of them -and to be ready for them when they are to be seen. - - - - -VIII - -HOW THE SUPERIOR PLANETS SEEM TO MOVE - - -The movements of the superior planets, Mars, Jupiter, Saturn, Uranus, -and Neptune, as they appear to us, are different from those of the -inferior planets in some important respects. Instead of swinging back -and forth east and west of the sun, and never appearing very far away -from it, as the inferior planets do, the superior planets make an -entire circuit of the heavens, and it is possible to see them at any -distance from the sun, and at any time during the night. Sometimes -they are, with relation to the earth, in that part of the sky exactly -opposite to the sun, and hence in line with it and the earth. At -such times they can be seen all night. They are then said to be in -opposition, and are in the best position for our observation. The -earth being, when in this situation, in a direct line between them and -the sun, we have the sun at our backs, as it were, shedding its full -rays on the disc of the planet under observation, which is then at its -nearest to us, and also at its brightest. For, since the orbits of all -the superior planets are outside of ours, the planets never get between -us and the sun, and, in consequence, never turn a dark side toward -us. Their entire discs are practically always illuminated, and their -changes in brightness depend largely upon their changes in distance, -which, as we have seen, is not the case with the inferior planets. - -Mars, the nearest of them, is at times somewhat gibbous (that is, shows -a little less than a full face, as the moon does when just beginning -to wane), and, in less degree, Jupiter also. But in neither case -is this departure from fullness sufficient to have any appreciable -effect on the planet’s brightness, and, moreover, it does not occur -when the planet is in the most favorable position for us to see it. -At opposition, therefore, we always have the full face of the planet -presented to us; and being, as we then are, on the same side of the sun -with it, we are ninety-three millions of miles (our distance from the -sun) nearer to it than the sun is. - -Being, when in opposition, exactly opposite the sun, the planet rises -just as the sun sets. After opposition it rises a little earlier each -evening, and is higher up in the sky at each succeeding sunset. When -we find it just half-way between the eastern and the western horizon -at sunset, it is at quadrature. After quadrature it appears nearer and -nearer the western horizon each evening at sunset, until it finally -is too near the sun to be visible. It is then traveling in that part -of its orbit which is beyond the sun from us. From opposition to this -situation it has been an evening star. - -When a superior planet is in line with the sun and the earth, and is -on the far side of the sun from us, it is said to be in conjunction, -and we are then one hundred and eighty-six millions of miles, or twice -our distance from the sun, farther from it than we are when it is in -opposition. But besides being placed at so much greater distance from -it, we have in this situation the bright sun excluding the planet from -our view. It will be readily seen, therefore, why the superior planets -are in so much better position for us to see them in opposition than at -conjunction. - -From conjunction to opposition the planet is west of the sun, and -will be below the horizon at sunset, and will rise some time during -the night. At first it will appear just before sunrise as a morning -star, but will gradually rise earlier each night until, when it reaches -opposition again, it will rise just as the sun sets. Half-way between -conjunction and opposition it is again at quadrature. - -From opposition to conjunction the planet will be east of the sun and -above the horizon at sunset. When a planet is in conjunction with the -sun, it passes the meridian, or the point half-way between rising and -setting, about noon, and is above the horizon with the sun during the -day. When it is in opposition it passes the meridian about midnight, -and is above the horizon during the night. When it is at quadrature and -moving toward conjunction, it passes the meridian about six o’clock in -the evening, and may be seen in the western half of the sky during the -early evening, and will set before midnight. When it is at quadrature -and moving toward opposition, it will rise some time between midnight -and sunset, and will be in view in the east during a part of the first -half of the night. The nearer it is to opposition, the earlier in the -evening it rises and the longer it may be seen. - -The main movement of the superior planets among the stars is from west -to east, and this is known as their direct motion. But not far from -opposition they seem to hesitate, then move more slowly, then finally -stop, remain stationary for a time, turn back on their tracks, and -start off in the opposite direction. This is their retrograde motion. -They do not continue in it as long as in the direct motion; but after -a comparatively short time they again hesitate, go more slowly, stop, -remain stationary, then turn back and swing off in the original -direction, and continue to move in this direction until they are again -approaching opposition. It is exactly in the middle of this sweep -toward the west that the planet is in opposition. Close observation -will show that the superior planets also make something of the same -sort of a loop in their path among the stars that the inferior planets -make, and for the same reason. The only difference is that when a -superior planet is retrograding we are passing it, and when an inferior -planet retrogrades it is passing us. - -In giving this rather rough outline of the way the planets in general -move among the stars, reaching in their wanderings these various -positions with relation to the sun and the earth, the intention is only -to fix some definite situations from which to consider the movements -of the individual planets. When we come to know each planet as an -individual, and to follow it as it comes and goes in the heavens, and -to watch its ever-wonderful changes in brilliancy, these situations -will have a much more definite meaning to us and a relatively greater -interest and importance. The planets as they appear to us all move -along pretty much the same path; but each has its own way of gracing -this path, and each its particular manner of changing in aspect. - - - - -IX - -THE PATH OF THE PLANETS - - -Though the planets are called wanderers, they are not by any means the -vagrants that the name might imply. They have a fixed course among the -stars from which they never deviate, and the ways of all of them, and -also of the sun and the moon, are confined to a comparatively narrow -strip in the sky. - -That strip is called the zodiac. It is only sixteen degrees wide, and -extends like a narrow band all the way around the heavens. It lies so -that it is always easy to observe; and, being so limited, very little -observation is necessary to become familiar with every part of it. -Within its limits all the movements of the sun, the moon, and the -planets take place. Through the center of it is the ecliptic, the great -circle that marks the annual apparent path of the sun through the -heavens. It is the standard circle from which we measure the paths of -the moon and the planets. Whatever degree their courses vary from the -ecliptic is what we call the inclination of their orbits. If the plane -of the orbit of a planet is tilted away from the ecliptic, the planet -will travel half the time on one side of it, and half the time on the -other. - -The orbits are, in fact, very little inclined to the ecliptic, and all -but one of the planets may always be found within three degrees of it, -most of them nearer than this. The one exception is Mercury, which -is sometimes as much as seven degrees from this central line of the -zodiac, but ordinarily it is not so far as this. Uranus is so nearly on -the ecliptic that an ordinary observer would not notice the deviation, -and particularly as Uranus can rarely be detected with the naked eye, -and can never be thus followed. Of the four planets which are the ones -we ordinarily see, Mars and Jupiter are never as much as two degrees -from the ecliptic, Saturn never more than two and a half degrees, and -Venus never more than about three degrees. They are all usually nearer -than these outside limits. The greatest distance of the moon from the -ecliptic is about one and a half degrees. - -Hence, with the exception of Mercury, all the planets and the sun and -the moon travel in a path six degrees wide, which is only one degree -wider than the distance between the pointers as we see them in the -Great Dipper. The fact that the zodiac is sixteen degrees wide, or -eight degrees on each side of the ecliptic, is due only to a very -generous allowance for the vagaries of Mercury, which he really does -not quite need. For Mercury is always as much as twice the breadth of -the moon, or one degree, inside of the zodiac, and usually more than -that. - -Because the earth is tilted on its axis twenty-three and a half degrees -from the perpendicular, the ecliptic runs through the heavens in an -oblique circle, crossing the line of the equator at two points called -the vernal and autumnal equinoxes. The equator in the heavens is the -great circle extending around the celestial sphere half-way between the -north and south poles. It is always practically ninety degrees from -the north star, and the points at which the ecliptic intersects it are -called the equinoxes. These are the only two points on the ecliptic -that are just ninety degrees from the pole. The word equinox is derived -from _equus_ (equal) and _nox_ (night), and when the sun is at the -equinoxes the days and nights are of equal length. - -From the vernal to the autumnal equinox the line of the ecliptic is -north of the equator, and hence high in the sky, reaching its highest -point midway between the equinoxes. It then crosses the equator again -and runs obliquely south to the lowest point in its path, and then -curves northerly back to the vernal equinox. The vernal equinox is the -point at which the sun arrives when spring begins. This results in the -sun’s being north of the equator from spring until autumn, and south of -it from autumn to spring. - -As the part of the zodiac that we can see best at night is that -opposite where the sun is, so in summer, when the sun is high, we see -best the part of the zodiac which is low in the southern skies in the -evening; and in the winter, when the sun is in the southern half of his -journey, the part of the zodiac best seen by us is high in the heavens. -No part of it, however, is ever as high as the zenith, or directly -overhead, and no planet is ever seen as far north as the zenith in any -place whose latitude is more than twenty-three and one-half degrees -from the equator. - -To know the paths of the planets it is necessary to know only twelve -constellations out of the seventy or more in the entire heavens; but -it is difficult to imagine any one’s learning these twelve without -becoming interested in and more or less acquainted with many of the -splendid stars and constellations that lie on each side of them. -The larger one’s acquaintance is with the appearance of the skies -as a whole, the easier, naturally, it will be to distinguish the -planets from the stars, and to follow their courses. But the planets -themselves may be intimately known quite apart from any but the twelve -constellations forming the zodiac. Happily, among them we shall find -some of the most beautiful constellations in the heavens, and some of -the most splendidly brilliant first-magnitude stars.[1] - -[1] The reader will find fuller descriptions of the stars in the zodiac -in _The Friendly Stars_, by the author of this book. - -The twelve constellations of the zodiac are as follows: - - Pisces, the Fishes. - Aries, the Ram. - Taurus, the Bull. - Gemini, the Twins. - Cancer, the Crab. - Leo, the Lion. - Virgo, the Virgin. - Libra, the Scales or Balance. - Scorpio, the Scorpion. - Sagittarius, the Archer. - Capricornus, the Goat. - Aquarius, the Water-Carrier. - -We shall begin at the point of the vernal equinox to trace the line of -the ecliptic through these constellations, and that line will mark for -us the path of the sun, the moon, and all the planets. It is convenient -to begin at this point, because it is where the sun crosses the equator -in the spring, and hence it is at the beginning of that part of the -ecliptic which lies north of the equator. - -The point of the vernal equinox is now situated in the constellation -Pisces. It is not marked by any bright star, but is not very difficult -to find. It marks the point on the eastern horizon where the sun -rises about March 21st, and about the 21st of September it is on the -eastern horizon exactly opposite that point in the western sky where -the sun sets. It is always ninety degrees from the pole, and if -one chances to know the constellation Cassiopeia, which is shaped like -a chair and is on the opposite side of the pole from the Big Dipper, -one can locate the vernal equinox by drawing a line from the pole-star -through the star which marks the lower part of the front of the chair, -and extending it until it is ninety degrees long. The ninety degrees -can be estimated by using the distance between the pointers in the -Dipper (which is five degrees) as a measure. The star mentioned in -Cassiopeia is about thirty-two degrees from the north star. - -[Illustration: MAP SHOWING THE CONSTELLATIONS OF THE ZODIAC AND THE -LINE OF THE ECLIPTIC RUNNING THROUGH THEM - -The paths of all the planets, save one, lie always within three degrees -of the ecliptic.] - -Having once learned the constellations of the zodiac and, -approximately, the line of the ecliptic, it is not necessary for the -ordinary observer to keep in mind the exact location of the vernal -equinox. It is, however, an important point for the student of -mathematical astronomy. - -Beginning at this point, the ecliptic runs through Pisces in a -northeasterly direction for about thirty degrees to Aries, the second -constellation of the zodiac. - - -ARIES - -Aries is best seen in the autumn when the sun is in the opposite side -of the heavens. It is marked by a small acute-angled triangle, with -the apex toward the north and the brightest star of the three at the -apex. This star is called Hamal, and, while not a first-magnitude star, -is a rather bright one of the second magnitude; and the triangle itself -is very distinctly marked. It is the only group of stars by which -to distinguish Aries, and it is sometimes confused with the little -constellation called Triangulum, which lies just west of it, or above -it, as it rises. With this in mind, Triangulum may be made to serve as -an identifying mark. They both rise just a trifle north of the exact -east early in the evenings of late September and October. Triangulum -rises first, with its apex toward the south. In less than an hour the -triangle of Aries arrives with its apex pointed north. The ecliptic -runs about five degrees below this triangle, and its path across Aries -is about twenty-eight degrees long. When one sees any very bright star -in Aries, one may be sure it is a planet. The sun is in Aries from -April 16th to May 13th. - -During the summer this constellation is not visible in the early -evening; but it may be seen every evening from September to April, -drawing all the time nearer to the sun, and setting earlier each -evening until the sun blots it out. From this constellation the -ecliptic runs into Taurus, the third zodiacal constellation. - - -TAURUS - -This constellation may be identified by the brilliant first-magnitude -star Aldebaran,[2] and the misty Little Dipper of the Pleiades. It is -a very beautiful and large constellation. About an hour and a half -after the triangle of Aries has risen, the soft-twinkling cluster of -tiny stars which form the Pleiades comes above the eastern horizon, and -about an hour later a V-shaped cluster of brighter stars, with a very -bright-red one at the end of the lower half of the V, appears. This -last cluster is the Hyades, and the bright star is Aldebaran. - -[2] See “Aldebaran” in _The Friendly Stars_. - -By these two clusters we may know the constellation. The ecliptic -passes across Taurus about four degrees east of the Pleiades, and -about seven degrees west of Aldebaran. The planets in passing through -this region often come very close to the Pleiades, and parts of the -group are sometimes occulted by the moon. Taurus is conspicuous in the -eastern evening sky from September until nearly January. From that -time on until May it may be seen in the evening, high up in the sky, -a little farther west each evening, until it disappears in May. Among -the four planets that we most see Mars is the only one that resembles -Aldebaran in color. They are both reddish, but Mars is always west of -Aldebaran near the line of the ecliptic, and also it does not have the -same twinkling face that Aldebaran shows; hence the star and the planet -need never be confused. Mercury, it is true, is reddish and twinkles, -but so seldom needs to be taken into account that it will not be -troublesome. The other planets when in Taurus will proclaim themselves -by their color and size. There is no very bright star in Taurus except -Aldebaran, which has been described. Any bright star north of it in the -constellation is sure to be a planet. - -Through Taurus the line of the ecliptic runs in a northeasterly -direction, and about fifteen degrees east from Aldebaran it passes -about half-way between two fairly bright stars which mark the tips of -the horns of Taurus, and from there on into the fourth constellation. - - -GEMINI - -Gemini lies northeast of Taurus, and is outlined by a box-shaped figure -something more than twenty degrees long and about five degrees wide. -The two stars marking the end of it farthest from Taurus are the famous -twins, Castor and Pollux.[3] Pollux is a first-magnitude star, and -Castor is very little less bright. They are both very charming stars, -and too conspicuous to escape easy identification. Castor is greenish -in tint, and rises between an hour and a half and two hours later than -Aldebaran. About fifteen minutes after he appears, Pollux, with a -yellow-tinted face, comes up over the eastern horizon. They rise about -thirty degrees north of the exact east. The ecliptic has reached its -highest point north just after passing through the horns of Taurus. -It then runs through Gemini in a southeasterly direction, curving -diagonally across the main figure and passing five or six degrees -below Pollux. Gemini can be seen from October to early June. It is -particularly charming in May in the northwest just after sundown, and -when any of the planets are going along this part of their path at that -season, they are sure to win one’s interest and admiration. - -[3] See “The Heavenly Twins” in _The Friendly Stars_. - - -CANCER - -After leaving Gemini the ecliptic passes through the small -constellation Cancer. Its way runs southeasterly for about twenty -degrees, passing just south of a charming little cluster of stars which -can be dimly seen with the unaided eye, but comes out brilliantly with -an opera-glass. It is called Præsepe, or the Bee-hive, and is the only -object to attract attention in Cancer. Fortunately, it is so situated -as to mark the line of the ecliptic through the constellation. The -Bee-hive rests almost exactly on the ecliptic. - - -LEO - -Leaving Cancer, the sun enters Leo, a large, well-marked constellation -known to many persons by the conspicuous figure in it of a sickle. At -the end of the handle of the Sickle is Regulus, one of the bright -first-magnitude stars. A little more than fifteen degrees east of the -Sickle the rest of the constellation is marked by a large triangle -formed by three rather bright stars. Both of these figures are -well marked and easily seen, making Leo one of the easiest of the -constellations to find. The sun crosses it in a southeasterly direction -which leads straight across Regulus. The star is often occulted by the -moon, and by the sun also, though that we cannot see on account of the -blinding light of the sun. - -Leo is visible nearly eight months in the year. It is in the eastern -sky early in the evening in the winter, and shines all night from late -in December until April. In May and June it is traveling westerly, -but high up in the sky. In July it is in the western sky in the -evening. The sun passes through it from August 7th to September 14th. -Regulus is a white star, and twinkles violently, so that it is easily -distinguished from any planet that is passing near it. In the other -part of the constellation the path of the planets runs about ten -degrees below the triangle. - - -VIRGO - -When the sun has passed Leo it enters the largest of all the -constellations, Virgo, and passes through it in forty-five days, from -September 14th to October 29th. The constellation is far from rich in -bright stars; but one may find the ecliptic, or path of the sun, by -following a curved southeasterly line from Regulus about sixty-five -degrees until it reaches Spica,[4] a very bright first-magnitude star -in this comparatively starless region. If there is any doubt about -Spica, it may be found by following the curve of the handle of the Big -Dipper about thirty degrees, which brings one to the splendid Arcturus, -and then about thirty degrees farther on, which points one to Spica. - -[4] See “Spica” in _The Friendly Stars_. - -Eight or nine days after entering Virgo the sun crosses the equator at -the autumnal equinox, and the rest of the ecliptic lies farther south. -Spica is about ten degrees south of the equator. - -Spica is in the east during the early evenings in April and May; -throughout June and July it may be seen in the south during the -evening. In October it sets at about the same time as the sun. - -The autumnal equinox, or the point where the ecliptic crosses to the -south of the equator, is in Virgo, and lies about fifteen degrees -northeast of Spica. - - -LIBRA - -Libra is the next zodiacal constellation, and it is a small one. The -sun passes through it in about twenty-three days. It may be known by -four fairly bright stars which form a more or less imperfect square. -The ecliptic passes along the southern edge of this figure. - -During the summer and early autumn, Libra is best seen. It is then -passing across the southern sky, drawing nearer the west each evening. -A planet passing across this constellation would always be easy to -identify, since it would always be so much brighter than any star in -this region. The sun enters Libra about October 29th, and it is not -visible in the evening during the rest of the year. - - -SCORPIO - -It is a joy to know Scorpio, quite aside from its connection with the -path of the planets. It is a brilliant constellation, best seen during -the summer and autumn, as it passes across the southern sky. It is the -most southerly of any of the constellations of the zodiac; but the -ecliptic passes through only a very small portion of the northern part -of it, so the sun does not reach the most southerly point in its path -while it is in this constellation. - -Scorpio may be best identified by its brilliant deep-red star -Antares,[5] which is supposed to lie in the heart of the Scorpion. The -whole figure makes a splendid serpent-like sweep toward the southern -horizon, and is one of the most conspicuous objects just west of the -Milky Way in the south in summer. - -[5] See “Antares” in _The Friendly Stars_. - -The line of the ecliptic runs about three degrees north of Antares; -hence the planets in their course sometimes pass very near it. Jupiter -has been in that region all this year (1912), and will not be far from -there the early part of 1913. Mercury and Mars both have something the -color of Antares; but this is not likely to result in any confusion. -The star is always there, and in the same relative situation with -reference to the other stars. When Mars is there, it will always be -above the star. Mercury can seldom be seen when he is in Scorpio. If he -is in greatest elongation while there, he will still be near the sun, -and the sun, as seen from the middle latitudes, is so far south and so -near the horizon when in that part of the ecliptic that the situation -will not be favorable for seeing the planet. Farther south, and -particularly in high altitudes, Mercury could be well seen in Scorpio, -but if the position of Antares is kept in mind, Mercury will easily be -recognized as a stranger in the constellation. - -The sun enters Scorpio about November 21st, and the constellation then -ceases to be visible in the evening sky until the following May. It is -in its greatest glory during the summer and early autumn. - - -SAGITTARIUS - -When the sun leaves Scorpio it crosses the Milky Way into Sagittarius, -and there reaches the lowest point in its path, twenty-three and -one-half degrees south of the equator. This constellation is best -distinguished by the little “milk dipper,” which is easily seen turned -upside down just at the eastern edge of the Milky Way. The line of the -ecliptic runs a little north of it. The constellation may be best seen -during about the same months that Scorpio is visible. The sun enters -it, and it passes out of view about the middle of December. - - -CAPRICORNUS AND AQUARIUS - -From Sagittarius the ecliptic runs in a northeasterly direction through -a region in which there are no very bright stars, nor any very distinct -outlines of figures. The two constellations through which it passes are -Capricornus and Aquarius. It then runs a few degrees into Pisces, and -there reaches the vernal equinox, where we began to trace its course. - -Although one cannot trace the line of the ecliptic with the same -definiteness in this region as in one where there are bright stars -to mark the way, yet when a planet is in this part of its path it -is perhaps more conspicuous and more easily recognized than when it -appears in any other part of the sky, because of the very absence of -other bright bodies. These constellations comprise all that region -running from the Milky Way east to the vernal equinox. It is a part -of the heavens easily seen during the pleasant evenings of summer and -autumn, and if a planet is crossing it during those seasons it is -particularly well placed for observation. - -The two brightest stars in Capricornus are of the third magnitude, and -lie about twenty degrees northeast of the “milk dipper.” The ecliptic -runs just under them. Through Aquarius it runs six or seven degrees -above a waving line of faint stars, which are supposed to represent the -water that Aquarius is pouring from his urn. - -If one will take the trouble to trace the line of the ecliptic through -the sky, and remember that it lies exactly in the center of the zodiac, -and that the planets are, therefore, within a very few degrees of it, -one will have no trouble in keeping track of them. The mere knowing of -these constellations is in most cases sufficient, since the planets -will disclose their identity in other ways than by position merely. - -The _signs_ of the zodiac are somewhat different from the -constellations. They are simply twelve equal divisions of thirty -degrees each, making in all three hundred and sixty degrees, which -is the whole number of degrees in any circle. They are so divided for -convenience in scientific observation and reckoning. About two thousand -years ago the signs and the constellations in the main coincided, and -they still bear the same names. The point of the vernal equinox was -then at the beginning of the sign and the constellation Aries. But, -owing to certain motions of the earth, this point shifts backward, -or toward the west, about one degree every seventy-two years. In two -thousand years it has shifted about twenty-eight degrees, until now -the sign Aries, with the vernal equinox at its western boundary, lies -almost wholly in the constellation Pisces, the sign Taurus corresponds -approximately to the constellation Aries, and so on around the circle. -It is important to know this in following the planets, because all -almanacs and scientific publications deal mainly with the _signs_ of -the zodiac, and not with the _constellations_. When a planet’s place -is said to be in Aries, Taurus, or Gemini, one will find it in Pisces, -Aries, or Taurus, respectively. And so it is with all the other signs; -they are each one constellation behind the one bearing the same name. -And this is why, beginning with the vernal equinox, Pisces is the first -constellation in the zodiac, while Aries is the first sign. - -The following is a list of the signs of the zodiac, with the -corresponding constellations. The symbols given in parenthesis are the -ones used for these signs in all almanacs: - - SIGN CONSTELLATION - - { Aries (♈) Pisces - Spring { Taurus (♉) Aries - signs { Gemini (♊) Taurus - - { Cancer (♋) Gemini - Summer { Leo (♌) Cancer - signs { Virgo (♍) Leo - - { Libra (♎) Virgo - Autumn { Scorpio (♏) Libra - signs { Sagittarius (♐) Scorpio - - { Capricornus (♑) Sagittarius - Winter { Aquarius (♒) Capricornus - signs { Pisces (♓) Aquarius[6] - -[6] For those who find rhymes an aid to memory, the following list may -prove useful: - - This is the way the spring begins: - First Aries, then Taurus, then the Heavenly Twins. - The first summer sign is the one we call Cancer; - The next two to Leo and Virgo will answer. - Then autumn brings Libra and bright Scorpio, - And next Sagittarius, with his strong bow. - Capricornus then ushers the winter in, - And near old Aquarius the year we begin. - Pisces comes next, and then winter is done; - And with Aries’s approach, a new spring is begun. - These are the _signs_; but bear this well in mind: - The sun lags in one constellation behind. - When his place is Aries, we’ll find him in Pisces; - When in Taurus he should be, in Aries he stays. - If Gemini’s his place, and to find him our wish is, - We must look back in Taurus to see his bright rays. - And so through the year, whatever his place is, - The bright group behind is the one that he graces. - - - - -X - -MERCURY - - -While Mercury is one of the five planets that can be seen with the -naked eye, it must be confessed that he does not play any important -part in the great spectacle of nature as we see it in the skies. But in -a certain way this only adds to our interest in him. The very rarity of -his appearances and the difficulty of finding him give a zest to the -search, and a sense of achievement, when it is successful, that one -does not have with regard to the other planets. It is something akin to -the feeling one has when, after a long tramp to some secluded recess in -the woods in search of the shy pink lady’s slipper, a splendid specimen -of that lovely flower suddenly comes into view hanging gaily on its -stalk, ready for the use of whatever fairy foot may tread its shady -groves. - -Then, too, the spring o’ the year is the most likely time to see -Mercury in the evening sky. He comes into his best position for this -view of him just when the evenings are growing longer and milder and -one begins to hunger for outdoor things, so that the quest of him at -that time has the gladness that goes with our first excursions into the -open after a winter’s housing, whether it be in search of flowers, or -birds, or stars, or simply the general loveliness of everything that -belongs to the beginning of the outdoor season. - -The reason Mercury is so elusive is that he is always very near the -sun, and in consequence his light is dimmed by the brighter light shed -by that luminary until it is well below the horizon; and after the -sun has set, the planet is so involved in the usual haziness of the -atmosphere near the horizon that the conditions must be very favorable -in order to see him. Though there are recorded observations of Mercury -as far back as nearly three hundred years before Christ, yet some of -the older of the modern astronomers, before the days of the perfected -telescope, are said not to have seen him at all; and the most important -observations of the planet nowadays are made in broad daylight, when -it is higher up in the skies and free from the mists of the horizon. -This can be done by means of a powerful telescope, because it is -possible in this way to shut off the light of surrounding bodies; -but, of course, the conditions are not as favorable as if midnight -observations could be made. Still, if one knows just when and where to -look, Mercury can be seen with the naked eye at least once or twice a -year, and sometimes oftener than this, especially if one chances to -live in one of the Western States, where the air is very clear and the -situation in latitude and altitude more favorable than, say, in New -England, or in the middle Atlantic States. In our Northern States, and -in the whole of England, this planet is more difficult to see, because -of the longer twilight in northern latitudes, and also because the line -of the ecliptic, over which it passes, seems there lower down in the -skies, while in the far South, say in Cuba or Porto Rico, the twilight -is shorter, the ecliptic runs high in the sky, and the situation is -favorable for a good view even though the atmosphere is no clearer than -it is farther north. - - -WHEN AND WHERE TO FIND MERCURY - -Mercury is never more than twenty-eight degrees from the sun, and is -brightest when the distance between them is somewhere near twenty-two -degrees, or about four times the distance between the pointers in the -Big Dipper. The direction in which to search for him must always be -along the line of the ecliptic obliquely above the sun. Since his orbit -is inclined seven degrees to the ecliptic, he will be some place within -seven degrees of this line, on one side or the other. Within this -narrow strip in the sky, fourteen degrees wide and twenty-eight degrees -long, Mercury will be found whenever he is visible at all. And this -strip may be further shortened by at least twelve degrees; for when the -planet is nearer than that to the sun it is futile to attempt to see -him with the naked eye, save in very exceptional conditions. The five -degrees between the pointers will serve as an aid in measuring these -distances. - -We can never see Mercury with the naked eye except when he is near one -elongation or the other; and even then he is visible only about an hour -after the sun is down in the evening or about an hour before it rises -in the morning. Three times each year he appears in the evening for -more or less than a week, according to the situation of the observer, -and three times a year he is visible in the morning for about the same -length of time. But, owing to his position with relation to us, the -evening exhibit that comes in the spring is the most favorable one for -a good view of him, and the morning appearance that is most favorable -is the one that comes in the autumn. - -The mean synodic period of Mercury is about one hundred and sixteen -days, or a little less than four months. That is, he returns to -greatest eastern elongation and can be seen in the evening sky about -every one hundred and sixteen days, and the same length of time -elapses between his appearances in the morning sky at greatest western -elongation. But this mean synodic period is made up of synodic periods -varying in different revolutions from one hundred and five to one -hundred and thirty-four days. So, though one may mark the dates at -which the various positions of the planet occurred during any one -revolution, one cannot so easily determine the exact time at which -he will be found in the same positions at the next revolution; that -is, whether the revolution will take place in less or more than one -hundred and sixteen days. The earth and the planet are each traveling -at varying rates of speed, according as they are near the sun or -farther from it, and obviously it is a situation that requires careful -mathematical work to compute. The almanac must be referred to for the -exact date. - -But, lacking an almanac, one will generally find that Mercury will -return to the same position relative to the earth and the sun within a -few days of his mean synodic period. Three periods, however much they -may vary individually, are almost always equal to three hundred and -forty-eight days, or three times the mean period. This is seventeen -days less than a year. Hence, if one is lucky enough to have seen -Mercury at eastern elongation one spring, and will look the next year -about seventeen days earlier, the planet will be found a little to -the east (about fifteen degrees) of where he was when first seen the -year before. He is there in the same position with relation to us and -the sun that he had the preceding spring, but in a slightly different -relation to us and the stars, because the sun lacks seventeen days of -having completed its apparent yearly journey around the zodiac. It must -still go through about one half of a constellation. - -When Mercury shows himself at eastern elongation, he may be seen in the -west as an evening star for somewhere near a week, each evening drawing -nearer to the sun. When he disappears from view he passes between us -and the sun, and about four weeks later appears in the morning sky -before the sun rises. Under favorable conditions he is again visible -for a week or more; and then, again approaching the sun, he can be -seen no more for about ten weeks, during which time he passes through -superior conjunction on the other side of the sun from us and comes -back to eastern elongation. - -Thus we can get, under very favorable conditions, six short views of -Mercury during the year--three in the evening and three in the morning. -So many views, however, are rarely secured by any but the professional -observer. The circumstances may well be considered felicitous if one -succeeds in getting a glimpse of him once or twice a year--at his -favorable situation in the evening in the spring and the morning -in the autumn. The sight of him, though, is truly worth a little -inconvenience--even to the extent of facing a cold evening wind in the -very early spring or getting out of a comfortable bed before dawn -during the first cool mornings of autumn. - -It is hardly possible to say exactly where one can find Mercury at all -times during a long succession of revolutions. Moreover, it is not -necessary. These computations are made anew each year by experts in the -employ of the government, and the result is published in the _Nautical -Almanac_. From there it finds its way into all almanacs, so it is easy -of access to any one. - -In the almanacs Mercury is represented by the sign (☿). It is a -conventionalized form of the caduceus, or wand, carried by the god -Mercury as a symbol of his power. - -The next seven eastern and western elongations of Mercury occurring -after the publication of this book are as follows: - - Eastern Elongation Western Elongation - (Evening Star). (Morning Star). - 18 November, 1912. 27 December, 1912. - 10 March, 1913. 24 April, 1913. - (Favorable for viewing.) - 7 July, 1913. 22 August, 1913. - (Favorable for viewing.) - 1 November, 1913. 10 December, 1913. - 22 February, 1914. 6 April, 1914. - (Favorable for viewing.) - 18 June, 1914. 5 August, 1914. - (Favorable for viewing.) - 15 October, 1914. 23 November, 1914. - - -DISTANCE AND BRIGHTNESS - -Of all the planets Mercury is nearest the sun. His average distance -is thirty-six million miles. He is nearly eighty times nearer than -Neptune, the outermost planet, and more than two and one-half times -nearer than we are. But his orbit departs so far from being a circle -that his distance from the sun varies as much as fifteen million miles. -When he is nearest the sun, or in perihelion, he is only twenty-eight -million miles from it; when he is farthest, or in aphelion, his -distance is forty-three million miles. There is even greater variation -in his distance from us. The difference between his least possible -and his greatest possible distance from us is as much as eighty-nine -millions of miles. For the earth has an elliptical orbit as well as -Mercury, and when we are at perihelion, which occurs in the winter, -we are three millions of miles nearer to the sun than we are in -mid-summer. If Mercury chances to be then at his greatest distance -from the sun, and also at inferior conjunction, or between us and the -sun, he is only forty-seven millions of miles from us. If, when we are -farthest from the sun, he also is at his greatest distance from it, -and is in superior conjunction, or on the other side of the sun from -us, he is one hundred and thirty-six millions of miles from us. - -These changes in distance from the earth have much to do with Mercury’s -changes in apparent brightness to us. At his brightest, when he appears -at greatest elongation and we can see him without a telescope, he is -brighter than Arcturus, the brilliant first-magnitude star in Boötes, -that swings over us nightly from early spring to late autumn. When seen -with the naked eye, he is also red in color, somewhat like Arcturus; -but through a telescope he is dull silver, like the moon, or even more -ashy in his paleness. As he goes farther and farther from us he becomes -dimmer and dimmer and can be followed only with a telescope until, even -with this aid to vision, he is lost in the rays of the sun at superior -conjunction. His apparent diameter as mathematically measured varies -from five seconds, when he is farthest away, to thirteen seconds, when -he is nearest. - -When he is at his nearest possible distance from us, light travels from -Mercury to us in a little more than four minutes. At his greatest -possible distance we could not receive the waves of light that he sends -out in less than twelve minutes. As a matter of fact, we do not receive -them at all, for, as we have seen, he is invisible when at his greatest -possible distance from us, being then on the far side of the sun. - -Another cause of Mercury’s apparent change in brightness is due to the -fact that, in common with Venus, he goes through phases from crescent -to full like the moon. This is, as we have seen, a result of his -shining only by reflected light and of his orbit’s being between ours -and the sun. If he shone by his own light, he would be at his nearest -approach to us a very brilliant body indeed. As it is, his dark side -is turned toward us when he is nearest, and when his full face is -illuminated he is on the far side of the sun. We see half of his face -when he is at greatest elongation; but he is brightest when we see -less than half, because he is then nearer to us, and the difference in -distance more than compensates for the difference in illumination. - -These phases cannot be seen with the naked eye, but it requires only -a small telescope to show them, and a very charming little moon-like -body Mercury is when we see them. His horns point toward the east when -he is coming toward us and nearing inferior conjunction, and when he -is backing away from us and going toward greatest western elongation -they point toward the west. It was through the blunting of one of these -horns when the planet was in certain positions that a mountainous -surface was suspected, so great is the significance of small details in -observations. - -As a mere place from which to view the other bright bodies Mercury -would be far superior to the earth. He not only has the sun nearly -seven times larger in appearance at its mean distance than we see it, -but, being himself nearest the sun, all the other planets are outer -planets in relation to him, and all have their discs fully illuminated. - -The earth and the moon, as seen from Mercury, would show as a splendid -pair of stars circling about each other, the earth more brilliant than -any first-magnitude star, and the moon of the third magnitude, or about -as bright as Phecda, the star at the bottom of the bowl of the Big -Dipper, just under the beginning of the handle. The earth would show a -disc of about twenty seconds, and the moon one of about eight seconds, -with a distance between them of about 871 seconds. Some idea of what -this distance is may be had if one knows Mizar, the star at the bend -of the handle of the Dipper, and its tiny shining attendant, Alcor. -These two stars are 708 seconds apart. The distance between them is -about equal to one-third of the diameter of the moon as measured from -the earth. It does not appear to be nearly so much as that, and some -persons have difficulty in separating the two stars; but the moon is -not only inconstant but deceptive, and owing to its brilliancy seems -always proportionately larger than it really measures. - -Venus would appear from Mercury as much as four times as large as -she seems to us--a veritable little moon, and always full, her size -varying slightly as Mercury speeded back and forth from the farthest to -the nearest point in his orbit, changing the extreme of the distance -between them from one hundred and ten million to less than twenty-four -million miles. If Mercury needed a moon, he could well find some -consolation for his lack of it in the presence of the lovely Venus in -his sky. - - -MERCURY’S SIZE AND THE CONSEQUENCES OF IT - -Mercury is the smallest of all the major planets. His diameter is about -three thousand miles. It is only about nine hundred miles greater than -that of our moon. The surface of Mercury is only one-seventh that of -the earth, and his volume only one-twentieth. Jupiter and Saturn each -have a satellite that is considerably larger. - -Mercury would make a splendid satellite or a giant asteroid, but -as a planet seems hardly to have had a fair chance in life. For -being a small planet means something more than being constructed on -smaller lines than some others are. It means a difference in physical -development. It means less power to hold the gases that compose an -atmosphere, which is the cover that shields the planets from the too -burning rays of the sun and keeps their internal heat from radiating -too quickly into space. It means less power to resist the tidal -friction that the parent body uses as a brake to retard rotation. It -means a shorter time of activity in life, and a long, dull, monotonous -old age. - -The nucleus that was detached from the great spiral, or the portion -of nebula that was separated in whatever way from the parent body, -to form Mercury chanced to be a small one. Being small, it was unable -to add materially to its mass by attracting other particles to it -through the power of gravitation, as a larger planet might do, and thus -Mercury was doomed to develop with the limitations that nature’s law -has decreed as inevitable in the small bodies of our solar system, be -they planets, satellites, or asteroids. Of these limitations the first -and most far-reaching in its effect is the feebleness of its force of -gravity, or power to attract other bodies. - -Mercury’s force of gravity is small. It is smaller than that of any of -the other planets. It is a little less than one-quarter that of the -earth. The same weight of feathers that would compose a pillow here -would make a whole feather bed on Mercury. Any object weighing one -hundred pounds here would weigh only twenty-four there. The materials -composing our earth and all the planets are held together only by the -force of gravity. The air we breathe would dart off into space with -almost incredible fleetness if the earth had not sufficient gravitative -force to hold it. Its particles are struggling all the time to get -beyond this power. The lightest of them do get beyond it and are lost, -and the less power we have to hold them the sooner they leave us. The -greater the mass of a body, the rarer the gases it can hold in its -atmosphere, for this mysterious force which pulls everything toward the -center of a planet depends upon its mass, or the quantity of material -in it. The planet may be very large because it is very much expanded. -It may be gaseous even, and its mass would then be very small in -proportion to that of a solid body of the same size. As it condenses, -the particles draw closer and closer together, the density increases; -but the mass is the same. It is only the size that diminishes. - -So a planet with a small mass starts out in life with a disadvantage. -It not only has little power to grow by drawing in particles from its -environment, but also has little power to hold such as by their nature -are volatile and swift of motion, as the molecules of gases are. The -mass of Mercury is not exactly known. The only way we have of measuring -the masses of the planets is by their influence through gravitation on -other bodies near them. When a planet has satellites, the movements -of the satellites tell the story, and by mathematical calculation the -amount of material in the planet can be determined. But Mercury has no -satellite, and the only way to determine his mass is by observation -of his influence on Venus, and on an occasional comet which passes -near enough to be disturbed by the planet. The particular comet which -has been useful in determining the mass of Mercury is Encke’s. On -passing near the sun it comes sometimes near Mercury, and the pull -it has repeatedly received from that little planet on such occasions -is thought to be largely responsible for the comet’s having become a -part of the solar system. The changes in its orbit caused by these -encounters show the power of Mercury, and hence the mass. - -In these ways the mass of Mercury has been found, with reasonable -belief in its accuracy, to be about three one-hundredths that of the -earth. Yet there are, indeed, considerable differences regarding it -among astronomers. The exact figures are not important to any but the -close student. It is certain that the mass of Mercury is very small--so -small that the planet probably never had much atmosphere, and almost -undoubtedly has none to speak of now. The planet could not hold any -molecule moving faster than two and forty-five one-hundredths miles -a second, and few gases move as slowly as this. The proportion of -light that Mercury reflects to that which he receives also points to -a probable scarcity of atmosphere. If he had an atmosphere, it would -have clouds. Clouds have a very high reflecting power, giving out -about seventy-two per cent. of the light that falls upon them. Mercury -reflects only fourteen per cent. of the light he receives, which -shows at least a lack of clouds, and something more. It indicates a -hard, dark, almost metallic surface, and a very considerable density. -Density, however, is the only quality in the possession of which -Mercury seems to occupy a middle ground among the planets, being -slightly less dense than either Venus, or Mars, or the earth. The earth -is the densest of all the planets, and it is about one-third more dense -than Mercury. Density is simply the closeness with which the particles -composing a body are packed together. A piece of gold, for example, is -denser than a piece of iron of the same size. - - -WHAT THE SUN DOES FOR MERCURY - -It is probable that Mercury has no alternations of light and darkness, -causing day and night such as we know them. That is, the planet does -not rotate on its axis in such a way as to turn first one side and then -the other toward the sun as the earth does. In this, as in some other -things, Mercury must accept the fate that overtakes many other small -bodies which revolve around large ones--that of our moon, for instance, -and the satellites of some of the other planets. Working under the law -of gravitation, which gives such power to the large bodies, the sun -has so retarded the rotation of Mercury that the planet now makes but -one rotation on its axis during one circuit around that central body, -and so keeps always the same face toward the sun. Some astronomers -do not regard this as having been wholly proved; but all the later -observations of Mercury strongly indicate that it is the fact, and it -is coming to be more and more regarded as established. - -But, even if this is the predicament into which Mercury has come, the -planet is probably not in so bad a plight as many another body to -which the same sort of thing has happened. The extreme eccentricity -of his orbit, which has given him the true mercurial temperament, -resulting in sprightliness, agility, and changeableness, is accountable -for some mitigating circumstances. The sun may hold him so that he -cannot turn his face away from that luminary; but it cannot keep him -from rotating on his axis at a uniform rate of speed, and from this, -combined with the vagaries caused by his eccentric orbit, come some -interesting things. - -Since Mercury is less than two-thirds as far from the sun at perihelion -as he is at aphelion, there is a corresponding variation in his rate of -speed. When he is nearest the sun, at perihelion, he darts along at the -rate of thirty-five miles a second; at aphelion, when he is farthest -from the sun, he travels only twenty-three miles a second. Twenty-three -miles in one second is not exactly a snail’s pace, terrestrially -considered, and it is faster than the earth moves at any time; but the -planet was named Mercury because of his swiftness, and we would not -expect much lagging even when he is moving at his slowest gait. This -difference in speed in different parts of his orbit causes what is -called the librations of Mercury. When he is traveling at his swiftest -pace he gets a little ahead of his rotation, the speed of which is -uniform, and thus throws the sunlight somewhat farther around on one -side. When his speed decreases, he falls behind his time of rotation, -and thus gets a little more sunlight on the other side. Thus, during -each revolution he juggles the sunlight a little farther around him -than he could if he were a more steady-going planet. - -These librations result in there being two strips on the surface of -Mercury--one on each side--which undoubtedly have a day and night, -varying in length in the different parts of the strips. The part that -lies nearest the illuminated side of the planet has alternate periods -of sunlight and darkness, each of considerable duration, while that -part nearest the dark side has merely a glimmer of sunlight every -eighty-eight days, which is Mercury’s sidereal year, or the time -required for him to make one revolution around the sun. These two -strips on which the light varies comprise about one-eighth of the -surface of Mercury. One half of his entire surface is always light, and -of the other three-eighths are always dark. It is this dark, cold side -that is turned toward us when Mercury is nearest to us. - -It is possible that on those parts of Mercury where the sunlight and -darkness are unstable there may be something resembling a tolerable -temperature. They are something more than a thousand miles in breadth, -and perhaps near the center of them the sun may give heat sufficient -to enliven and yet not burn. More than likely, they are alternately -scorched and frozen. For it takes more than the mere presence of -sunlight to make a climate tolerable. Atmosphere is what is necessary, -and we have seen that Mercury has probably lost practically all his -atmosphere long, long ago. An atmosphere absorbs much of the radiant -energy that comes from the sun before it reaches the more solid parts -of a planet, and it also acts as a blanket in preventing the too rapid -escape of such heat as the planet may have acquired. Thus it has the -doubly beneficent office of tempering the rays that would otherwise -be scorching and of hindering a radiation that would leave the planet -stiffened and frozen. - -Stiffened and frozen is what the dark side of Mercury undoubtedly is. -The sun has never shone upon it since Mercury became a solid body. All -the inherent heat it had has long since passed off into space, and its -temperature must be somewhere near the absolute zero. The absolute zero -is the point in temperature where all known substances become solid. It -is more than 450° below the Fahrenheit zero, or more than 350° lower -than any temperature recorded in our arctic regions--a degree of cold -unthinkable to any but the scientist. - -On the other side of Mercury the heat is beyond anything we have any -notion of. With an equal atmosphere it would receive from the sun six -thousand times as much light and heat as Neptune on an equal space, -and, on an average, seven times as much as the earth. At Mercury’s -distance from the sun his hot side would be more than 300° above -zero, if there were absolutely no atmospheric protection. Even though -tempered by a thin atmosphere, as it may be, the heat on this side is -still probably enough to boil away any water that might be there and to -change some other substances from what we regard as their normal state. - -Stability, at least, is a quality of the hot and the cold side of -Mercury. Scorched and seared and desolate of life, as we know it, the -one side lies under a blazing, dazzling sun. Cold and hard and bleak, -and no less desolate, the other side turns its face toward the darkness -of space. Thus they will remain until the end of time. And let us hope -that, when the final catastrophe occurs and a new nebula is formed, the -matter composing Mercury may find a place in a larger mass, and in its -new incarnation have a fuller and larger life. - -It is the atmosphere also which causes twilight, as well as the gradual -changing from heat to cold. With no atmosphere, we would drop from full -daylight to the darkness of starlight at the setting of the sun. So, -with the thin air that Mercury probably has (if he has any), the two -zones which are alternately light and dark, and hot and cold, are not -much better off than the parts which are permanently either light or -dark. They are plunged alternately from the temperature and light of -the hot side of Mercury to the temperature of the cold side, with few -gradations to prepare them for such extremes. Thus the only part of -the planet that might be expected to have any variations of seasons -fulfils the expectation with little satisfaction. - -The only changes in climate which may have an appreciable effect are -mainly those caused by the eccentricity of Mercury’s orbit, which -carries him so near the sun at certain times and so comparatively far -away at others. When he is nearest the sun he receives more than twice -as much heat and light as when he is farthest away. At aphelion he -receives four times as much heat and light as the earth. At perihelion -the amount of heat and light is increased to more than nine times that -of the earth. Since it takes Mercury a little more than twelve weeks -to make one revolution around the sun, he passes from nearest distance -to farthest, or the reverse, every six weeks. And thus, as viewed -from the planet, the sun expands gradually for six weeks until it has -increased its diameter two and one-half times, and the next six weeks -it diminishes in the same proportion. At such times, of course, the -amount of heat is more or less according to the planet’s distance from -the sun; but all the time it is very great. - -Moreover, it is believed that the axis on which Mercury rotates stands -perpendicular to his orbit. This being the case, there would be on -Mercury no change of seasons such as the earth has. The earth’s axis -is inclined a little more than twenty-three degrees to its orbit, and -from this we get the sun’s rays in a great variety of directions and -different degrees of obliquity, causing the seasons, as we know them, -in grateful variation. With the axis perpendicular, as it probably is -in the case of Mercury, the sun’s rays fall on the face of the planet -always with the same degree of directness, the only relief from their -greatest heat being when the planet backs away from the sun every six -weeks, and when in his librations he turns first one sun-burned cheek -and then the other toward the coolness of space. - -Thus we must regard the smallest of our family of planets, Mercury, -as always the dwarf among us, with never a fair chance to develop a -rich and luscious life according to our ideas of such a life. Beaten -by the sun’s hard rays, and with no sufficient atmospheric protection; -pulling always at his tether, but held firmly with his face to the -center; circling at times with mercurial swiftness and thus cheating -the sun into sending its rays farther toward the dark, cold side of -him than it otherwise would, and with all his defects from a human -point of view, we may still regard him as a right merry, roguish little -planet, after all. He may be prematurely aged, he may have missed many -experiences that the larger planets are having, he may have a long time -to wait for the final change that will reunite us all; but he is not -lying in sluggish inactivity until it comes. - -In view of the fact that he is the only planet that twinkles, may it -not suggest, when we see his ruddy face peering through the thick -atmospheric mists near our horizon, that the impish little body is -winking at us, and that it may be with planets as it is with people: -they may not always be in an unfortunate plight because their fate is -different from ours? - - -TRANSITS - -Occasionally Mercury passes at inferior conjunction between us and -the disc of the sun, appearing like a black spot against the sun, and -thus makes what we call a transit. Because the planet is so small, his -transit across the sun cannot be seen with the naked eye; but it is -an interesting phenomenon to those who can view it with a telescope, -though, apparently, astronomers do not regard it as having any great -scientific importance. It is during a transit, however, that we watch -for confirmation of the theories concerning Mercury’s atmosphere, -which, if it were a reality, would show a diffused light about the -planet; and until this question is settled beyond any dispute it will -always come up at the time of a transit of Mercury. At nearly every -transit some observer sees these indications of an atmosphere; but -the better the telescope, the less they seem to be seen. Hence it -is probable that there is an illusion somewhere either of eye, or -instrument, or mind, and that the majority opinion, which accords to -Mercury practically no atmosphere, is about the correct one. - -These transits occur at intervals of seven, thirteen, or forty-six -years, according to the position of the earth. They would occur every -time that Mercury passed inferior conjunction if the earth’s orbit -and that of Mercury were in exactly the same plane. But the orbit of -Mercury, we have seen, is tilted out of the plane of the ecliptic, -which marks our orbit, seven degrees, so that the only time the earth -and the planet are anywhere nearly in the same plane is when they are -at or near the points where their orbits cross each other. - -The earth is near the two points where Mercury crosses the ecliptic -about May 8th and November 9th, so that transits can occur only near -these dates. Mercury passes these points four times every year, or once -in each revolution around the sun. But the earth is not always there -at the same time, and it is because of this that transits occur only -in periods of seven, thirteen, or forty-six years. They occur more -frequently in November than in May. The last transit was in November, -1907. The next will be on November 7, 1914, and there will not be -another in November until 1927, an interval of thirteen years. But at -the point where the May transits occur there will be one on May 7, -1924. - - - - -XI - -VENUS - - -Of all the planets lovely Venus is the one that is best known and -most admired. It far exceeds all the other planets in brilliancy and -beauty when as an evening star it hangs in gracious silvery softness -above the sun, which has just passed below the horizon; and it is not -less surpassing in loveliness when as a morning star it comes into -view shortly before the sun rises, its glowing face still silvery and -bright, but yet tinged with the rosy flush of the eastern morning sky. - -In either position it never twinkles as Mercury sometimes does, but -shines so steadily and softly that at times its disc can almost be -seen with the naked eye, and it has such brilliancy that its light can -often be seen in the daytime, if one knows when and how to look for the -planet. At its brightest it frequently throws a light sufficiently -strong to cast a shadow, as one may easily prove by holding a book or -some other opaque object between Venus and a white background, such as -the wall of a white house. It is six times as bright as the brightest -of all the fixed stars, Sirius, the beautiful dog-star, which we see in -winter chasing across the southern skies after Orion. - -Venus’s superior brilliancy is due in part to the fact that it comes -nearer to the earth than any other planet; but it is also intrinsically -brighter than any of the others. From equal areas it reflects almost -four times as much light as Mercury and three times as much as Mars. - - -WHEN AND WHERE TO SEE VENUS - -When Venus appears in the sky she is not often mistaken for any other -planet. Among all the planets she is the most readily recognized and -the easiest to find. This is due largely to her extreme brilliancy and -a peculiar silvery appearance that none of the other planets have; -but also, in part, to her limited range in the sky, and her favorable -situation for observation. Unlike Mercury, she is far enough away from -the sun to be seen above the horizon for as much as three hours after -sunset, and is then sufficiently high in the heavens to be seen free -from the vapors of the atmosphere at the horizon. Yet, being one of the -inferior planets, with her orbit smaller and nearer the sun than that -of the earth, she can never get so far from the sun as to be at any -uncomfortable height for viewing, and hence, when she can be seen at -all, is always an obvious bit of brilliancy and a joy to the beholder. -She is never higher in the sky than forty-five degrees, which is -half-way between the horizon and the zenith, and is never farther away -from the sun than forty-eight degrees. One frequently sees a bright -planet higher up in the heavens than this; but it is never Venus nor -Mercury. - -We first begin to notice Venus in the evening sky about six weeks after -she has passed superior conjunction. She is then very near the sun, and -sets a little less than half an hour after sundown. Evening by evening -she grows gradually brighter, mounts higher and higher in the sky and, -consequently, sets correspondingly later, until in a little more than -seven months after superior conjunction, and about six months after we -have begun to watch her, she reaches her greatest elongation east from -the sun. At that time she is usually somewhere near forty-five degrees -above the sun, and is a very lovely and conspicuous object in the -evening sky, setting a little more than three hours after sundown. - -From this point she begins to travel back toward the sun, still -becoming brighter each evening, because she is really coming nearer to -us; and in about four or five weeks she attains the greatest brilliancy -that she will have as an evening star during the particular revolution -she is making. About twelve days after her brightest she will reach the -point where she seems to be stationary for a time. This is when she -is about to overtake us in our journey around the sun. After a short -pause she will move on gradually, her course among the stars then being -retrograde or westward; but what we most notice is that she is drawing -nearer to the sun, setting earlier each evening, and becoming more -and more difficult to see. At the end of about three weeks she is in -inferior conjunction, on a line between us and the sun, and invisible. -She has run her course as an evening star for nine and a half months, -and has been visible anywhere from seven to eight months, the time -of her invisibility depending upon the eye of the observer and the -conditions of situation and atmosphere. - -A week or two later we shall find her a splendid morning star, rising -nearly an hour earlier than the sun. About three weeks thereafter she -will be at her brightest as a morning star, and will continue to be -very brilliant for some weeks. In about five more weeks she will have -reached her greatest elongation west of the sun, and will rise about -three hours and a half before dawn. Then she will begin to retrace her -path, moving eastward, growing smaller all the time as she goes farther -away from us, and showing a slower apparent movement, which gives one -an agreeable sense of a reluctant parting, until after a little more -than seven months she will have reached the sun and will again be in -superior conjunction. She has then been a morning star for nine and a -half months, and has been visible for about the same length of time -that she was when she shone as an evening star. - -This is a brief outline of a typical journey of Venus through one -synodic revolution. She began one of these journeys on July 5, 1912, -being then in superior conjunction. During the autumn of this year and -the winter of 1912–13 she may be seen shining with great brilliancy -in the west at sunset, and a few hours thereafter. Early in November, -1912, she and Jupiter will both be in Scorpio, where they will approach -within two degrees of each other; and there is no doubt that their -presence will add much charm to that region of the sky during the -entire autumn. - -About the middle of February, 1913, Venus will appear half-way up to -the zenith at sunset. She will then be at her greatest distance east -of the sun, and will be very bright; but, though a little nearer the -sun, she will be still brighter shortly after the middle of March. A -month later she will be invisible, and inferior conjunction will occur -on April 24th. During most of May and all of June and July she will -be a morning star, and her brilliant beauty will well repay an early -morning outlook. She will get back to superior conjunction on February -11, 1914, and in that year she will be in an ideal situation for us -to cultivate a more intimate acquaintance with her. From the latter -part of March to November, 1914, she will be the brightest star in the -western evening sky, and will do much to enhance the beauty of the -pleasant summer evenings of that year. The sturdy, red-faced Mars will -meet her on August 5th, a little more than a month before greatest -eastern elongation, and might almost kiss her pale cheek as they pass -within one-sixth of a degree of each other, a distance equal to less -than one-third of the diameter of the moon. - -The next long period when Venus will shine as an evening star will -comprise the spring and early summer of 1916. She will be at her -greatest distance from the sun during the last week of April, and will -not pass from view until about the first of July. Then again she will -be an evening star, and so seen in the west during the autumn of 1917 -and the winter of 1917–18, reaching greatest eastern elongation during -the first few days of December, 1917. Her next return to the evening -sky will be for the first eight months of 1919, and the next will be -for the winter of 1920–21 and the spring of 1921. - -The synodic period of Venus is nearly five hundred and eighty-four -days, or a little more than one year and seven months. That is, the -planet returns to the same position with relation to the sun and -the earth at intervals of about that length. The intervals do vary, -however, as much as a week or more, owing to the various motions and -situations of the planet and the earth. But every eight years Venus and -the earth come around to almost exactly the same relative position with -each other and the sun and the stars, and thus the appearances of Venus -at the various seasons practically repeat themselves every eight years. -The full splendor that she is to offer us in the summer of 1914 will -be repeated in 1922, just as that of 1914 will but repeat that which -she showed in 1906. And in each of the intervening years she will have -again the same appearances that she had eight years before. - -With the following table as a guide, the appearances of Venus can be -followed through a number of years with sufficient accuracy for any -but a close student of her movements. The exact dates of elongations -and conjunctions will vary a few days, but for at least two or three -multiples of eight years not enough to make any material difference in -her various aspects. - - 1913--1921--1929--1937 - - Greatest eastern elongation, February 12th. Inferior conjunction, - April 24th. Greatest western elongation, July 3d. - - 1914--1922--1930--1938 - - Superior conjunction, February 11th. Greatest eastern elongation, - September 17th. Inferior conjunction, November 27th. - - 1915--1923--1931 - - Greatest western elongation, February 8th. Superior conjunction, - September 14th. - - 1916--1924--1932 - - Greatest eastern elongation, April 26th. Inferior conjunction, July - 5th. Greatest western elongation, September 14th. - - 1917--1925--1933 - - Superior conjunction, April 28th. Greatest eastern elongation, - December 2d. - - 1918--1926--1934 - - Inferior conjunction, February 11th. Greatest eastern elongation, - April 22d. Superior conjunction, November 25th. - - 1919--1927--1935 - - Greatest eastern elongation, July 6th. Inferior conjunction, - September 14th. Greatest western elongation, November 25th. - - 1920--1928 - - Superior conjunction, July 5th. - -The meetings of Venus with the other planets do not, however, occur -with this delightful regularity. They all are moving about in their own -ways, and engaged in their own affairs, and only the earth gets back -to repeat the meeting with her in just eight years. These eight-year -cycles are due to the fact that Venus makes thirteen revolutions -around the sun while the earth makes eight. Her journey around the -sun requires a little less than two hundred and twenty-five days -(224.70), and the earth completes its revolution in a little more than -three hundred and sixty-five days (365.25). So at the end of about -two thousand nine hundred and twenty-two days--which equals eight -years--they come into almost exactly the same relative positions in -their orbits with which they started out, and begin the cycle anew. - - -DISTANCE AND BRILLIANCY - -The mean distance of Venus from the sun is 67,269,000 miles. Her orbit -more nearly approaches the form of a circle than that of any other -planet. It is, like the orbits of the other planets, an ellipse, but -of such small eccentricity that the difference between her greatest -and least distance from the sun is scarcely more than a million miles. -Light, traveling as it does, at the rate of a little more than one -hundred and eighty-six thousand miles a second, goes from the sun to -Venus in about six minutes. It takes something more than eight minutes -for light-rays to come from the sun to us. When Venus is nearest the -earth, her silvery beams come swiftly across to us in a little more -than two minutes. When she is farthest from us, the rays of light -require a few seconds more than fourteen minutes to travel over the -distance. She is, when at her greatest distance, more than one hundred -and thirty-five million miles farther from us than when at her nearest. -This difference is due not to any great eccentricity in her orbit, or -in that of the earth, such as causes Mercury’s great variations of -distance, but to the situation of the two bodies in their orbits: they -are nearest together when they are on the same side of the sun, and -farthest apart when on opposite sides. - -Usually at inferior conjunction Venus is a little more than twenty-five -million miles from the earth. At her nearest possible approach to us, -however, which takes place at inferior conjunction, when the earth -is nearest the sun and Venus is farthest from it, a situation which -occurs only once or twice in a century, the distance between us and -the planet is only a little more than twenty-three million miles. This -is nearer than any other heavenly body ever approaches us, except the -moon and, so far as we now know, one small asteroid. Also, it is nearer -than Venus comes to any other heavenly body except perhaps Mercury. -Her nearest approach to that planet is also about twenty-three million -miles. - -Unfortunately, our comparative proximity to this beautiful planet does -not much aid us in learning anything about her personal peculiarities. -Shining only by reflected light, and being, like Mercury, situated -nearer to the sun than the earth is, when she comes around to the -same side of the sun on which we are, her unillumined side is turned -toward us, and at the point of very closest approach she is absolutely -invisible to the naked eye. Through a telescope, however, she can be -seen up to the very point of inferior conjunction. What we see then is -a mere curved line of light, so thin is the crescent she presents; but -it is always apparent except when the planet makes a transit. During a -transit she is actually in our line of sight with the bright disc of -the sun, and is neither above nor below it, as at the ordinary times -of inferior conjunction. The slender crescent that we ordinarily see -offers a very narrow field for observation. - -If there is any one on Venus who is studying the earth, he has a -much easier task than we have in our effort to learn something about -her. The earth is not only somewhat larger than the planet, but when -the two bodies are nearest together the disc of the earth is fully -illuminated, and so must show a splendid face; and then, our atmosphere -probably interferes less with close observation than that of Venus. -This little terrestrial system would undoubtedly shine as a magnificent -pair of stars if observed from Venus. At that distance our moon would -appear considerably larger than Venus appears to us when at superior -conjunction, the earth would seem much larger than Venus ever does to -us, and the distance between them would seem to be a little more than -the apparent diameter of the full moon as we see it. The light of the -earth must cause much more of a shadow than we ever get from the light -of Venus. - -It has been suggested that light from the earth is responsible for a -dusky illumination of the dark side of Venus, which is occasionally -seen, and which enables us to distinguish her entire outline even when -only the merest line of a crescent is really illuminated. It is known -to be earth-shine that causes what is apparently the same phenomenon -often seen by us on the moon; but it seems that there is no reason to -think that our earth, at its distance, would be sufficiently brilliant -to illuminate Venus even so slightly. The cause of the illumination is -not known; but it is thought that it may have some electrical origin, -probably similar to that of our aurora. - -Venus has the same phases that Mercury has. She shows her full face -when at superior conjunction, and is then farthest away and smallest -to our view. As she moves toward us she first becomes gibbous, and -then, at eastern elongation, like a half-moon. As she comes nearer to -inferior conjunction, and hence nearer to us, she becomes a thinner and -thinner crescent, and as she goes from inferior to superior conjunction -these phases are repeated in reverse order. We see less than half of -her face when she is at her greatest brilliancy, a phase which usually -occurs when she is about forty degrees from the sun, as she is a few -weeks before and after inferior conjunction. A very small glass will -show the phases of Venus. They have occasionally been seen without -artificial aid to vision by an exceptionally good eye. They were -not known, however, until they were discovered by Galileo after the -invention of the telescope in 1610. - -Venus would be many times brighter than she ever appears if the -entire disc of the planet could be seen when it is nearest to us. The -apparent diameter of the disc at that time is nearly seven times larger -than when we see it at the planet’s greatest distance from us. When -Venus is in superior conjunction and farthest from the earth the disc -measures only ten seconds, while at inferior conjunction its measure -is nearly sixty-seven seconds. The diameter of the moon is about 1,868 -seconds, so one could string across the diameter of the moon one -hundred and eighty-six such planets as Venus appears to be when at her -smallest, and only twenty-seven of the size that she appears to be -when at her largest. Between these two extremes of size she changes -gradually, day by day, from large to small and small to large, in -ceaseless succession, as she approaches the earth and recedes from it -in her orbital journey. Apparent diameter is determined by an actual -measurement of the disc of a planet, and in the case of Venus indicates -nothing as to brightness. When the apparent diameter is largest she is -not visible to the naked eye. - -[Illustration: THE LOVELY CRESCENT THAT VENUS SHOWS WHEN TO OUR VIEW -SHE IS AT HER GREATEST BRILLIANCY - -This remarkable photograph was made at the Yerkes Observatory by E. E. -Barnard.] - -[Illustration: RELATIVE APPARENT SIZE OF VENUS AT DIFFERENT PHASES OF -ILLUMINATION - -She shows the full disc when farthest away. As she draws nearer she -shows first the half moon and then the smaller crescent. She is nearest -when she shows the larger crescent. She is brightest, though, when she -shows the smaller crescent.] - - -VENUS’S LIKENESS TO THE EARTH - -The fact that of all the planets Venus most resembles this good little -earth on which our present lot is cast gives us a strong feeling of -kinship with her, and a more lively interest in all her affairs than we -might otherwise have. She and the earth are so nearly of one size that -they are often referred to as twin sisters. There is a difference of -less than three hundred miles in their diameters, the earth’s diameter -measuring 7,917 miles, and that of Venus 7,629 miles. The surface of -the planet is about ninety-three per cent. as extensive as that of -the earth; its mass is a little more than eighty per cent., and its -volume about ninety per cent. as great as the earth’s. Differing so -little in these particulars, it follows that it must differ very little -in density and gravity. The earth is the densest of all the planets, -and Venus is only one-tenth less dense than the earth. Its force of -gravity is not quite nine-tenths that of the earth. A removal from -the earth to Venus would make just a comfortable reduction in one’s -weight. A person weighing one hundred and seventy-five pounds here -would weigh on Venus one hundred and fifty-four. If through strength -of appetite and weakness of will one should take on two hundred pounds -of too, too solid flesh here, transportation to Venus would bring about -an instantaneous reduction to a solid one hundred and seventy-six -pounds--as much of a reduction as would be compatible with health. - -Venus must have begun her career in much the same way that the earth -began its career. The nebula that formed her nucleus was probably -nearly the same size (contained about the same amount of matter) as -that with which the earth began its existence. The two bodies have -succeeded in capturing about the same amount of loose material, and -their gravity is such that they can hold within their bounds particles -traveling at about the same rate of speed. No molecule of gas coming -within the range of Venus’s attraction and traveling more slowly than -six and thirty-seven hundredths miles per second can escape from Venus, -and the earth can hold only such as move, when coming within its own -attraction, with a less speed than six and ninety-five one-hundredths -miles per second. - -The earth has a moon, and Venus has none; but that may be because, -like Mercury, Venus is too near the sun to be permitted to retain such -a luxury. It is likely that if, in her earlier history, she had within -the limit of her gravitative attraction the nucleus of a satellite, it -would have been taken away from her by the stronger attraction of the -sun. The same thing would have happened to us if we had been a little -nearer the sun. And yet in 1645 a moon belonging to Venus was supposed -to have been discovered, and it was thought to have been seen three -times within the rest of that century, and four times within the first -half of the following century. The last supposed view of it was in -1791; it has never been seen since. There is little doubt that it was -an illusion of some kind. Perhaps, though, Venus has not the same need -of a moon that we have. - - -ATMOSPHERE, DAY AND NIGHT, AND SEASONS - -There is no doubt that Venus is in much better plight than Mercury, the -other inferior planet, in regard to atmosphere. Until recently no one -has questioned the belief that her atmosphere is very extensive--twice -as heavy, perhaps, as that of the earth, dense, and full of clouds. -The luminous ring about her, shown when she is making a transit across -the face of the sun, points to a heavy atmosphere; and no less certain -indications of it are given in the faint light which stretches beyond -the termination of the horns when she is in the crescent phase, near -inferior conjunction. Her very high reflecting power is also indicative -of an atmosphere laden with clouds. White clouds form one of the most -highly reflecting surfaces known, and the peculiar brilliancy of Venus -is thought to be in great part due to the presence of large masses -of clouds in her atmosphere. By the spectroscope, and in other ways, -the water necessary to form clouds is shown to be abundant in her -atmosphere. Even those astronomers who doubt the long-current belief -in the large extent of her atmosphere concede an atmosphere of more or -less density, though by one authority it is characterized as somewhat -gauzy. - -There is one vital point concerning the development of Venus upon which -we have as yet no positive knowledge: the length of time in which she -rotates on her axis. This is unfortunate, because until her time of -rotation is known we cannot know much about her physical condition. -Her rotation, we know, determines the length of her day and night, or -whether, indeed, she has any. The time of it has been calculated to be -anywhere from a little less than one of our days to two hundred and -twenty-five, the latter being also the time of her revolution about -the sun. Astronomers of equal reputation have come to exactly opposite -results in their investigations. To one, the spectroscope has indicated -the short day and night; to another it has shown no day and night, -but a planet with one face forever toward the sun, like Mercury. What -appeared to be stable surface markings have been observed, but have -indicated under the eyes of different observers both the short day and -no day at all. The disc has been measured during a transit, and shows -so little flattening as to indicate a slow rotation and the long day. -On the other hand, the best authorities think it unlikely that at the -distance of Venus the sun could so retard the planet’s rotation as to -make it coincide with its time of revolution. Thus the question is -still an open one. - -The truth may be that, owing to the density of her atmosphere, the -surface of Venus has never been seen at all, and that the apparently -stable markings are but clouds more or less lacking in stability. The -difficulty of observing Venus will probably make it impossible to -determine this point by visual observation. It may some day be settled -beyond a doubt by the spectroscope. In some way it will surely be -settled. Astronomers have too often made possible what seemed to be -impossible for us to doubt that some one will find a way to discover -this secret of Venus. With them a failure to prove a conclusion does -not mean to abandon the subject, but to try some other means of getting -at the truth. - - * * * * * - -The sun viewed from Venus would appear considerably larger than it does -to us. Its apparent diameter to us is a little more than thirty-two -minutes, while on Venus it would be something more than thirty-eight -minutes; that is, it would appear about one-fifth larger on Venus than -it does to us. This is enough to make a material difference between -the two planets in the amount of heat and light they receive. Venus -receives nearly twice (1.9) as much heat and light from the sun as we -receive, but less than one-third as much as Mercury. If she had no -atmospheric protection, there is no question but that she would have a -climate disastrously warm for a race of beings constituted as we are. -The normal temperature of an unprotected body at the distance of Venus -is about 158° Fahrenheit (70° Centigrade). - -If Venus is finally proved to have no alternations of day and night, -she is still better off than Mercury, who has practically no atmosphere -to protect him from the intense heat of the sun. How much protection -she has depends altogether on the extent of her atmosphere. It is -probably not enough to make the hot side comfortable from our point of -view; and Venus, being undoubtedly a solid body with no internal heat, -the cold side must be cold beyond anything we have any conception of. -But there may be a very considerable part on each side that, owing -to the refraction of light by the atmosphere, is more or less well -lighted, and is also more or less protected by this same beneficent -atmosphere from deadly extremes of heat and cold. In this situation -there would undoubtedly be lively currents of air from the heated side -to the cooler; but even these may in some way carry with them some -tempering effects on the climate, as we know such currents do here on -the earth. - -If it should prove that the length of the day and night on Venus is -something near that of the earth’s (and this seems not unlikely), she -would then be indeed more like a twin sister to us. Being next to each -other in our distances from the sun, and of nearly the same size, -differing but little in density, mass, volume, and force of gravity, -with her greater normal heat probably reduced by her heavier atmosphere -to a temperature producing climatic conditions not very unlike ours, -and with not very different alternations of day and night, we might -well be considered more nearly related than any of the other members of -the solar family. - -The seasons, however, on Venus and the earth would not have much -resemblance to each other. The axis of the earth is inclined to the -ecliptic nearly twenty-three and one-half degrees, so that we receive -the sun’s rays with varying degrees of obliquity during our yearly -journeying around it, which is the cause of our agreeable change of -seasons. Venus travels with her axis so slightly inclined to her orbit -(a little more than three degrees) that each particular parallel of -latitude receives practically the same amount of sunlight every day -in the year, though at different parallels the sun’s rays strike with -varying degrees of obliquity. However delightful or disagreeable the -climate may be, there are no changes of seasons to speak of, and one -could find variety only by going from place to place on the planet. She -receives no compensation for this monotony by alternately receding from -and approaching the sun as Mercury does, or by librations, such as he -has. Her orbit being, as we have seen, so nearly circular as to permit -of only small variations in her distance from the sun, and her axis so -nearly perpendicular to her orbit, it follows that she has nothing to -mark the year; and, whether she turns on her axis many times or only -once during a revolution, life on Venus would be very monotonous to -any one accustomed to our delightful variety of climate and seasons. -Still, there is nothing in this monotony to prevent Venus from being a -fairly comfortable habitation in some parts for such beings as inhabit -the earth. The only real obstacle to habitability on Venus would be her -lack of rotation and all that it involves. - -Since we are not sure that we can see the surface of Venus, we cannot -say what that surface is. Nevertheless, there is some reason to -suspect that we would find there mountains of vast height. Certain -irregularities have been observed at times, of a kind to indicate -mountains covered with snow, extending beyond the clouds. They have -been estimated to be many miles higher than any mountains we have on -earth, their height depending somewhat upon the temperament of the -observer. But inasmuch as these same high mountains have sometimes been -thought to be only masses of clouds, it seems hardly safe to pronounce -definitely upon them. - - -TRANSITS - -On rare occasions, when Venus is in inferior conjunction, she makes a -transit, and can then be seen as a black dot moving over the bright -face of the sun. Transits can occur only when the earth and the planet -are near the point where their orbits cross each other. The earth is at -this point every year on June 7th and December 7th; but the orbit of -Venus is such that she is there on the proper dates only four times in -a period of two hundred and forty-three years. In every two hundred and -forty-three years four transits take place. They occur in pairs, eight -years apart, and in the same month. If a pair occur in June, it will be -one hundred and five and one-half years after the last one of the pair -until we have the first of the December pair of transits. After that it -will be one hundred and twenty-one and a half years until we have the -first of another pair of June transits. - -The first transit of Venus that was scientifically observed was in -December, 1639. It was the last of a December pair, there having been -a transit eight years before, in December, 1631. One hundred and -twenty-one and a half years later, in 1761, a June transit occurred, -and in 1769 another one took place in June. Then there were no more for -one hundred and five and one-half years, when we had a December pair in -1874 and 1882. The next ones will be in June, 2004 and 2012. - -Great importance was attached to those transits that occurred in 1874 -and 1882, because they were expected to be useful in determining with -greater exactness the distance of the sun. Extensive preparations -were made for scientific observation of them; but the results were not -satisfactory, largely because the atmosphere of Venus prevented her -from showing a sharp outline at the moment of entering upon and of -leaving the face of the sun. The main scientific value of a transit of -Venus now is in the opportunity it may offer to investigate the nature -of her atmosphere. Even though that interesting question may have been -practically settled before another transit takes place, it will be -important to know to what degree the phenomena observed at the next -transit confirm the decision. - - * * * * * - -On account of the surpassing brilliancy of Venus, the brightest of all -the heavenly bodies after the sun and moon, she was to the ancients -the most important of all the stars and planets. She was the supreme -evening and morning star. As evening star she was known as Hesperus, or -Vesper; as a morning star she was called Phosphorus, or Lucifer, and -under all these names she is frequently mentioned in Greek and Latin -and kindred literatures. - -The symbol of Venus is ♀, a figure which is nothing more than the -conventionalized form of a looking-glass, an article that is often -pictured in the hands of the goddess for whom our beautiful planet was -named. In her general aspect she is as placidly splendid and charming -as ever a goddess could be, and it is not strange that the happy ears -that could hear such strains should find her, as they did, singing a -rich contralto in the music of the spheres. Jupiter and Saturn, under -this mythological apportionment, sang bass, Mars took care of the tenor -strains, and the high soprano was carried by our little dwarf Mercury. - - - - -XII - -MARS - - -The planet that varies most in the beauty of its aspect is Mars. It is -as much as fifty times brighter when it is nearest to us than it is at -its greatest distance from us. At its brightest it is many times more -brilliant than any of the first-magnitude stars; but when it leaves our -neighborhood and goes far off into space in its journey around the sun, -its glory is so dimmed that it becomes not brighter than an ordinary -second-magnitude star, such as the pole-star, and less brilliant than -the brightest stars in the Big Dipper. - -These extreme changes of brightness are due not so much to any great -distance that Mars goes from us in comparison with other planets as -to its coming so very near to us at times. It is, after all, a small -body, and no great distance, as heavenly distances go, is required to -make it show so. But the eccentricity of its orbit brings it sometimes -very near us, and its near approaches are at a time when we can see -its entire disc, and not a mere crescent, such as we see when Venus -is nearest to us. Mars does not come quite so near to us as Venus -comes, but when he is in the best position to be seen he is much nearer -than she is when in her best position. For we have seen that Venus is -brightest before she reaches her nearest position to us, while Mars is -brightest when he is at his nearest to us. When Venus is at greatest -elongation she is three times farther away than Mars is at his nearest. - - -HOW TO IDENTIFY MARS - -But with all his variations in brilliancy and beauty Mars remains ever -a charming, rosy-hued planet, shining always with a steady, clear -light, and when once we have come to know him is not easily mistaken -for any other planet, or for any of the brilliant stars that may more -or less resemble him in color. Red in varying degrees of intensity is, -perhaps, the most obviously distinguishing mark of Mars; but his own -characteristics are never more distinct than when his path takes him -into the region of the two best-known red stars in the heavens. These -are Antares, the glowing star in the constellation Scorpio, which we -see in the southern sky during the summer, and ruddy Aldebaran, which -shines in the head of Taurus and under the Pleiades through the bright -wintry nights. On every journey around the skies Mars passes near these -two stars. They are both in the constellations of the zodiac, and are -often quite near to Mars, as well as to the other planets and the moon. -The stars, though of the same color as Mars, are much more jewel-like -than the planet. Mars is less sparkling. When it is small, it shows -a placid, rosy little disc, without much gaiety, and not in any way -suggesting anything martial; but at its largest, it has a distinctly -flame-like aspect, which easily suggests why it was named for the god -of war. - -[Illustration: THE TWO PHASES OF MARS - -We see its full face when it is opposite the sun. When half-way -between opposition and conjunction it becomes gibbous, as shown in the -photograph on the right. These photographs were made at the Mt. Wilson -Observatory.] - -Mercury is the only planet that in color even suggests Mars, and for -Mercury it can never be mistaken after one has once seen the two -planets. Mercury, we know, is always very near the sun; but when -visible at all is, even in that unfavorable situation, always as -bright as a first-magnitude star. Mars is near the sun, to our view, -only when it is approaching conjunction, and it is then so far from -us that it always appears as a rather small star, and, while never -insignificant, is, in this situation, quite inconspicuous even as -compared with the rarely visible Mercury. - -On seeing a planet, then, sufficiently high above the horizon to -attract one’s attention, one may be sure that it is Mars if it is red, -and equally sure that it is not Mars if it does not show this color. -Under certain atmospheric conditions the sun, moon, and all the planets -sometimes appear red when they are very near the horizon; but in this -situation there is always something other than color that marks them. - -If its color is not a sufficient mark by which to identify Mars, a -still further difference between it and the stars is its markedly rapid -movement. A single night will make a sufficient change in its position -to show the planet as a wanderer. On an average, it travels over about -four-tenths of a degree in the heavens in one day. This equals more -than half the diameter of the moon, a change of position sufficiently -great to be easily detected. - - -WHEN AND WHERE MARS MAY BE SEEN - -Unlike Mercury and Venus, which are never far from the sun, and can be -seen only for a comparatively short time either early in the morning -or in the evening, and are never very high up in the skies, Mars may -be situated so that it can be seen at any time of the night, and also -at any distance from the sun. When it is in opposition it rises just -as the sun sets, and is then in view all night. At this time it is -nearer, larger, and brighter than at any other time in the particular -revolution it is then making, and, consequently, is in the best -position to be viewed by us that it will have during that revolution. - -Oppositions differ, however, in different revolutions, and some show -us the planet more splendidly brilliant than it appears at others. The -oppositions at which Mars shows most brilliant take place, fortunately, -in the summer and early autumn--the seasons which are most agreeable -for outdoor observation. He is then traveling through that region of -the sky, sparse in stars, that lies between Sagittarius and Aries; and, -since the ecliptic there runs rather low in the sky, he can easily be -observed at any time in the night without any neck-breaking postures. - -These favorable oppositions occur in the summer because the earth is in -line in the latter part of August with that point in the orbit of Mars -where the planet makes its nearest approach to the sun. Oppositions -never occur when Mars is exactly at that point; but they do occur when -he is very near it, and at such times we see him in his greatest glory. -This happens once every fifteen or seventeen years. But at any summer -or early-autumn opposition Mars is not very far from this nearest point -to the sun, so that at any oppositions during these seasons he is very -brilliant and almost as bright as when he is at his best. - -The earth is in line in the winter with that part of Mars’s orbit which -carries him farthest from the sun, and at opposition then he is much -less bright than at the summer oppositions. He is at the same time in -those constellations which pass nearly overhead in the sky, and cannot -be quite so comfortably seen at all times in the night as he can be -in the summer. The very best and most brilliant oppositions occur in -the latter part of August or in the early part of September; the -least favorable ones occur in February. The others vary in brilliancy -according to their distance from these favorable and unfavorable dates, -all the summer ones being quite brilliant, and all the winter ones much -less so. At any opposition, though, however unfavorable, the planet is -much nearer to us and much brighter than when in conjunction. - -It is worth one’s while, even at some inconvenience, to see Mars at -whatever time he is in opposition, for he is a delight to the observer, -and always notable in the part of the skies through which he is then -passing. There are some aspects of the planet that are so charming -at a winter opposition that it is a positive loss not to have seen -him at such times. He is more isolated and conspicuous in the summer; -but he fits well in that gay company of winter stars that shine more -brilliantly than any others, and we can easily feel something akin to -family pride as we watch him moving so graciously among them. - -Mars makes a complete circuit of the skies, and comes back into the -same position with relation to the sun and the earth on an average -every seven hundred and eighty days, which makes his synodic period -longer than that of any other planet. Owing to the great eccentricity -of his orbit, and his consequent unequal motion in the various parts -of it, the synodic period varies as much as thirty-five or thirty-six -days. One cannot say, therefore, without computation of some length, -just exactly how many days will elapse between any two single -oppositions. - -For mere purposes of naked-eye observation the variations in the -synodic period of Mars do not make any difference, for the planet is in -view practically all night for many nights before and after opposition, -with changes of brightness too small to be noticed by an untrained -eye. For at least two months at the time of opposition it has almost -the same aspect to us. At that time it is always in the east early in -the evening, and shines all night. For nearly nine months afterward it -is visible and conspicuous in the evening sky, appearing each evening -nearer and nearer to the western horizon, until finally, in a little -more than a year after opposition, it passes behind the sun and becomes -a morning star. But, as it then rises before the sun and passes across -the heavens in the daytime, it is invisible to us. It is pleasant, -however, at such times to know that as the sun passes across the skies -in its daily journey Mars is up there, within a certain distance from -it, making the same journey with it, beaming down upon us with the same -lively light that it shows at night, and could be as well seen at any -time but for the too dazzling rays of the sun. - -Mars will be in conjunction in November of this year (1912), and will -not be visible in the evening during 1913 until toward the end of the -year. The next opposition after the publication of this book will occur -in January, 1914. From that time until the following autumn the planet -may be seen in the evening. In 1915 Mars will not be visible in the -evening sky until late in the year. After November it will be in the -east in the evening, rising earlier each evening, until at opposition, -early in 1916, it will rise at sunset and will be visible in the -evening during the entire summer and autumn of that year, but will not -be extraordinarily bright. In 1917 it will be again invisible in the -evening. In 1918 it will be in opposition in the early spring, and will -shine in the evening all the rest of that year. It will not be visible -in the evening in 1919, but will be in opposition again in the latter -part of April, 1920, and will shine in the evening all of that year and -the early part of the next, when it will again disappear from evening -view and will not emerge again until it is nearing a fine opposition -that will take place just at the beginning of the summer of 1922. The -planet will then be in the constellation Scorpio, not far from Antares, -and this will afford an excellent opportunity to see these two ruddy -bodies near together. - -In 1924 there will be an exceptionally brilliant opposition, which will -occur during the last week of August, and the planet will then be about -as brilliant as it ever appears, and will be very favorably situated -for observation in the constellation Pisces. We shall then see Mars in -the flame-like phase of his beauty, and he will dominate the evening -sky during the whole of that summer. At oppositions such as this one -Mars is more favorably situated for observation from the earth than any -other heavenly body except the moon. - -The next oppositions will take place the last week in October, 1926, in -December, 1928, January, 1931, early March, 1935, the middle of May, -1937; and then we will have two more splendidly brilliant oppositions -in July, 1939, and early October, 1941, respectively. - -During the years that Mars does not appear in the evening we need not -be deprived of a sight of the planet if we will look for it in the -morning sky. A few months after conjunction it may be seen as a morning -star, rising shortly before the sun. It rises earlier each morning, -and hence can be seen each morning for a longer time. After its hour -of rising has reached midnight it then passes into the evening sky and -rises earlier each evening until it reaches opposition. - -The movement of Mars among the stars, as we see it, is generally toward -the east, and we can see by looking that it changes its place among -the constellations in that direction, going from Aries to Taurus, from -Taurus to Gemini, and so on. On each side of opposition, however, the -planet appears for a few weeks to be moving westward among the stars. -This is the retrograde motion which an outer planet appears to have -when we are overtaking and passing it, and which has been explained in -the chapters on the movements of the planets. - - -SIZE, ATMOSPHERE, AND TEMPERATURE - -In size Mars is one of the smallest members of our solar family. Its -mass is a little more than one-ninth that of the earth, and its entire -surface is only about one-third as great as ours. It is the merest -trifle more dense than Mercury, but only about sixty-six one-hundredths -as dense as the earth. Its force of gravity is about thirty-six -one-hundredths as powerful as that of the earth. A man weighing -two hundred pounds here would be relieved of about one hundred and -twenty-four pounds of his weight if transported to Mars, weighing there -only seventy-six pounds, which would greatly increase his efficiency if -he were in other respects the same. - -It would necessarily follow that Mars, having such small force of -gravity, could not long retain a heavy atmosphere, even if it had set -out with such a one. No molecule of gas moving at a greater speed than -three and thirteen-hundredths miles a second could be held by Mars in -its atmosphere, and so much as it may have had of the rarer gases -which move with great rapidity must have escaped long ago. But it did -not begin life with an atmosphere heavy in proportion to that which the -larger planets have. We have seen, in the case of Mercury, that being -one of the small planets entails many restrictions in development. Such -planets not only lose their atmosphere more quickly than the larger -ones, but it is less dense to begin with. The atmosphere of Mars is -probably no denser than we have at the tops of our highest mountains, -more than likely not even so dense as that. There is some water vapor, -and there are a few clouds most of the time; but in the main the -atmosphere is so clear and thin that we can without any doubt see the -actual surface of the planet. It is not certain that the clouds we -see are formed from water vapor, as clouds of the ordinary kind are. -It has been suggested that they may be simply dust-clouds. But this -is as yet not much more than a suggestion, and nothing convincing has -been offered to substantiate the idea. Even dust-clouds would need -currents of air to create and carry them; so, whether dust or vapor, -the presence of clouds implies an atmosphere. - -The famous white polar caps, which furnish so many news items to the -journals, are also of uncertain origin, and their true nature can be -determined only by a fuller knowledge of the atmosphere of Mars. They -appear in the winter season on the planet and disappear in its summer, -so there seems to be no doubt that they are dependent in some way on -the temperature in the polar regions of Mars. If they are hoar-frost -or snow, they are condensations of water vapor; and, in that case, -when they disappear there must be sufficient heat to melt them. It has -been contended that the sun’s rays fall too obliquely on the poles of -Mars to melt more than a few inches of snow, but that the caps may be -light snow or frost, and thus capable of being dissolved by even such -oblique rays of sunlight as they receive. Also it has been suggested -that the deposit resembling snow may be carbon dioxide, which condenses -into a white substance at a temperature more than a hundred degrees -(-109° Fahr.) lower than is necessary to produce snow and melts at a -correspondingly low temperature. What the nature of the phenomenon seen -at the poles of Mars is depends largely upon what the temperature is; -and the temperature in turn is dependent in some measure on the density -and constitution of the atmosphere, as well as the planet’s distance -from the sun. - -The normal temperature of an unprotected body at the distance of Mars -from the sun is about thirty-two degrees blow zero (Fahrenheit); and -since we know Mars has no dense atmosphere to retain the heat it -acquires, it is natural to suppose the existence there of a very low -temperature, and one incompatible with our ideas of life and growth. -The most favorable conclusions do not place the mean temperature higher -than forty-eight degrees Fahrenheit. It is certain that the planet -must be subjected to great extremes of temperature within its range, -since its filmy robe of atmosphere cannot protect it to any extent from -the direct rays of the sun during the day, nor prevent the heat from -escaping with great rapidity at night; so that, whatever heat it may -gain in the daytime, it probably loses much of it during the night. -Until we know more of the constitution of the atmosphere of Mars we can -know nothing certainly about its temperature beyond the fact that it is -much colder than ours and more subject to variations. Anything much -more definite than this is speculative at present. But with all the -observation that is now given to Mars, and with the always increasing -facilities for the work, many uncertainties regarding the planet are -likely to be made clear before long. The spectroscope will probably be -the final resort for facts concerning the atmosphere. - - -DISTANCE AND BRILLIANCY - -Mars is, on an average, about one and a half times farther from the sun -than we are. Its mean distance is, in round numbers, one hundred and -forty-one million miles; but, since its orbit is very eccentric--more -eccentric than that of any other of the planets except Mercury--its -distance from the sun varies as much as twenty-six million miles. -At its nearest the planet is a little more than one hundred and -twenty-eight million miles from the sun. Its greatest distance from -that luminary is one hundred and fifty-four million miles. At its mean -distance something more than twelve and a half minutes are required for -light to travel from the sun to the planet. - -The sun becomes quite a medium-sized object as viewed from Mars, and -must lose some of the majesty of aspect that it has to us. Its apparent -diameter is about twenty-one minutes, which would make it less than -two-thirds as large as we see it. The average amount of light and heat -that it furnishes to that poor, lightly clad little planet is less than -half as much as we receive, though when the planet is at perihelion the -sun’s radiance is forty per cent. more powerful than when it is at its -greatest distance from the source of these life-giving forces. - -The eccentricity of the orbit of Mars is the cause also of his great -variations in distance from us, and hence of his extreme changes in -brilliancy. These changes are many times greater with reference to -the earth than to the sun. At the planet’s nearest approach to us it -comes a little nearer than thirty-five millions of miles. This is when -it is in opposition in August. When opposition occurs in February, it -is as much as sixty-two millions of miles from us; and when it is in -conjunction, and on the other side of the sun from us, it is sometimes -two hundred and forty-eight million miles distant. At his nearest -approach light leaps over to us from Mars in about four minutes and -eighteen seconds; at his greatest distance it cannot reach us in less -than twenty-two minutes. The apparent mean diameter of Mars is about -nine and fifty-six hundredths seconds, but varies from three and -six-tenths seconds, when the planet is farthest away, to twenty-five -seconds when it is nearest to us. - -While Mars does not exhibit the phases of the inner planets Venus -and Mercury, by showing a disc sometimes at half-full and sometimes -at crescent it is sufficiently near us to be, in certain positions, -gibbous, or to show a little less than a full face. When this occurs -Mars is about half-way between opposition and conjunction, and the -earth and the sun are so situated that we are slightly to one side -of the fully illuminated face of Mars. This phase, however, is not -sufficiently marked to make any material difference in the brilliancy -of the planet. It is not apparent without the aid of a telescope. - -From Mars the earth shows all the phases that Venus shows to us. When -Mars is flaming down upon us in his position of greatest brilliancy we -present to him a thin crescent. When he sees our full face we are on -the opposite side of the sun from him. It would be necessary to have -a more brilliant electrical illumination than any we have yet seen to -lighten the dark side of the earth and exchange signals with Mars when -we are nearest to him--if, indeed, our atmosphere would permit from -Mars any view at all of the surface of the earth, which is not at all -certain. In spite of its phases, the earth must shine on Mars at times -in a very attractive way. It is not so bright, perhaps, as Venus is -to us, nor as we are to Venus; but with our moon circling about us we -may well be, when in a favorable situation, a very interesting double -star, the distance between earth and moon appearing on Mars about equal -to one-fourth of the apparent diameter of the moon. - -[Illustration: MARS: DIFFERENCE IN ITS APPARENT SIZE AT ITS NEAREST, -MIDDLE, AND FARTHEST DISTANCE FROM THE EARTH - -Mars appears fifty times brighter when nearest than when farthest away.] - - -DAY AND NIGHT, AND SEASONS - -Owing to the undoubted permanent markings on the surface of Mars, -astronomers have been able to determine the length of its day with -much less likelihood of error than in the case of any other planet -except the one on which we dwell. It rotates on its axis in twenty-four -hours, thirty-seven minutes, and twenty-three seconds, which makes its -day nearly forty minutes longer than ours. In our greed for all too -fleeting time we may feel a little envy of these extra minutes, which -would mean so much to us if added to our day. But they do not seem so -important when we consider that while Mars is having six hundred and -seventy of these days we are having six hundred and eighty-seven of -ours, which, after all, seems to give us eighteen days more of time. -Our attitude toward the situation depends upon the point of view. - -The axis of Mars is inclined to its orbit about twenty-four degrees -and fifty minutes. This is but little more than the inclination of the -earth’s axis, which is twenty-three degrees and twenty-seven minutes. -Mars, therefore, has seasons very much like ours. They are, however, -slightly more marked than ours, because of the somewhat greater -inclination of the axis of the planet; and they are nearly double -the length of ours, because it takes Mars nearly two of our years to -make its journey around the sun. Its seasons, then, are nearly six -months long, while ours are but three. It has frigid, temperate, and -torrid zones, practically the same as the earth has. Its greatest -inequalities of season are caused by the eccentricity of its orbit. -It is, like the earth, farthest away from the sun when it is summer -in the northern hemisphere; and in this situation it travels so much -more slowly than when it is near the sun that summer in its northern -hemisphere is seventy-five days longer than the same season in the -southern hemisphere. The northern summer and the southern winter are -each three hundred and eighty days long, while the reverse seasons in -each hemisphere are only three hundred and six days long. The northern -summer is not only longer but also cooler than the southern, and -the northern winter is shorter and warmer than the southern. Which -hemisphere has the more favorable climate depends upon what is needed -on Mars to maintain life. It may be that in this regard the shorter, -hotter, southern summer is the best season the planet affords. - - -SURFACE ASPECTS OF MARS - -Seen through a telescope, Mars is not so red as it appears to the naked -eye. One of the best observers of it has compared it to an opal, and it -surely has some of the qualities of an opal in the diversity of aspect -that it shows to different observers from different points of view. No -other planet has been so subjected to controversy over what appears -on its surface. This is partly due to its being the only planet whose -surface is without doubt open to our view and in a situation where it -can be minutely studied, and partly to the fact that the controversy -involves questions concerning life and intelligence, which are always -of intense human interest. Matters of this vital sort are never -accepted without dispute. That is one way of getting at the truth. -In the intensity of the discussion the question of the existence of -the phenomena and that of the meaning ascribed to them are sometimes -unnecessarily made to depend upon each other. In the case of Mars it -may well be that there is less difference of opinion as to what is -really seen on its surface than as to the meaning of the phenomena. - -There are recorded observations made of Mars as early as 272 B.C., more -than two thousand years ago, and it has been nearly two hundred and -fifty years since the snow-caps were first seen. Through the telescope -not only the snow-caps are plainly visible at the proper seasons, but -there are also visible dark patches over the surface, showing a variety -of color, and in certain parts changing somewhat as the seasons change. -It is one of these patches, the outline of which suggests a somewhat -twisted eye, that is known as the “eye of Mars.” The main surface of -the planet is reddish yellow in color; the patches on it are variously -described as gray, grayish green, or blue, colors which in combination -could easily take on a tone of any of them according to the eye of the -observer, and this portion of the planet’s surface does, in fact, show -first one and then the other of them predominating. - -When the planet’s differences of color were first observed, the -reddish-yellow portion was supposed to be land, and the areas of -varying bluish-green and gray were thought to be the waters of the -ever-changing seas. A little after the middle of the last century -some keen eyes saw a few streaks or markings of some sort across the -land areas, and in 1877 a close study of the planet by an eminent -Italian astronomer, Schiaparelli, brought to his view many greenish -streaks, all directed toward the so-called seas, and sometimes seeming -to intersect there. In publishing this discovery Schiaparelli called -these streaks _canalli_, which is properly translated “channels,” -but appeared in English as “canals.” Since “canal” with us means -artificially constructed waterways, the discovery became at once one -of universal interest; for artificial waterways mean human beings to -construct them, and it was an intensely interesting thing to know -that Mars was probably inhabited with beings at least somewhat after -our own kind. It was a new world. The little planet became a topic of -absorbing interest to all of us. And thus began the controversy over -the habitability of Mars, and the meaning of its surface features, in -which astronomers, seeking only for the truth, have taken a much more -dignified part than they have sometimes been more or less sensationally -represented as doing. The discoverer of the so-called canals himself -believed them to be natural waterways cutting through the land after -the manner of our straits and channels, and had very little to say in -explanation of them. But his work gave a new impetus to the study of -this little brother world of ours. - -In our own country the observatory at Flagstaff is the one the best -known among those doing research work on Mars; but it is not the only -one. The observatory there is finely situated in the thin, clear -atmosphere of Arizona, the mechanical facilities for such work are -good, and there seems no doubt that there are there some observers -who have eyes that were made for seeing. All that the sharp vision of -Schiaparelli saw has been seen there, and much more. Several hundred -canals have been discovered, and at certain seasons many of them have -appeared to become double. Their courses have been followed, and their -appearances and disappearances have been watched. Somewhere near six -hundred of them have been mapped. According to these maps, the canals -seem to be laid out with a geometrical precision such as nature is -not likely to follow; they run across some regions that were formerly -supposed to be water, and they have points of convergence every here -and there, forming at such points large dark areas. - -Naturally, when a person has discovered any new and curious phenomenon -in nature he seeks to determine the exact meaning of it. It would have -very little interest for him if he did not, and it would be a dry lot -of facts that did not arouse a desire to do this. The interpretation -put upon what has been seen at the observatory at Flagstaff is, in -brief, about as follows: - -The surface of Mars has no oceans or mountains. The reddish areas, -which form the larger part of the surface, are deserts. The blue-green -streaks are ribbons of vegetation along each side of artificially -constructed waterways, which are of immense length and cross and -recross each other until they somewhat resemble a network of lines -over the desert surface of the planet, and are used for irrigating -this arid region. The points where the canals converge and form the -large dark spots are oases made by the water carried by the canals. -The water is supplied by the melting of the caps of snow at the poles -during the Martian summer, the expanding of the lines of vegetation -seeming to occur at periods corresponding to the time required for the -water of the melting snow to reach the oases. The presence of this vast -system of artificial waterways covering a large part of the surface of -Mars makes it seem probable that “Mars is inhabited by beings of some -sort or other,” that these beings are not men such as we know anything -about, but that “there may be a local intelligence equal to or superior -to ours.” - -These conclusions concerning what is seen on Mars are not held by -any one to be completely proved, but are thought by their author -to follow reasonably from the phenomena as observed. By persons -of a different temperament they are regarded as too complete an -explanation, particularly as the data upon which they are founded are -not undisputed. Some of the best astronomers have not been able even -to see the multitude of fine lines, much less to give any explanation -of them. Others do not regard it as certain that they are so geometric -in their outlines as to suggest anything more than cracks or clefts -in the surface of Mars, such as might be made by nature, and consider -that, instead of indicating life, human or other, they may be the marks -of age, such as similar lines or cracks which have been observed on -Mercury seem to be. - -Also, it is not at all certain that there is sufficient water vapor in -the slight atmosphere of Mars to furnish the snow necessary for this -great irrigating system, nor the heat to melt it at the proper season. -The natural temperature of Mars would be, as we have seen, very low, -and unless it is modified in some way not yet indicated everything -points to a frigidity too intense to permit the continuance of life and -growth of any sort known to us. - -These things must all be reckoned with before anything certain can be -known of the surface of Mars. The difficulty of pronouncing upon the -minute details is impressively indicated by Professor Moulton, who says -that, even under the finest conditions and with the best telescopes, it -is like viewing “a perfectly accurate relief map of the whole United -States made on such a scale that it would be only three inches in -diameter and held at a distance of three feet from the eye.” Under such -a near limit of vision, we can well see that differences of opinion -might arise. - -The mere fact that some astronomers have not seen the lines on Mars -does not mean that they deny their existence. Some eyes have greater -defining power than others, as well as some telescopes, as every one -knows. But while all the lines and patches of color that are claimed -to have been seen on Mars doubtless have been seen by some persons, -yet it is not necessary to accept the interpretation of them given -by lively-minded observers when it is not convincing. There may be -vegetation on Mars, and even intelligent beings. We do not know; and -thus far there is not much to support, even by inference, the view -that there are. If we want the truth, we are brought no nearer to it -by giving full credence to a speculative theory simply because it is -interesting and pleasant; and thus far all theories advanced as to the -nature of the surface markings on Mars are speculations, though there -is no doubt that the marks are there. It is pleasing, however, to -contemplate the idea of there being on Mars, or on any other planet, an -active intelligence of any sort resembling what we have here on earth, -and it is not strange that such a wide-spread popular interest should -attach to Mars, in view of what has been suggested by the markings on -its surface. - - -THE SATELLITES OF MARS - -Mars has a little family of two moons. Tiny little bodies they are, the -smallest in the solar family except, perhaps, an occasional asteroid. -Neither one of them is more than ten miles in diameter, and the two -together are smaller than any other known satellite. They can only be -seen when Mars is in opposition, and then only with a fairly large -telescope. They were discovered in 1877, and named Phobos and Deimos, -the names of the two attendants of the god of war. Phobos is the -brighter and the nearer to the planet. It is less than four thousand -miles from the surface of Mars; and on account of its being so near -and the shape of Mars being a spheroid, like that of the earth, the -little satellite can never be seen from Mars beyond sixty-nine degrees -of latitude on each side of the equator. Within these limits it shows -great activity. It makes a complete circuit around Mars in seven and -a half hours; and this swift revolution, combined with the motion of -Mars on its axis, makes Phobos seem to rise in the west and set in the -east, pass over the heavens in less than twelve hours, and go through -all its phases, from “new” to “full,” one and a half times every night. -Its light is rather insignificant, being about sixty times less than we -receive from our satellite; but, on the whole, it must be a rather gay -and pleasant little moon. - -Deimos is not any larger than Phobos, and not as bright; but it is -slightly less difficult for us to see, because it is between two and -three times farther away from Mars than Phobos is, and thus not so -much lost in the light of the planet. It circles around Mars in a -little more than thirty hours, and this, being only six hours more -than Mars consumes in turning around on its axis, results in requiring -more than two days for the satellite to pass from rising to setting. -Between rising and setting it goes through its phases four times. It -can be seen from all parts of Mars, but gives very little light to the -planet--more than a thousand times less than our moon gives us. - -The symbol of Mars is ♂, a conventionalized figure representing a -shield and a spear--implements of war appropriate for the use of the -deity especially connected with warfare. - - - - -XIII - -JUPITER - - -One never feels so impressed with the power of the sun as when one -contemplates it in relation to Jupiter. Great Jupiter, he may well be -called, nearly five hundred million miles out in space, almost a sun -himself, the center of a system containing bodies larger than the sun’s -nearest planet, Mercury; and yet just Jupiter, one of the planets, -held firmly in leash like the others by the sun’s overwhelming force -of gravity, forever compelled to revolve about that parent body with -the rest of its offspring, to stay at home within the bounds of the -sun’s domain, to keep within certain limits in his own orbit, forced to -hasten on when he comes nearest the power that controls him, and unable -to keep up the same rate of speed when he is farther away. One may well -wonder at the immensity beyond comprehension of the stars, among which -our sun is but a very small one, when one considers how even this small -one can thus swing huge Jupiter about. For Jupiter is, after the sun -itself, the mammoth member of our system. In volume he is larger than -all the other planets put together, and in mass he is more than double -as large as the combined mass of all the others. He is about equal to -the sun in density, and about one-fourth as dense as the earth. - -There is less difference in size between Jupiter and the sun than -there is between Jupiter and the earth. His diameter is eleven times -greater than that of the earth. The sun’s diameter is only ten times -greater than Jupiter’s. His surface is one hundred and sixteen times -that of the earth; the sun’s own surface is only a hundred times larger -than his. Jupiter weighs more than three hundred times as much as the -earth; the sun weighs only six times more than Jupiter. At the equator -his diameter is about ninety thousand miles; but, as the planet is -much flattened at the poles, the diameter from pole to pole is only a -little more than eighty-four thousand miles. This flattening is due -to the very rapid spinning of the planet on its axis, a motion that -will always cause a plastic body to bulge at the equator, and thus -flatten at the poles. - -[Illustration: JUPITER, THE MAMMOTH MEMBER OF THE SOLAR FAMILY--LARGER -THAN ALL THE OTHER PLANETS PUT TOGETHER - -This photograph shows the flattening at the poles and also the belts -encircling the planet. It was photographed at the Yerkes Observatory.] - -The force of gravity on Jupiter is about two and one-half times greater -than on the earth. A fairy-like figure weighing here only a hundred -pounds would be held to the surface of Jupiter with a force equal to -two hundred and sixty pounds. This tremendous power makes Jupiter the -greatest disturbing body among all the planets. He gives Saturn a -mighty pull when the two planets come near each other; he draws some of -the little asteroids five or six degrees out of their course when it -carries them into the field of his influence; and there are as many as -thirty comets that have become permanent members of the solar system, -because through his great power of attraction he has made them captive. - -Jupiter is so much farther from the sun than we are that his orbit is -about five times larger than that of the earth. In consequence also of -his greater distance from the sun, he moves much more slowly than the -earth. His average velocity is about eight miles a second. It requires -more than four thousand days, or nearly twelve of our years, for him -to make one revolution around the sun, and he thus consumes more -than ten thousand of his own days. He travels through about one sign -of the zodiac each year, and is thus not very difficult to keep trace -of, since the signs and the constellations of the zodiac so nearly -coincide. His synodic period, or the period from one opposition to -another, is a fraction less than three hundred and ninety-nine days, -or about one year and a little more than a month. His daily motion in -the skies is almost too small for us to detect it without observation -for more than a day. It is in one day about equal to one-sixth of the -apparent diameter of the moon; but in a month he has moved a distance -about half as great as that between the two pointers in the Big Dipper, -as can be easily seen by comparison with the stars near him. - - -JUPITER’S PLACE IN THE SKY - -Jupiter is now (1912) in the constellation Scorpio, and he will be -in this region, and thus a summer star, for several years to come. -In 1913 he will be in opposition early in July, and will then be in -Sagittarius, not far from the little “milk dipper,” and will be a -gloriously beautiful object during all the summer. He will be in -opposition about August 10, 1914, in Capricornus, and will again be -the most brilliant object in the summer sky. In 1915 he will be in -opposition a little after the middle of September, and will then be -situated on or near the eastern edge of Aquarius, where he will be a -very distinguished star during all the charming evenings of late summer -and the autumn. He always seems particularly splendid when in this -season of the year he reaches opposition. The insistent brilliancy of -his disc brings him then into view before the sun is fairly down; and -he hangs, placid and alone, in the southeastern sky during the autumn -twilight, and later in the evening shows to advantage his dominating -beauty, with Antares on the west of him and Fomalhaut below him, no -less charming in their own way, but far less brilliant than this -splendid planet. - -In 1916, when opposition will occur not far from Hallowe’en, Jupiter -will be about on the eastern border of the constellation Pisces, and, -rising then just as the sun sets, will enliven the evening view for the -rest of that year. He will appear at his very best at this time, for -he will be at about his nearest to the sun; and all that this situation -can do for him in the way of enhancing his brilliancy may then be seen. - -In 1917 he will be in opposition to the sun about the first of -December, in Taurus; and for the next few years he will be a winter -star, moving majestically along his path in the zodiac, never more -than one and a half degrees from the ecliptic, and passing in turn the -Pleiades, Aldebaran, Castor and Pollux, and the little Bee-hive in -Cancer. There will be no opposition in 1918; but one will occur early -in January, 1919, when Jupiter is in the eastern half of Gemini; and -toward the middle of February, 1920, another will take place, when the -planet is in Cancer, with Castor and Pollux, the sparkling twin stars -in Gemini, to the west of him. - -During part of 1920 and all of the next three years Jupiter will be -journeying across Leo, Virgo, Libra, and Scorpio. He will be opposite -the sun in 1921, a little after the middle of March; in 1922, in the -latter half of April; and in 1923, toward the very last of May. He will -pass near Regulus, the sparkling star in the handle of the Sickle, in -the summer of 1920; near Spica in 1921; and he will not be far from -Antares in 1923. - -In 1924 Jupiter’s cycle of twelve years will be completed, and he will -be in opposition again early in July, and situated near the western -edge of Sagittarius, not far from where he was in 1912. - -These cycles do not repeat themselves exactly; but the planet lacks -only four days of having been in opposition eleven times during twelve -of our years, so that it is not difficult to keep track of him through -a long series of years. For exact dates, such as one needs in a very -close study of the planet, an almanac must be consulted; but this is -not necessary for mere recognition, which is all that is needed to -enjoy the acquaintance of great Jupiter. - -Every year Jupiter is an evening star for more than six months. For -two months before opposition he rises somewhat after sundown; at -opposition he appears exactly at the setting of the sun; and thereafter -he is found in the evening sky, appearing farther toward the west each -evening, until, when nearing conjunction, he is lost to our view for a -time. He is a morning star for an equal length of time, and for about -three months can be seen between midnight and six in the morning; but -much of the rest of the time he is obscured by the daylight. - -Jupiter retrogrades in his motion for about two months before and after -each opposition; but, since he changes his place to the extent of only -two and a half degrees a month, the whole apparently backward movement -amounts only to ten degrees a year. Still, it is very interesting to -watch him swing back and forth over this ten degrees before he starts -out on each yearly journey. - - -DISTANCE, LIGHT, AND HEAT - -Jupiter is nearly five times farther from the sun than we are. His -mean distance from that orb is four hundred and eighty-three millions -of miles. His orbit is not so eccentric as that of Mercury or of Mars, -but the eccentricity is sufficient to make his distance vary by as much -as forty-two millions of miles. His distance is five hundred and four -millions of miles when he is farthest from the sun, and four hundred -and sixty-two millions when he is nearest to it. On account of his -orbit being outside of ours, we are at times nearer to him and at -others farther from him than the sun ever is. At his best situation -when in opposition, we are three hundred and sixty-nine million miles -from him. This is more than ten times farther than we are from Mars -at that planet’s most favorable oppositions, and yet Jupiter is much -brighter at such times than Mars ever appears to be. At the times of -conjunction he is five hundred and ninety-six millions of miles from -us, but is still always brighter than a first-magnitude star like -Capella or Vega. - -Although the distance of Jupiter from us varies thus two hundred and -twenty-seven million miles, there is never in him the marked difference -in brilliancy that we see in Mars. He is at all times so far away that -the variation in distance does not count for as much, though we can see -the effect of it plainly enough, even with the naked eye. Light, with -all its marvelous speed, consumes more than fifty-three minutes in its -journey from Jupiter to the earth when we are most widely separated -from him. When we are nearest to him light comes to us from the planet -in twenty minutes less time. At his average distance from the sun it -requires about forty-three minutes for light to pass from the sun to -Jupiter. - -Notwithstanding the sun’s great power over Jupiter in shaping his -course, it does not give him much in return for his subserviency. So -far as light and brilliancy are concerned, it is to Jupiter a very -small sun indeed. To an observer on Jupiter the sun would not appear -to be more than one-fifth as large as it seems to us. The light it -furnishes to Jupiter is twenty-five times less than we receive; and if -the planet depended entirely upon the sun for heat, his temperature -would be more than two hundred degrees below zero, Fahrenheit. There -is every reason to believe that the little heat the sun gives to -this mighty planet does not count for much one way or the other at -the planet’s present stage of development. Jupiter does not need the -nourishing that the smaller terrestrial planets must have, or die. He -is probably almost a sun himself. We are not at all certain that the -planet is even so far cooled as to have a solid surface. If it has, -there is reason to think that the surface is at least red hot, and -gives to the planet a temperature higher than anything we have any -comprehension of. Jupiter’s atmosphere, too, is extremely thick and -dense, so that the planet is probably so protected that it gets very -little heat from the sun and loses very little of its own. - -It is certain, however, that this great planet is not so much of a sun -as to shine by its own light. The light we receive, though it is very -brilliant, is reflected sunlight. This is shown by the fact that the -planet does not furnish light for its own satellites. When they pass -into its shadow the sunlight is shut off from them; and if they receive -any light from Jupiter, it is too dusky to be perceptible to us. That -the planet may have a red glow, though, is also suggested by the action -of the satellites. When they pass between us and Jupiter they sometimes -cast less of a shadow on his surface than would be expected, thus -indicating that the surface is not altogether dark, though it may only -dully glow rather than shine. - - -DAY AND NIGHT, SEASONS, AND ATMOSPHERE - -Jupiter accomplishes one rotation in a little less than ten hours; but, -curiously enough, all parts of the planet do not rotate in the same -length of time. A day at the equator is nine hours and fifty minutes in -length. In some of the higher latitudes it is nine hours and fifty-five -minutes, and this notwithstanding the equator is so much larger in -circumference than any other part and any one point on it has farther -to go in a revolution. As many as eight different rates of rotation -have been observed; and even in the same zones some parts seem to lag -behind others, taking a little more time to complete the rotation than -other parts surrounding them. This is another indication that Jupiter -is not a solid body. The surface features are none of them permanent, -though some of them remain practically the same for years. It is -through this occasional stability of them that it has been possible to -mark the planet’s time of rotation. - -In the matter of seasons Jupiter has very little variety. The axis of -the planet is inclined but little more than three degrees to its orbit, -so that whatever amount of heat the sun’s radiance affords must be -very nearly uniform during the entire Jovian year. Its distance, too, -is at all times so great that there would be no appreciable change in -temperature between its perihelion and aphelion positions. - -There is every indication that Jupiter has an extraordinarily dense -and deep atmosphere. It has been sometimes estimated to be as much -as a thousand miles in depth, and the spectroscope shows it to be -heavily laden with vapor. But beyond these very general facts not much -is definitely known about it. It is certain, though, that it is very -different from our atmosphere. The spectroscope shows in it elements, -or compounds of elements, which are not familiar to us. The enormous -gravitative power of Jupiter would enable him to hold gases rarer than -the earth, or the smaller planets like the earth, ever acquired. A -molecule of gas would have to move more rapidly than thirty-seven miles -a second to escape from Jupiter. The earth, as we have seen, cannot -hold any gases moving faster than seven miles a second. So there are -many gases which may forever remain in Jupiter’s atmosphere and yet -have never had a place in ours. - - -SURFACE FEATURES - -Seen through a telescope, Jupiter shows the loveliest variety of -colors, with the reddish ones always most conspicuous. The slightly -pink-tinted steady light that we get from the planet with the naked eye -in no way suggests the turbulent, flame-like aspect that a telescopic -view opens to us. The telescope also reveals very clearly that -flattening at the poles which has already been spoken of. - -With so dense an atmosphere as Jupiter most likely has, it is sometimes -doubtful whether his surface can be seen by us at all. But it is -certain that we see something apparently much more dense and stable -than an atmosphere is supposed to be; and hence it is thought that, in -spite of its thickness, the atmosphere may be only partially opaque, -and that it may be in some places even more or less transparent. - -It does not seem probable that the markings on Jupiter are wholly -atmospheric. Some of them indicate that the substance we see has -considerably more consistency than a mere gas. The whole surface of -the planet is covered with belts and spots of various colors and -varying shapes. The belted appearance is particularly marked. It has -been noticed for more than two hundred years, and can be seen with a -comparatively small telescope. Sometimes as many as twenty or thirty -belts have been seen at one time. All of them are parallel with the -equator. - -Two broad red belts on each side of the equator, called the tropical -belts, are very distinct, and sometimes retain the same shape and -color for months at a time, though sometimes they change rapidly in -both color and outline. Between them is the equatorial belt, which -is also a semi-permanent feature, remaining often for a considerable -period unchanged. These belts, and the spots that sometimes appear on -and near them, have been closely watched, because about the equator, -and especially just south of it, is the region of greatest activity on -Jupiter’s surface. - -One feature that more nearly suggests solidity and permanency than -anything else on Jupiter is the famous great red spot which lies in the -southern hemisphere just below the southern tropical belt. It appeared -about thirty-five years ago, in July, 1878, as a pale pink spot, grew -brighter for two or three years, and then faded, until, at the end of -two or three more years, it was almost invisible. In another year it -came again, and increased in brightness for five or six years. Then it -grew a little fainter, and has since remained a rather faint red spot, -but plainly visible. - -In shape the great red spot is an immense oval as much as thirty -thousand miles from east to west and seven thousand miles from north -to south, which gives it a surface four or five times as large as the -land area on the entire earth, and larger even than the whole surface -of the earth including the oceans. Although retaining its own shape, -it seems to drift about among its surroundings, showing that it is not -attached to any solid surface; and yet it has a suggestion of solidity -in itself, which was shown when it and another smaller spot were seen -to be drifting toward each other, and then finally to meet. Instead of -colliding or going over or under, they calmly drifted to one side and -went around each other. - -Appearances such as this have suggested the idea that the great spot -might be a continent in process of formation. Such an idea is at best a -speculation; but it would be interesting if it should prove that we are -witnessing on Jupiter the process through which our own earth must at -one time have passed when its crust began to solidify in patches, as -one of the steps in the long period of evolution which has prepared it -for our uses. It is not at all certain that Jupiter will ever be just -like the earth. The differences between its atmosphere and ours may -have some influence in its development that we have little knowledge -of at present, and there are some other fundamental differences -between the two planets which may in some way effect a difference in -development. But in a general way we know that the planet will in time -become more condensed than it now is and will finally solidify. Whether -the processes will be carried on in just the same way in which they -have been here on the earth is not so certain. - - -JUPITER’S SYSTEM OF SATELLITES - -Jupiter is the center of a superb system of satellites, eight in -number. Four of them were first seen in 1610, and have the honor to -be the first heavenly bodies discovered by means of the telescope. -The fifth one was not discovered until 1892. The sixth was first seen -in 1904, and the seventh in 1905. After three years an eighth was -discovered (in 1908). - -When the first four satellites were discovered they were named -respectively, in the order of their distances from Jupiter, Io, Europa, -Ganymede, and Callisto. Ganymede is not only the largest of the four, -but is also the largest satellite in the solar system. It is larger -than Mercury, and not much smaller than Mars. Callisto is next to -Ganymede in size, and is about the size of Mercury. Io is about the -size of our moon, and Europa is not much smaller. Under very favorable -conditions Ganymede and Callisto can be seen by the naked eye; but -a good many persons think they see the moons of Jupiter when they -see only some small stars in that region. They are invisible to most -people, but probably could be seen oftener if it were not for the -glaring light of the planet, which more or less obscures anything so -near it. - -After the discovery of Jupiter’s fifth satellite, astronomers seem -to have become possessed with that dull spirit of orderliness such -as is sometimes exhibited by city councils in substituting numbers -for historic and beautiful names in designating streets. No more of -Jupiter’s satellites were given names such as might be appropriate for -members of this Jovian family; but all were given numbers--the first -four in order of their distance from Jupiter, the others in order of -their discovery. Io, Europa, Ganymede, and Callisto are now designated, -respectively, I, II, III, and IV, while V, VI, VII, and VIII have never -had any designation other than these numbers. - -The fifth satellite, discovered in 1892, is the nearest to Jupiter, -and the smallest of all his satellites. Its diameter is probably not -more than one hundred and twenty miles, but its exact size can be -estimated only by the amount of light it reflects. It is too small to -show a measurable disc, and cannot even be seen when it makes a transit -across the planet. It would seem then a mere speck, if we could see -it at all. It makes one revolution about Jupiter in less than twelve -hours (eleven hours and fifty-seven minutes), and is only a little more -than twenty-two thousand miles from the surface of the planet at the -equator. It appears to us as a star of about the thirteenth magnitude, -and cannot be seen except with a large telescope. Owing to the great -curvature of the planet, and to the satellite’s being so near him, it -cannot be seen from the surface of Jupiter beyond sixty-five degrees -of latitude. It moves faster than any other satellite in the solar -system, going at the rate of sixteen and a half miles a second. It -does not make a revolution in as short a time as Phobos, the little -satellite of Mars, does, but it has a much longer distance to travel -and goes at a faster rate. The fact that Jupiter rotates in ten hours -and the satellite makes a revolution around him in twelve hours results -in the satellite’s taking five of Jupiter’s days to cross from the -eastern horizon to the western. It would go through all its phases four -times during that period if it were not that, being so near the planet, -his huge form cuts off the sunlight from the little satellite for -nearly one-fifth of the time, and it is never seen “full.” - -This satellite is very difficult for us to see on account of its -diminutive size and its nearness to the shining disc of Jupiter; yet -it was discovered by means of the telescope, and not by photography, -as so many small bodies are discovered nowadays, and by a man who thus -far has not been able to see the fine line markings on Mars, which some -other astronomers think they can see--a fact that is very interesting -as showing the difference between observers even of great keenness of -vision. From this satellite Jupiter would seem an enormous body, nearly -eighty-five times larger than our sun appears to us, and, no doubt, a -splendid object. But the little satellite pays rather dearly for the -view by suffering numerous and long-continued eclipses. - -The sixth and seventh satellites are also very minute bodies, measuring -probably less than one hundred miles in diameter. They circle about -Jupiter at a distance nearly thirty times more remote than our moon -is from us. They are about seven million miles from the planet, and -probably not more than barely visible from it. It takes them two -hundred and sixty-five days to make one revolution, which is more -than five hundred times as long as the period of Jupiter’s nearest -satellite. These two satellites are so nearly of one size and revolve -so nearly in the same time and at the same distance from Jupiter that -they are thought to have had a common origin. Just what their relation -is has not yet been determined. - -The eighth satellite, discovered in January, 1908, is certainly -no larger, and is perhaps still more tiny, than the sixth and the -seventh, though it is a little brighter than either one of them. It is -about three times farther away from Jupiter than the seventh satellite, -and with eyes such as ours would not be visible from Jupiter. It -shows to us as about a seventeenth-magnitude star, which is almost -at the limit of our vision with even the largest telescope. It seems -to revolve about Jupiter in a direction exactly opposite to that of -the other satellites--a retrograde motion that appears in the solar -system in only two or three other cases and has not yet been entirely -accounted for. - -Jupiter’s satellites have played an important part in astronomical -discoveries and investigations. It was through observation of their -transits that it was discovered that light occupied time in passing -through space. When Jupiter was near us in his orbit, the eclipses -occurred too soon for their calculated time; when he was farther -away, they occurred too late. It was found that these irregularities -were due to the fact that light is not transmitted through space -instantaneously, and further investigation showed that it travels at -the rate of 186,400 miles a second. The eclipses of Jupiter’s moons are -carefully computed and recorded in the _Nautical Almanac_, and it is -through observations of them that chronometers are corrected at sea. - -Ganymede and Callisto have been found to keep always the same face -toward the planet, as our moon keeps always the same face toward us; -and it is thought that all of Jupiter’s satellites probably do this. - -The symbol of Jupiter is ♃, a hieroglyph for the eagle, which -was the bird of Jove. - - - - -XIV - -SATURN - - -Among the four planets that we commonly see, the easiest, perhaps, -to keep track of is Saturn. Its peculiar aspect is very distinctly -marked. It appears as a large, pale, yellow star shining with a soft, -misty light that sometimes barely escapes being dull. It is always as -bright as a first-magnitude star, but not always as bright as Sirius, -and never as brilliant as Mars, Jupiter, or Venus when they are at -their brightest. The general effect of it is as a large rather than a -brilliant star. - -The only time it loses these very marked characteristics is when it -is drawing in toward the sun, and thus nearing conjunction. At such -times we see it each evening lower in the rosy glow of the setting sun, -and more and more obscured and changed in color by the surrounding -atmosphere. Then it sometimes seems as red as Mercury, and sometimes -even twinkles a little in a sort of farewell gaiety as it backs away -from us into the rays of the dazzling sun and finally disappears for -a time from the evening sky. Proximity to the sun and entanglement in -the atmosphere of the horizon has this effect more or less on all the -planets, as we know, but it always seems unexpected in Saturn, because -it is so out of keeping with his ordinarily large, pale, placid face, -which suggests softness and gentleness rather than vivacity. - -But there is no mistaking the planet even under this aspect if we but -stop to think where he is. And it is through knowing where he is that -it is so easy to keep track of Saturn. For nearly two years and a half, -on an average, he remains in the same constellation, passing slowly -over about one degree a month, or a little more than twelve degrees in -a year, occupying almost thirty years in making one circuit through the -constellations of the zodiac. One has, therefore, ample time to get -well acquainted with him before he has wandered far from the position -in which one first found him. - -For nearly six months each year Saturn shines as an evening star, and, -returning each year as he does with such slight changes of position, -he comes to have something of the stability of a fixed star. Having -seen him one year, we can count on his returning the next only about -thirteen days behind time, and but little farther from his original -position than twice the distance between the pointers in the Big Dipper. - -The one degree a month which he travels along the ecliptic is toward -the east, except for a little more than two months before opposition, -and the same length of time afterward, when he has the slight apparent -retrograde motion due to our overtaking and passing him, which has -been explained. With Saturn this motion is so slight--only four -degrees--that it does not put him much out of position, and it is, in -fact, not much noticed except by close observers. He has all the time -been going steadily on toward the east (for the retrograde motion is -only an apparent motion), and the annual change of twelve degrees in -position is always in this direction. - -My first acquaintance with Saturn was when he was traveling through -Pisces and Aries, where there are no first-magnitude stars to mark -the path of the wandering bodies in the heavens. It was then that I -was most impressed with the fixity and reliability of his return. -Every autumn then for five years we watched Antares passing toward -the west, followed by the little “milk dipper” in Sagittarius; and -then Fomalhaut, crossing the sky in the same direction, though below -the constellations of the zodiac; and then turned our eyes toward -the east, knowing that the next bright body to come peeping over the -tops of the trees would be Saturn. And when the first frosts began to -strip the leaves from the trees we found the compensation that nature -always gives when she destroys one beauty: we could see earlier in the -evening, through the bare branches, that lovely yellowish disc, with -its suggestion of aloofness and grandeur that is peculiar to it. For -the face of Saturn, while never what we would call cold, has little -in it of that bright, warm, friendly aspect which is at times so -characteristic of Venus, Mars, and Jupiter. - - -AROUND ONE CIRCUIT OF THE SKIES WITH SATURN - -Saturn is now (the autumn of 1912) in the first part of his path -through Taurus, and he will be in that constellation during all of 1913 -and the greater part of 1914. - -From 1912 to 1920 he will be a beautiful object in the winter sky, -threading his way slowly through that splendid galaxy of stars that -blazes across the glittering sky peculiar to the cold winter nights. He -will pass between the Pleiades and Aldebaran, and will be in opposition -in that region on November 23, 1912. Farther east in the constellation -he will be in opposition in the first week of December, 1913. Almost -on the border line between Taurus and Gemini he will be in opposition -during the third week in December, 1914; and, as this is very near the -perihelion point in Saturn’s orbit, the planet will then be at his -brightest. - -In 1915 he will not be in opposition at all; but sometime within the -first two or three days of 1916 he will reach that position, and -will then be well on in his journey across Gemini. For these four -years--from 1912 to 1916--he will be visible during the entire night, -at the times of his opposition, and in his best condition. The rings -that surround him will then be placed so that we will get a broad -expanse of light from them, as well as from the planet itself, which -greatly increases its brightness. - -Saturn will then continue to move across Gemini, passing in the early -part of 1917 under Castor and Pollux, and very near to Neptune--a -meeting which, unfortunately, cannot be seen with the naked eye. During -this year (1917) he will begin his journey through the smallest of -all the constellations of the zodiac, Cancer, passing near the lovely -cluster of stars we call the Bee-hive, and will reach Leo early in -1919, where he will remain until about the end of 1921. While in this -region he will be visible during the winter and all of the spring and -the early summer. All three of these constellations--Gemini, Cancer, -and Leo--while seen in the winter, are particularly lovely in the -spring. Gemini, in the beautiful evenings of May, hangs with its two -splendid stars in the northwest above the setting sun; and with the -soft face of Saturn near them, these stars will be more than ever -charming in the two seasons that the planet remains with them. - -In 1917 Saturn will be in opposition in the region of Gemini, about -the middle of January. In 1918 opposition will occur about the last of -January, and Saturn will then be in Cancer. The next year he will be -in opposition sometime during the second week in February, and will -then be situated between the Bee-hive, in Cancer, and the brilliant -first-magnitude star Regulus, in Leo. The next two oppositions will -be in Leo, about thirteen days later each year. Saturn will then pass -during the first half of 1922 into Virgo, which is the largest of all -the constellations, and he will remain there until three oppositions -have taken place, about thirteen days later each year. - -About a year after passing Spica, the white, sparkling, first-magnitude -star in Virgo, Saturn will enter Libra, crossing that constellation -near the lower part of the square in it. From there he will go through -Scorpio and Sagittarius, passing above Antares and the “milk dipper,” -and in about 1932 will have reached that comparatively starless region -which includes a part of Sagittarius and all of Capricornus, Aquarius, -Pisces, and Aries. For the next nine and a half years he will give -distinction to this part of the heavens, and thus complete his circuit -of twenty-nine and a half years, and, with never resting, never -changing movement, will start on a new round, with a new generation of -eyes following his fair face along the great circle of the ecliptic. - -Saturn is brightest when he is in Taurus, not far from Gemini, as he -will be in 1914, and again when he is in Scorpio, as he will be between -fourteen and fifteen years later. The recurring times at which we can -get an evening view of him at his greatest brightness thus alternate -between midwinter and midsummer. He is least bright when he is in -the last half of Leo and when he is in that part of Aquarius above -Fomalhaut. Between these positions he gradually waxes and wanes in -brightness, changes that are largely due to the position of his rings. - - -DISTANCE AND SIZE - -Saturn is almost twice as far from the sun as Jupiter, and between nine -and ten times farther than we are. His mean distance from the sun is -eight hundred and eighty-seven million miles; but his distance varies -nearly one hundred million miles between perihelion and aphelion. His -orbit is only a trifle more eccentric than that of Jupiter, but the -variation in miles is so much greater because the orbit is so much -larger. - -His average distance from the earth at opposition is seven hundred and -ninety-four million miles, but at the most favorable opposition it may -be fifty million miles nearer than that. At conjunction his average -distance is nine hundred and eighty million miles; but his greatest -possible distance at such times may be as much as one billion miles. -When he is in this situation it takes light a little more than an hour -and a half to pass from him to us. At his nearest we receive light from -him in about an hour and six minutes. At his average distance from the -sun, light requires about an hour and twenty minutes to go from one to -the other. - -While Saturn is next to Jupiter in size among the planets, he is not -as large as Jupiter by two-thirds, but his mass is almost three times -greater than that of all the other planets put together except Jupiter. -It is ninety-five times greater than that of the earth. In diameter -Saturn is 72,772 miles; but it is more flattened at the poles than any -other planet, and in consequence there is a difference of nearly seven -thousand miles between its polar and its equatorial diameters. - -The density of Saturn is less than that of any other planet, and it is -ten times less than that of the earth. No other planet is less dense -than water; but Saturn would float in water, and is not more dense -than cork. On account of its mass its gravity is greater than that -of the earth by about one-tenth. This is not enough to make a very -interesting difference in the weight of objects on Saturn and on the -earth. The average person weighing one hundred and fifty pounds here -would weigh only one hundred and sixty-five pounds on Saturn. The -numerous penny-in-the-slot weighing-machines vary almost that much. -Saturn has eighty-three times more surface than the earth, and more -than seven hundred and fifty times the earth’s volume. - - -SURFACE ASPECTS AND CONSTITUTION - -It is not at all certain that Saturn, more than Jupiter, has any solid -surface. Indeed, it is almost certain that it has not. It is surrounded -by an atmosphere of great density, and we do not at any time see the -surface of the planet. It is believed probable that it is at least -largely in a liquid state, if not to a great extent even gaseous. - -The planet is certainly not in any way dependent on the sun for the -extraordinary heat that everything indicates it to have, and its -surface is brighter than it is believed it could be if shining only -by the reflected light of the sun. This does not mean that Saturn is -self-luminous; but it is nearly certain that it is extremely hot and -glowing, and its brightness may be in part due to its own internal -fires and the extremely luminous and dense atmosphere that surrounds -it. It receives one hundred times less heat and light from the sun -than we do. If it depended entirely upon the sun for its heat, -the temperature would be nearly three hundred degrees below zero, -Fahrenheit. It is probably not only very hot itself, but its heavy -atmospheric envelope perhaps allows comparatively little heat to escape. - -Its surface is belted and spotted somewhat after the manner of -Jupiter’s, but, being so much farther from us than Jupiter, it does not -disclose its surface features with the same distinctness. Apparently -it is much less turbulent than Jupiter; but even this we are not quite -certain of, and it may seem more placid because we do not so well see -its agitations. - -Like all the outer planets, it differs in its constitution from the -earth and the other inner planets. Its atmosphere contains compounds -with which we are not familiar, and the body of the planet itself is -rarer and lighter, and less condensed, and in a much earlier stage of -evolution than the earth and the small planets so comparatively near us. - - -DAY AND NIGHT - -The length of Saturn’s day, or its period of rotation on its axis, is -about ten hours and a quarter. Like Jupiter, it has slightly different -rates of rotation in different latitudes, thus showing its lack of -solidity. The rate of rotation has been determined, as in the case of -Jupiter, by observation of the spots on its surface, which, while they -are not exactly permanent, yet remain apparently in the same positions -for months and even years at a time, and are thus sufficiently stable -to measure a rotation of so short a time as ten hours. - -Whirling over at this rate would cause the sun to appear to skim across -the sky very swiftly as viewed from Saturn. In size, it would not seem -more than three times as large as Venus at her brightest seems to us, -and every minute it would cover a distance about equal to the diameter -of the full moon as we see it. In an hour it would seem to move more -than six times as far as the distance between the “pointers.” At the -time of Saturn’s equinox the little five-hour day, followed by the -equally short night, must present a lively aspect with the sun racing -thus swiftly across the sky in daylight and the stars sweeping as -swiftly over at night. If things remain as they now are, it will be a -splendid panorama for the people there when, in the far-distant future, -Saturn may have cooled and solidified sufficiently to maintain life -somewhat as we know it. The earth, though, and Venus and Mars would be -from Saturn only telescopic objects to eyes like ours, and Jupiter no -brighter than he is to us. Thus does our brother Saturn pay the price -of his remoteness from the rest of the solar family. - - -THE RINGS AND MOONS OF SATURN - -But the circling stars and the swift-moving sun are the least part of -the splendid spectacle that might be seen from Saturn. He is surrounded -with no less than ten moons of more or less imposing size, and in -addition has three rings circling around with him, composed of myriads -of small satellites, together forming a band the outer diameter of -which is something more than twenty-one times broader than the diameter -of the earth. These are the famous rings of Saturn, the only objects -of their kind in the solar system, intensely interesting to scientific -observers, wonderful to the curious, and splendidly beautiful to -everybody. It is this profusion of rings and moons that entitles Saturn -to be called, as he often has been, the most spectacular of all the -planets. - -The outer ring is nearly ten thousand miles broad, and is separated -from the next one by a space of about seventeen hundred miles. The -second ring is nearly eighteen thousand miles across. It is very bright -on the outer edge, but gradually grows less so, until, with a not -very perceptible division, it fades into the inner ring, which is but -slightly luminous, and is called the crape ring. This is about nine -thousand miles broad and nearly ten thousand miles from Saturn. This -gradual fading of the rings to a dusky hue toward the center, and then -the blackness of the space between them and the planet, gives them from -certain points of view a nest-like appearance; and my first impression -of Saturn, when I saw him through the telescope, was that he was -nestling in a concave body of light--an appearance that is intensified -by his extreme flatness at the poles. - -Notwithstanding the imposing breadth of these rings, they are less -than a hundred miles in thickness. They are, in fact, nothing more -than an untold number of tiny satellites revolving about Saturn in -the same plane and close enough together to appear, at the distance -they are from us, as if they were one body. Just how close they are -together, and how they appear when near by, we do not yet know. It -was first shown by mechanical laws that they _must_ be composed of -separate bodies; the spectroscope shows that they _are_; and it has -recently been thought that they have even been _seen_ to be so through -a telescope. - -Being all in the same plane, they form a flat, broad, thin ring, so -thin that when the edge of the ring is turned toward us we cannot see -them at all. We never see them at their full breadth. If we did, Saturn -would be much brighter at times than he ever is. The plane in which -they revolve is the plane of Saturn’s equator; and the axis of Saturn, -with the rings, has a tilt of twenty-seven degrees in his orbit. The -result of this is that at the time of Saturn’s equinoxes the edge of -the rings is turned toward us, and they practically disappear. Half-way -between the equinoxes they are open again as far as they ever are to -our view. This is why Saturn alternates in brightness. The times of his -equinoxes occur every fourteen and eight-tenths years, and he is then -alternately in Leo and Aquarius and is least bright. The times at which -the rings are most open occur at intervals of the same length, and he -is then alternately in Scorpio and Taurus and at his brightest. - -[Illustration: SATURN AND ITS RINGS - -Photographed at Mt. Wilson by E. E. Barnard, the six exposures being -made on one plate.] - -It is believed that Saturn’s rings were never a part of the planet, but -are mere particles of cosmic materials which happened to be left over, -and which he has gathered up by his force of gravity and compelled to -revolve about him. - -Saturn, more fortunate than Jupiter, has escaped the unimaginative -naming of his moons by number, though one would think that, having -such a numerous offspring, a shortage in names would be more likely to -occur in his than in any other planet family. They all have names more -or less connected with the great god whose name the planet bears, and -are, in order of their distance from Saturn: Mimas, Enceladus, Tethys, -Dione, Rhea, Titan, Hyperion, Japetus, Phœbe, and Themis. The -largest and brightest of them all is Titan. It is larger than our moon, -which is one of the large moons in the solar system, or than Mercury, -and is not much smaller than Mars. It is more than three-quarters as -large as all the other moons of Saturn put together. Naturally, it -was the first to be discovered, and was under observation as long ago -as 1655. Rhea and Japetus are next in size, and were discovered in -1671–72; Dione and Tethys were both discovered in 1684, and Enceladus -and Mimas in 1789. - -Until 1848 seven moons were all that were known to belong to Saturn. -In that year little Hyperion, whose diameter, it is thought, can -hardly exceed two hundred miles, came into our view. A little more -than fifty years later (in 1898) Phœbe made her bright mark on a -photographic plate at Harvard, and was caught. By tracing her from one -plate to another her orbit was computed, her probable size determined, -and practically all that is known about her was found out before she -was seen, which was not until 1904. She is not much larger than a -good-sized mountain, but is a unique and interesting little satellite -that, far outside of the paths of any of the other moons, circles in an -eccentric orbit around Saturn in an opposite direction from the rest of -the satellites, and thus gives rise to many interesting astronomical -speculations. Themis, also a tiny body, was discovered in the same way -in 1906, and is thought to be the smallest body in the solar system. -Titan is the only one of this group of satellites whose true disc we -can see even with a telescope. Only one other (Rhea) can be seen in -transit across the planet. The others are not much more than bright -points of light, while Phœbe and Themis are almost at the limit of -visibility. - -On account of their great distance from the sun Saturn’s moons are, -of course, not very bright, and all of them put together do not give -one-tenth as much light to Saturn as we receive from our moon. But, -such as they are, they may some day be very useful to Saturn as a -means of illumination. Receiving as he does a hundred times less light -from the sun than we do, he may be some day much in need of the light -reflected from all his rings and moons. - - -SEASONS - -The seasons on Saturn are somewhat like ours in the succession of -spring, summer, autumn, and winter; but the inclination of its axis -to its orbit being twenty-seven degrees instead of twenty-three and a -half, as ours is, each season is much more accentuated than ours. The -sun climbs higher during the northern summer, and sinks correspondingly -lower during the winter. But in length Saturn’s seasons are very -different from ours. Like his year, they are about twenty-nine and -one-half times as long as ours. Each one is more than seven years long. -Even the agreeable seasons might grow monotonous to one in that time; -but to be spinning through the rapidly alternating days and nights of -Saturn during seven long years of winter is a situation that one does -not care to contemplate. It is with world personalities as with human -personalities: however much we may admire their superior grandeur, when -we consider details we would not change places with them. - -The symbol of Saturn is an ancient scythe (♄), which gets its -appropriateness from the fact that the deity of that name was the -special protector of agriculture. - - - - -XV - -URANUS - - -Venus, Mars, Jupiter, and Saturn, brilliant beauties that they are, -have always been distinguished features of the heavenly view. The -records of Mercury do not go back so far as those of these more easily -seen planets, yet there is no reason to think that he has not been -always known, though less widely, perhaps, than the four planets more -frequently in view. To Uranus belongs the distinction of being the -first planet that was _discovered_--a distinction that one cannot help -but feel was too long delayed, for it did not come until 1781. For ages -and ages his lovely pale beams had been shining down upon us from his -little disc, no fainter in brilliancy than many a sixth-magnitude star -(a degree of brightness which we think is within the limit of good -vision, even in these days), and no human being had been conscious that -this bright body was only another member of the solar family, circling -with the rest of us around our parent, the sun, and having nothing in -common with the far-off stars among which we had numbered him. Nineteen -times he had been charted as a fixed star before his identity was -suspected, and after he became known to us as a planet he was, by means -of these charts, traced back for one hundred and thirty years, and much -information was thus gained concerning his orbit and movements. - -Uranus was not, however, discovered through observation of his movement -among the stars. A view of his actual disc was caught by the musician -and astronomer, Herschel, as he gleaned with his telescope in that part -of the sky where the planet lay, one hundred and seventy-one years -after the invention of that aid to vision. It was at first thought that -a comet had been discovered, but later investigation showed a much more -important member of the solar system, and the discovery of a new planet -was announced. - -George III. was then King of England, and the loyal Herschel called -the planet _Georgium Sidus_ in honor of that monarch. Fortunately, -the world-wide interest in this newly discovered body saved it from so -local an appellation, and it finally came to be called after Uranus, -the father of Saturn, a name somewhat more in keeping with its place -among the planets. In England, however, a very commendable loyalty to -Herschel has resulted in the planet’s sometimes being called Herschel, -after its discoverer, and we see this name often in English books on -astronomy, especially the older books; but Uranus is now the generally -accepted name. - -The symbol of the planet as it appears in all almanacs--at least in -all English almanacs--is a capital H with a planet swinging from the -cross-bar in the letter, thus ♅. And to this extent the discovery of -the planet by Herschel is commemorated. In American almanacs the symbol -is contracted into this figure ♅. - -It is a matter for regret that Uranus does not come more easily within -our view; for he is a very beautiful planet, pale green in color, and -unlike any of the others in his aspect. There are, however, very few -persons nowadays who can see him without the aid of at least a small -glass, and to most of us he must ever remain a body with which we can -have no personal acquaintance. None the less he must have an interest -to us such as attaches always to anything so closely related to us, -and sharing with us a common origin and a common destiny. To those who -have unusually keen vision--or a small telescope--there will be much -pleasure in viewing the planet. But even to those who have not these -facilities for seeing, it ought to be interesting to know in what -region of the skies this far-off member of our family dwells, what his -wanderings are, and something of his personality and habits. - -It requires a few days more than eighty-four years for Uranus to make -one revolution around the sun, so that he moves even more slowly than -Saturn from one constellation to another; and if we could only see him -more easily, he would be scarcely more difficult to keep track of than -a fixed star. He remains in each constellation somewhere near seven -years and his change of place in the skies amounts in one year to but -little more than four degrees, which is less than the distance between -the pointers. - -Since Uranus was discovered he has made one circuit of the skies, which -he finished in 1865, and he is now (1912) more than half-way around -on another. His position now is in Capricornus, nearly twenty degrees -east of the “milk dipper” in Sagittarius, and for the next quarter of a -century he can be seen by any who have eyes, or a glass, to accomplish -this during the summer evenings. Each year he will be about seven -degrees farther east. He is, however, still pretty far south of the -equator, and not so easily seen as he will be when he reaches that -part of the ecliptic which runs somewhat higher in the skies. Even an -opera-glass will bring Uranus into the view of many persons. His path -deviates very little from the line of the ecliptic--never quite so much -as half a degree. The knowledge of this makes it less difficult to find -him. - -The synodic period of Uranus is about three hundred and sixty-nine -days, so that an opposition occurs about four or five days later each -year. He was in opposition this year (1912) on July 24th. In 1913 an -opposition will take place on July 29th, and in 1914 on August 2d, and -oppositions will occur about four days later each year thereafter. - -Uranus is twice as far from the sun as Saturn is, and nineteen times -as far as the earth. Its mean distance from the sun is 1,784,732,000 -miles, and at this distance more than two hours and a half would be -required for light to travel from the sun to the planet. Viewed from -the planet, the sun would appear only about two and a half times -larger than Jupiter appears to us, and the earth would be a very small -telescopic body, if, indeed, it would be visible at all. Even at this -great distance from the sun, and with the sun showing so small as it -does, the planet would still have more than a thousand times as much -light as we get from our moon, and so in this respect might be fairly -comfortably provided for even for eyes constructed like those of human -beings. The heat the sun’s radiant energy furnishes to Uranus is, from -our point of view, almost a negligible quantity. If there were no -other source of supply, the normal temperature of the planet would be -more than three hundred degrees below zero, Fahrenheit. There is no -reason to think, however, that this is the temperature that prevails on -Uranus. As far as we can tell, it has a dense and extensive atmosphere, -and probably very considerable internal heat. - -Uranus is smaller than either Jupiter or Saturn; but it is much larger -than Mars, Venus, Mercury and the earth combined. Its diameter is -nearly thirty-three thousand miles. Its volume is sixty-five times -as great as that of the earth; but its mass is only about fourteen -times the mass of the earth, which shows it to be a very much expanded -body. It is slightly more dense than water, but only about two-tenths -as dense as the earth. Its force of gravity is small for so large a -body--only about nine-tenths that of the earth. - -There is every indication that the planet is not a solid body at all, -and that it is, perhaps, largely vapor. We undoubtedly cannot see the -surface of it; but through the telescope it faintly shows the same -belted appearance that we see on Jupiter and on Saturn, though it is -difficult to see the belted region, which is near the equator, because -the axis of the planet is so inclined to its orbit that much of the -time the poles are pointed almost toward us. The spectroscope indicates -something of the same materials in its atmosphere that the other large -and faraway planets have, and there is no reason to doubt that the -planet is in a much earlier stage of development than any of the -terrestrial planets. - -We really know nothing certainly about the rotation of Uranus; but -there seems to be some indication that, like Jupiter and Saturn, it -revolves swiftly--in perhaps ten or twelve hours, and hence has a very -short day and night. The great inclination of its axis must make its -seasons so abnormal, from our point of view, that it is difficult to -understand what they are. Moreover, the planet is, at this stage of its -development, so far from being a habitable body, for beings such as we -know anything about, that the subject of its seasons seems not very -important or interesting. - -It seems but fitting that this vapory, pale green planet should have -satellites with the fairy names of Ariel, Umbriel, Titania, and Oberon. -One can forgive a good many utilitarian feats in nomenclature for -the sake of these charmingly appropriate names for the satellites of -Uranus. Titania and Oberon were discovered in 1787 by Herschel, the -discoverer of the planet. They are not very much farther from Uranus -than our moon is from us, and are easily seen with a telescope. -Titania, the nearer to Uranus and the larger, is probably about one -thousand miles in diameter; and Oberon is not very much smaller. In -1852 Umbriel and Ariel were discovered. They are both smaller and -nearer to Uranus than either of the two first discovered, and are seen -with considerable difficulty, because of their proximity to the larger -and brighter body of the planet. There is not, however, very much -difference between any of the four in real brightness. - - - - -XVI - -NEPTUNE - - -It is rather curious to what extent we have a feeling of kinship with -Neptune, notwithstanding he dwells forever in far-off space where -we cannot expect even to have a glimpse of him without the aid of a -telescope. Uranus, the other very distant planet, is so nearly within -the limit of ordinary vision that we have always a hope that, by some -lucky chance of situation or atmosphere, we may some day be able to see -him face to face, and know for ourselves what manner of planet this is -which, though a member of our own cosmic family, remains always just -beyond easy exchange of glances with us; and so we in a measure keep a -lookout for him that gives us a sense of his reality. - -With Neptune there can be no feeling of this sort to keep us with a -lively interest in him, and yet he is hardly less real to us than -Uranus, and we have a more intimate sense of nearness to him than we -have for any fixed star. Far away as he is, the distance between us is -short compared with the many trillions of miles farther that we must -go to reach the nearest star, and in thinking of him we always have a -sense of this. Then, however aloof he may keep himself from this cozy -little bunch of planets near the sun, of which the earth is one, he -is still of the same parentage with us, and his life history is part -of our family history, so that we can never feel indifferent to what -concerns him. - -Close as Neptune is to us in kinship and distance, as astronomical -distances go, we never knew of his existence until sixty-six years ago. -He is to us almost a recent arrival in the solar domain, but we know -that he has been here as long as we have; and whether he was detached -before we were from the great nebula which gave birth to us all, or at -about the same time, we know that for long ages before there were eyes -on the earth to see him he was, as he still is, circling slowly and -majestically around our common center of control. - -The discovery of Neptune in 1846 created truly a sensation in -astronomical circles. And, unlike most sensational happenings, it -fully justified the extreme interest it aroused. The computation that -led to it was a mathematical triumph, and the final result was a most -splendidly convincing proof of the theory of gravitation. For the place -of this hitherto unknown planet was found by means of computations -based on the fact that at certain times Uranus went a little out of his -way, thus showing some disturbing body outside of his orbit pulling -him slightly from the course he would otherwise take. The deviation -was not much--only about one and three-fourths of a minute, which is -equal to about one-seventeenth of the apparent diameter of the moon, or -one-sixth of the distance between Mizar and Alcor, situated at the bend -of the handle of the Big Dipper, two stars that it is difficult for -some eyes to separate.[7] But this slight irregularity of Uranus was -enough to set at least two able men at work in an effort to locate the -disturbing cause. These two men were Adams, of England, and Leverrier, -of France. - -[7] See, in _The Friendly Stars_, “The Seven Stars of the Dipper.” - -The result of Adams’s work was announced to the Astronomer Royal in -England in the autumn of 1845; but the actual search for the planet -in the place predicted was delayed until the following summer. In the -mean time Leverrier had completed his work and had communicated with -astronomers in Berlin, directing them where to look for the planet. -The facilities for that sort of work were then better in Berlin than -in England; and within half an hour after the search was begun, on the -night of September 23, 1846, the new planet was discovered a little -more than half a degree from the exact position Leverrier had found for -it. It was first recognized as having a sensible disc, and within a day -its motion was apparent. No wonder the astronomical world was thrilled -by this achievement! - -Although the planet was actually discovered by following the directions -of Leverrier, it was found that it might have been seen months before -if the English astronomers had shown more promptness in using the -computations of Adams; and there has always been a disposition among -astronomers, both in France and in England, to give both men credit for -their extraordinary achievement, though, naturally, there is somewhat -more stress laid upon the work of each in his own country. The newly -discovered body was at first named for its discoverer, Leverrier, but a -sense of justice to Adams prevailed to such an extent that in the end -a less commemorative name was chosen, and the planet was called after -Neptune, the son of Saturn and the brother of Jupiter--a name more -fitting, on the whole, for a member of this planet family, whose other -members all bear the names of some of the ancient deities. The trident -(♆), Neptune’s three-pronged spear, is the symbol of the planet. - -The mean distance of Neptune from the sun is more than two and a half -billion miles (2,790,000,000), and his orbit is so nearly circular that -the variation between his perihelion and aphelion distance is only -about fifty million miles. His orbit is, in fact, less eccentric than -that of any other planet except Venus. His immense distance from the -sun, of course, deprives him of any great amount of heat or light from -that source as compared with the other planets. The sun would appear to -an observer on Neptune a little smaller than Venus appears to us. But -so great is the intensity of its radiance that even as so diminutive a -sun as that it would give to Neptune more than six hundred times as -much light as our full moon gives to us. This, however, would be as -much as nine hundred times less light than we get from the sun. Such -light as the planet receives from the sun reaches it after a journey of -a little more than four hours. - -Of the heat the planet has, either inherent or acquired from the sun, -we do not know much. The normal temperature at that distance from the -sun would be more than three hundred and sixty degrees below zero, -Fahrenheit, and there is not much to indicate in what state the planet -is with reference to its own heat. Investigations thus far made do -not show it to be so intensely hot as Jupiter and Saturn undoubtedly -are; but with its heavily vapor-laden atmosphere it could not have the -frigidity normal to a black, unprotected body at its distance from the -sun. - -Neptune is thought to have an immense atmosphere, and, like the -other outer planets, one of a composition not wholly familiar to us. -Consequently we do not know as yet just what this atmosphere does for -the planet. It has a fairly good reflecting power, though the planet, -on the whole, is darker in color than Jupiter or Saturn. Its color -is of that bluish cast which sometimes suggests a leaden appearance. -The color, as well as the fact that Neptune is denser than any of the -other outer planets, indicates that it may be in a more advanced stage -of development than at least Jupiter and Saturn are, and perhaps than -Uranus is. - -That Neptune has made greater progress toward solidity (though it -is still very far from that state) than the other outer planets is -suggested also by its size; for, as we have seen, the smaller planets -develop more rapidly than the larger ones. The diameter of Neptune -is a little less than thirty thousand (29,827) miles. The planet is -somewhat smaller, therefore, than Uranus, and much smaller than Jupiter -or Saturn. But as compared with the earth, the largest of the inner -planets, it is a vastly greater body. Its mass is seventeen times more -than that of the earth; its surface is as much as sixteen times more -extensive than the earth’s; and its volume is more than eighty times -greater than the volume of the earth. - -Of the time of Neptune’s rotation on its axis very little is known. -That little, however, indicates a slower rotation than the other -planets seem to have, and the alternations of day and night on Neptune -are, therefore, probably less swift than on Jupiter and on Saturn. The -planet is too far away for us to see its surface markings with any -distinctness, but there are indirect processes by which we can get -approximate information concerning the facts about rotation. One of -these processes is by observation of the motions of the satellites. Of -these useful bodies Neptune, fortunately, has one--a very excellent -moon about the size of our own. It has some eccentricities, such as -revolving about the planet in the opposite direction from that which -the more conventional satellites follow, and having an orbit a good -deal inclined to the plane of the equator of the parent body. But it is -a very interesting moon to astronomers, and will no doubt in time help -to make clear some things in the history of Neptune which are now not -quite understood. - -Being so far from the sun, Neptune moves, of course, very slowly in -comparison with the nearer planets, though his speed is at the rate of -three and a half miles a second, which, after all, does not denote any -high degree of sluggishness. His change of position in the sky amounts -to a little more than two degrees a year; so that in an ordinary -lifetime he does not make any very great progress along the zodiac. - -When Neptune was discovered he had just left the constellation -Capricornus, and in the sixty-six years that his movements have been -followed he has passed through Aquarius, Pisces, Aries, Taurus, and is -now (1912) in Gemini, very near Castor and Pollux. The time required -for his circuit around the sun is nearly one hundred and sixty-five -(164.6) years, so that he remains for about thirteen years in each -constellation. He will complete one sidereal period, dating from the -time of his discovery, in the year 2011. - -The apparent motion of Neptune is direct a little more than six months -in the year, and retrograde a little more than five months, so that it -seems to present the old mental arithmetic problem of the climber that -fell back so much every time after he had climbed a certain number of -feet. But the falling back in the case of Neptune is an illusion, as we -know. He keeps straight on in his journey, as we may see if we watch -him from year to year, and his change of position is so slight during -any year that the change of direction is hardly noticeable. - -Neptune is as bright as an eighth-magnitude star, and it is possible to -see him with a good field-glass. The difficulty is in distinguishing -him from a star, for his disc does not show except through a telescope. -If one has such a glass, however, it will be worth while to direct it -toward that part of the ecliptic just under Castor and Pollux any time -within the next two or three years, and a sight of this yet strange -brother planet may be the reward. He will be in opposition on January -14, 1913, and thereafter about two days later each year. - - - - -XVII - -THE LITTLE PLANETS, OR THE ASTEROIDS - - -The asteroids, or minor planets, are situated almost wholly in the vast -space between Mars and Jupiter. Their orbits are very irregular, both -as to shape and situation; but, so far as is known, only two of them -pass beyond the orbit of Jupiter, and only one has been discovered -which at any point in its journey around the sun comes nearer than the -orbit of Mars. - -The minor planets are called by astronomers almost indifferently -asteroids or planetoids. “Asteroids” is probably the name by which they -are most popularly known. But because they are in fact simply little -bodies that revolve about the sun as the planets do, “planetoids” seems -to be more truly descriptive of them, and it is the word I have chosen -to use here. - -It was early noted that, except in one instance, the planets seemed -to show in their distance from the sun something like a mathematical -progression. Struck by this appearance, an astronomer named Bode worked -it out into a formula, known ever since as Bode’s law, though the idea -seems to have originated with another astronomer. One almost always -sees it mentioned in any work dealing with this phase of planetary -history, and it is especially interesting because of the part it played -in the discovery of the planetoids. It was as follows: Beginning with -nothing for Mercury, add three for Venus, twice three, or six, for the -earth, twelve for Mars, and continue thus to double the number for each -planet out to and including Saturn. Then to each one of the numbers so -obtained add four, and the numbers resulting will very nearly represent -the relative distances of the planets from the sun. Thus: - - 0 3 6 12 24 48 96 192 384 - 4 4 4 4 4 4 4 4 4 - ------------------------------------------- - 4 7 10 16 28 52 100 196 388 - -The exception was that at the fifth number, 28, there was no planet to -correspond, and Jupiter was nearly twice as far away from Mars as it -should have been to conform to the law, thus leaving room for another -planet to occupy the allotted position and fill out this very beautiful -progression. - -About nine years after this law was set forth Uranus was discovered -circling out in space far beyond Saturn, and was found to conform to -the law in a most satisfactory manner, its distance being approximately -twice that of Saturn. With such close accord between the actual -distances and the prescribed distances of the planets from the sun, and -with the one exception leaving almost exactly the space allotted by -Bode’s law for another planet, astronomers naturally had a very strong -feeling that there must be another planet between Mars and Jupiter. -They accordingly set to work to prove this, if possible, and to find -what had become of this lost member of the planet family, if it ever -existed. - -As a result of this work, on January 1, 1801, the first planetoid was -discovered, and in rapid succession many like it were found, until -now many hundreds are known to astronomers. Their discovery seemed at -first almost a certain confirmation of Bode’s law, and the fact that -where one large planet should have been found there proved to be such -a swarm of small ones could be accounted for in no other way than -to suppose that something had happened in the making of the planet. -At any rate, the promulgation of Bode’s law was the direct cause of -the search for the missing planet which led to the discovery of the -planetoids. And this is the only reason why Bode’s law has continued -to be mentioned in the history of the planets. For it was no real -law, it had no scientific foundation, and its conformity to the facts -of the relative distances of the planets was only one of those very -interesting and singular coincidences that startle one for the moment -into thinking that there is some scientific significance in them. -Another example of such a coincidence is in the fact that the mass of -any given planet exceeds the total mass of all the planets of any less -mass than itself. - -In less than half a century after the discovery of the first planetoid, -Neptune was discovered at a distance not at all corresponding to that -indicated by Bode’s law. It was not nearly far enough away, and yet, -strangely enough, it was by taking Bode’s law into consideration that -the position was indicated which finally led to the discovery of the -planet. So while Bode’s law has been found to be no law at all, it -is, nevertheless, entitled to some mention because of its having thus -stimulated research that has had such important results. - -No really satisfactory and final explanation of the present state of -the planetoids has ever been given. At one time it was suggested that -another planet had originally existed in the space between Mars and -Jupiter, and through some catastrophe had been shattered into the -small bodies that now occupy that space. But this has been shown to be -impossible. - -It is now thought probable that in the original nebula the matter -forming the planetoids might have been prevented from condensing -into a planet by the powerful gravitative influence of Jupiter. This -influence, however, was not sufficiently strong to bring them entirely -under his control. Even yet he pulls some of them five or six degrees -out of the path they otherwise would take when they venture within the -limits of his domain; but he does not capture them, so they have been -left to circle around the sun as mere fragments of bodies, with no -force to combine and make a world, no mass to hold an atmosphere, and -with nothing to prevent them from quickly condensing and from radiating -all their heat into space. They are, in the main, just cold, dark, -lifeless rocks and lumps of matter whirling through space in a maze of -interlacing orbits, some of them almost as far from the sun as Jupiter -and some almost as near as Mars--one, indeed, a little nearer than -Mars at certain times--but most of them swarming more thickly about -half-way between Mars and Jupiter, not far from the place that Bode’s -law assigned to a planet. - -After the first planetoid was discovered and had been observed for -a few weeks, it was lost and had to be rediscovered by means of -mathematical computation of its orbit. Where this computation showed -that it ought to be, there it was found, on the very last day of the -same year, 1801. Early the next year another body of the same sort was -discovered, two years later another was found, and still three years -later a fourth came into view. These four were the only ones known in -this branch of the solar family for nearly forty years thereafter. - -In 1845 another period of discovery commenced, and has ever since -continued, until there are now between six and seven hundred of these -little bodies that have disclosed their right to be known as members -of the sun’s family. It is probable that there may be still many -more of them, since a new one comes to light every now and then on a -photographic plate, and there is no indication of any limit to the -number that may thus appear. - -It is likely that about all have been discovered that can be seen even -with a telescope, for a fairly systematic and thorough search has been -made of the heavens for this purpose during the last half-century. This -work has resulted in a continually decreasing number of discoveries, -until this method of search has finally been practically abandoned. But -it not infrequently happens that in photographing the stars a little -trail of light is discovered on the plate, showing that some heavenly -body with sensible motion has been caught on it. And this usually -proves to be a new planetoid. No matter how long a photographic plate -is exposed, the fixed stars imprint themselves on it only as points of -light. When the impression is a little streak of light instead of a -dot, the object is shown to be in motion, and is either a planetoid, a -satellite, or a comet. The fixed stars would make a trail also if the -photographic apparatus were not regulated by clockwork, so as to follow -the star in its apparent daily motion across the skies. The planets -and other bodies in the solar system are sufficiently near to have a -sensible motion in addition to the motion caused by the rotation of the -earth, which is the only motion we have to take into account in dealing -with the aspects of the stars. - -The first planetoid discovered was called Ceres, the next one Pallas, -the third Juno, and the fourth Vesta. This pretty custom of naming them -after the gods and goddesses of mythology was continued, with some -variations, until perhaps three hundred had been so christened. But the -number of them became too prodigious; and when so many began to swarm -into view, waiting to be named, the utilitarian method of designating -them simply by numbers in the order of their discovery was adopted. The -only distinguishing feature of so numbering them is that each number -is placed in a little circle. Thus Ceres is ①, Pallas ②, and so -on. Those of them that have any special claim to distinction, however, -are still referred to by their own names, if they have any, in spite of -this most orderly attempt to make them fit for easy reference in a list. - -There are so many of the planetoids, and they are so minute, that -even after they have been discovered they are frequently lost again. -Hence it is sometimes uncertain when they register themselves on the -photographic plates whether they are really new to us or have been -known before. In such cases they are named temporarily after the -letters of the alphabet, and, when the alphabet is exhausted, a second -letter is added. Thus A to Z, then AB to AZ, BC to BZ, and so on in a -sort of “round.” Sometimes these combinations of letters become the -fixed designation of a planetoid, as a nickname sometimes clings to a -person. And thus it happens that we sometimes read of one in particular -of these little bodies that is conspicuous for the great eccentricity -of its orbit, called “WD.” The letters are not its initials, but its -nickname. It really has no name other than its number in the list; but -it became famous while it was temporarily designated as “WD,” and thus -it continues to be called. - -The aid of a telescope is necessary in order to see the planetoids, -though it is said that Vesta, under very favorable conditions, -sometimes comes within the limit of visibility. She is the brightest of -them all, though not the largest, and her brilliancy is the subject of -much interesting speculation among astronomers, who have not yet been -able to account for it. She seems from her excessive brightness to be -covered with clouds; and yet it is manifestly impossible that so small -a body could have held an atmosphere throughout these long ages, though -clouds presuppose an atmosphere. No doubt, in time this mystery of -Vesta’s brilliancy will be made plain. Bright as she is in proportion -to her size, and even if she sometimes can be seen, one cannot -reasonably expect anything very brilliant to our view from a body not -much more than a hundred miles in diameter, shining by reflected light, -nearly two hundred million miles away. - -Ceres, as far as we yet know, is the largest of the planetoids, and -may be something more than four hundred miles in diameter. Juno is -somewhere near the same size. Pallas is about two hundred miles in -diameter, and Vesta about one hundred and eighteen. No doubt, these -four were the first to be discovered, because they are the largest -and so the easiest to be seen. At any rate, no others yet seen exceed -them in size, and some of the more lately discovered are not more -than fifteen or twenty miles in diameter. Many of those discovered by -photography are doubtless even smaller than these, and are, perhaps, -mere meteors in size. The combined mass of all those discovered up -to this time is far smaller than that of any of the large planets, -or even than that of our moon. Their mass cannot, of course, really -be measured, because they are too small to have any perceptible -gravitative effect on other bodies, and mass can only be determined -by the influence of one body on another. But we do know that their -aggregate mass, if it exceeded a certain limit, would show some -disturbing effect on Mars; and, since it does not do this, we know that -all of them taken together would make an extremely insignificant body. - -While the planetoids all revolve around the sun in the same manner and -in the same direction as the planets do, yet they are very erratic -in their courses, and do not all keep within the narrow limits of -the zodiac through which--happily for our convenient observation--the -larger bodies travel. The orbits of many of them are extremely -elliptical, while some are almost circles; and their inclination to the -ecliptic varies from almost nothing to nearly fifty degrees. If one -could catch from one side a view of them all together, they would have -much the appearance in space of a flock of swallows, the individuals -darting this way and that, passing above and below one another in such -intricate sweeps and sinuosities that it would be impossible to keep -track of them separately. And yet time has brought these apparently -tangled orbits into such nice adjustment that the little bodies can -continue to cross and recross each other’s paths with no danger of -interference from each other. Such collisions as there may have -been occurred in the very beginning of their careers. Such of them -as came into collision then traveled on together as one body until -accommodation was made for all. - -One of the most wide-wandering of these tiny bodies has been named -Eros, after the little god of love, more commonly known as Cupid. -It has a particular interest for us, because of all the heavenly -bodies it at times comes nearer to us than any except the moon and an -occasional comet. At its nearest it is within fourteen million miles of -the earth, which is more than ten million miles nearer than the closest -approach of Venus, the nearest of the large planets. - -This little body was thus near us in 1894; but we did not then know -this, for Eros was not discovered until 1898. After its discovery, -however, it was traced back on many photographic plates, and the fact -that it had been in our neighborhood was learned. For untold ages it -has been making these visits to us every thirty-seven years, and we -have known nothing of them. Its next near approach will be in 1931, and -it will continue to come thereafter every thirty-seven years. Now that -we know about them, these visits are not only pleasant to contemplate, -but it is expected that when they occur the planetoid will be of -great scientific value to us in helping to determine more surely and -accurately the exact distance of the sun. - -The planetoids, though so minute and of no value as a spectacle, have -been, and still are, very useful little bodies to us in a scientific -way. In addition to furnishing an easy means of measuring the distance -of the sun, they promise to throw some light on various questions of -physics in which the planets, too, are involved. The brilliancy of -Vesta, for instance, which has been mentioned, and the unaccountable -variability in the brightness of some others of them have yet to be -adjusted to known physical laws. Even the extreme eccentricity of some -of their orbits, and the large tilt of some of them to the ecliptic, -may be suggestive in finally solving certain planetary problems, for -these impish little bodies are far from conforming to the regular ways -of the planets, and there is, of course, some mechanical reason for -their apparent waywardness. - - - - -XVIII - -CONCLUSION - - -The great variety of beauty that the planets present to us is -sufficient to keep us always interested in them, when once we have -acquired an acquaintance with them. Rarely is there an evening when -some one of them does not enhance the charm of the splendid spectacle -of the sky in which all the heavenly bodies save the sun have a part. -Their greater brilliancy often brings them into view before the stars -have begun to glow in the evening, and prolongs our sight of them after -the rays of the sun have blotted out the light of the stars in the -morning. Thus they are always single in their loveliness, and always -hold a distinguished place in the midst of the brilliant company of the -stars. - -Having considered these brilliant bodies individually and in detail, -as we have, we ought by this time to be able to identify any one of -them that shows itself in the evening sky, and to have a pretty fair -notion of the general character and peculiarities of each. But even -if one does not much care for detailed information concerning them, -or, before seeking that, prefers first to become familiar with their -appearance, a quick and sure recognition of them may be had by noting -their positions and their very striking individual aspects as set forth -in the preceding chapters. - -On seeing a bright object in the sky that does not seem to be a -familiar star, simply stop and look at it. Does it twinkle? If it does -not, it is a planet. If it is more than forty-five degrees from the -sun, or if it is seen at a time when the sun has been down more than -three hours, then it is neither Mercury nor Venus, and must be either -Mars, Jupiter, or Saturn. Is it very bright and pinkish in tone? Then -it is Jupiter. Is it very bright and quite red? It is Mars, not far -from opposition. Is it not very bright, but small and rosy? Then it is -Mars going toward conjunction. Is it yellow in tone and, while large -and conspicuous, still not so very brilliant? It is Saturn. - -If the planet we seek to name is nearer to the sun than forty-five -degrees, but is still well above the horizon, it may be either of these -three--Mars, Jupiter, Saturn--or it is Venus. If it is very bright and -silvery, it is certainly Venus. If it is very low in the sky and very -near the sun, it may be any one of the five visible planets. In such a -position Mars will always be very small, and the others always larger -than a first-magnitude star; and they may all twinkle a little--Mercury -almost as much as a star. Their size will show them all (except Mars) -as planets, but it will be somewhat more difficult to tell which is -which than it is when they are higher up in the sky. The best thing to -do in such circumstances is to look up their positions either in this -book or in an almanac. The almanac will serve as a footman to announce -them. The book, it is hoped, has so recorded their peculiarities and -habits that either their appearance or their place will be sufficient -to make them known. - -In any event, the problem of identification in this position will not -keep one long, for in a situation presenting these greater difficulties -the planet will be visible for less than an hour after sundown. -Besides, it is not likely at such times to attract one’s involuntary -attention, but when under observation in such a situation is usually -sought out by those already somewhat informed as to the planet’s habits -and appearance, which will betray its identity. It is information -of this sort that I have endeavored to give in these pages, and it -is hoped that the reading of them will be the beginning of a long -and intimate acquaintance with these charming and always interesting -individuals. - -Individuals the planets inevitably become to any one who learns to -know them during the long, quiet nights in the country, or wherever an -opportunity is afforded really to contemplate their peculiar traits and -features. Like individuals of whatever kind, they impress different -persons in different ways. As I have watched them from year to year I -have come to have a very distinct impression of Jupiter as slow and -majestic, and yet not lacking in joviality; Saturn as friendly, but -reserved; Mars as sturdily brisk and busy; Venus as always gracious -and smiling; and Mercury as irresponsible and roguish. Others might -have an entirely different feeling in regard to them; but an intimate -acquaintance with them, which is not wholly scientific, cannot fail to -stamp them as in some sort individuals. - -And when we consider that these interesting individuals are closely -related members of our cosmic family, their ever-changing beauty of -aspect, the history of their development and their affairs generally, -gain a significance to us that no other heavenly bodies can have. The -two groups of planets--the inner and the outer--are like two sets of -children in a family: born of the same parent, but under very different -circumstances, and in very different surroundings. Mars, the earth, -Venus, and Mercury are all, as compared with the outer planets, small -and dense, with more or less thin atmospheres and an abundance of heat -and light. They all lie comparatively near to the sun, and are composed -of the denser material lying near the center of the great nebula, which -was the original form of the entire solar system. Probably denser to -begin with than the others, they have, on account of their diminutive -size, developed more rapidly and are further advanced toward the final -state of solidity which we shall all attain in the end. Mercury, the -smallest, is already old and seamed and hardened. Mars, the next in -size, is well advanced, but still has an atmosphere and some other -signs of vitality. Venus, though we know so little about her, has -probably a long period of development yet before her; while this warm, -nourishing earth, which seems to us the best one of them all, will -probably for a still longer time than Venus hold its atmosphere and -remain green and flourishing. - -On the other side of the vast space which divides the two groups of the -sun’s family dwell Jupiter, Saturn, Uranus, and Neptune. They are all -tremendous in volume, enveloped in immense atmospheres, far, far from -our common source of heat and light, of comparatively slight density, -and probably formed from the lighter material composing the outer edges -of the parent nebula, and, because of their immense size, still in a -very early stage of development. The two groups could scarcely seem -more widely different if they belonged to different systems; but the -members of each are all closely akin, and each one in its own way, -determined by its size and environment, is developing toward the same -end. - -If there is life on any of these outer planets, it must be of a -sort of which we have no conception. Jupiter and Saturn are probably -red-hot, and could sustain nothing more cold-blooded than a race of -salamanders, though why a race of intelligent salamanders should or -should not exist there, is a question that one might make bold to -answer according to one’s fancy. Uranus and Neptune are smaller, and -perhaps less hot than Jupiter and Saturn; but we really know very -little about the state of their domestic affairs, and the little -we do know in no way indicates a place of abode for any sort of -intelligence conceivable to us. We can, however, conceive of a time -in the far-distant ages when these four hot and vaporous planets may -have become sufficiently condensed to have a solid crust, and yet -have sufficient internal heat to moderate the frigid temperature that -would be normal at their distance from the sun, and they might then -support life even somewhat resembling and perhaps even more gloriously -beautiful than that with which we are familiar. - -Of the existence of life somewhat similar to ours on the smaller, -near-by planets we may have something nearer a reasonable conception, -though we are nowhere near the possession of any real knowledge -concerning it. Mercury, we have every reason to think, cannot support -life, mainly because of his lack of atmosphere; but also because of -his long rotation, which affords no alternations of day and night, but -leaves him with one side always burning-hot and the other inconceivably -cold. Venus might very well have a climate not utterly unlike ours, and -hence be habitable for beings somewhat resembling us, if she has, as -she has long been thought to have, a heavier atmosphere than the earth -has, and if she has alternations of day and night. But we have seen -that, owing to the obscurity of the surface of Venus, our knowledge -in regard to these conditions is far from certain, and we have little -reason to have even speculative ideas concerning life there. With Mars -it is a more open question. We can see that planet, and see it fairly -well. It has an atmosphere and changes of seasons, and while it may not -afford a climate that would be exactly attractive to us as a place of -transmigration, it is not particularly unreasonable to let our fancy -play over the rather pleasant speculation concerning the presence there -of beings at least understandable by us, even if not wholly congenial. - -Whatever each planet affords in the way of life and human interests, -all of them must ever be to us the most interesting things in all -nature, outside of our own earth, in the two regards already pointed -out: first, as the most beautiful objects of vision among all the -starry hosts, and, second, as our nearest kindred in this universe -of suns and systems of worlds. Together the earth and they circle -ceaselessly around and around the sun, following in nicely adjusted -orbits that great luminary as it sweeps majestically on through space -toward the beautiful Vega, itself a sun, and, so far as we now know, in -this close companionship we shall continue until every planet and the -sun itself has become cold and dark and lifeless. And then, perhaps, -or even before the light of our system is finally extinguished, we may -meet another wandering sun, and in the marriage of the two great bodies -another system of worlds may be evolved of which we and the planets -shall form a part. - - -SYMBOLS USED IN ALMANACS - - ☿ = Mercury. ⚫ = New Moon. - - ♀ = Venus. ☽ = First Quarter. - - ⊕ = Earth. ⚪ = Full Moon. - - ♂ = Mars. ☾ = Last Quarter. - - ♃ = Jupiter. ☉ = Sun. - - ♄ = Saturn. ☌ = Conjunction with the - sun; or, in the case - ♅ or ⛢ = Uranus. of two planets or a - planet and the moon, - ♆ = Neptune. near together. - - ☍ = Opposition. - - □ = Quadrature. - -Examples: - - ☌ ♂ ♀ = Mars and Venus near together. - - ☍ ♃ ☉ = Jupiter in opposition. - - ☌ ♃ ☉ = Jupiter in conjunction. - - ☌ ☿ ☉ Inf. = Mercury in inferior conjunction. - - ☌ ☿ ☉ Sup. = Mercury in superior conjunction. - - ☌ ♀ ☽ = Venus and Moon near together. - - - - -INDEX - - - Adams, 236–238. - - Alcor, star in Great Dipper, 105, 236. - - Aldebaran, first-magnitude star, 79–80, 153, 188, 210. - - Antares, star in Scorpio, 86, 153, 160, 187, 189, 209, 212. - - Aquarius, constellation of the zodiac, 76, 88–89, 91–92, 187, - 212–213, 221, 242. - - Arcturus, 24, 84; - color of, 102. - - Ariel, satellite of Uranus, 232–233. - - Aries, constellation of the zodiac, 76–78, 90–92, 212, 242. - - Asteroids, 244–257. - - - Bee-hive, 82, 211–212. - - Bode’s law, 245–249. - - Boötes, star of first magnitude, 102. - - - Callisto, satellite of Jupiter, 200, 205. - - Cancer, constellation of zodiac, 76, 82, 91–92, 188, 211–212. - - Capella, star of first magnitude, 191. - - Capricornus, one of the twelve constellations of the zodiac, 76, - 88–89, 91–92, 187, 212, 229. - - Cassiopeia, constellation, 77. - - Castor and Pollux, 81, 188, 211, 242–243. - - Ceres, first planetoid discovered, 251, 253. - - Constellations of the zodiac, 75–92. - - - Deimos, satellite of Mars, 180–181. - - Dione, satellite of Saturn, 222. - - - Earth, relation to planets, 11–15, 19; - nearness to sun, 19; - terrestrial planet, 41; - movement of, 51; - position in regard to Mercury, 120–121; - likeness to Venus, 138–140. - - Enceladus, satellite of Saturn, 222. - - Encke’s comet, 109. - - Equinox, derivation of word, 74. - - Eros, small planet, 255–256. - - Europa, satellite of Jupiter, 200–201. - - - Flagstaff, Arizona, observatory of, 175–176. - - Fomalhaut, 187, 209, 213. - - - Galileo, 136. - - Ganymede, satellite of Jupiter, 200–201, 205. - - Gemini, constellation of the zodiac, 76, 81–82, 91–92, 188, - 210–211, 213. - - George III., Uranus first called _Georgium Sidus_ after, 226. - - Great Dipper, 73, 77, 84, 96, 104, 105, 186, 236. - - - Hamal, star in constellation of Aries, 78. - - Herschel, discovery of Uranus by, 226–227, 232. - - Hyades, the, 79. - - Hyperion, satellite of Saturn, 222. - - - Inferior planets, 40. - - Io, satellite of Jupiter, 200, 201. - - - Japetus, satellite of Saturn, 222. - - Juno, planetoid, 251, 253. - - Jupiter, color, 5; - attraction between Saturn and, 15; - distance from sun, 19; - size and importance of, 20; - movement, 25, 65; - satellites, 34, 106, 199–205; - long known, 38; - superior planet, 41; - space between Mars and, 42; - influence on comets, 44; - gibbous, 66; - distance from ecliptic, 72; - near Antares, 86; - in Scorpio, 127; - size and velocity, 183–185; - place in sky, 186–190; - distance, light, and heat, 190–193; - seasons and atmosphere, 193–195; - surface features, 195–199; - symbol, 205; - compared to Saturn, 213–214, 215–218; - nearness of asteroids to, 244; - how to recognize, 259–264. - - - Laplace, nebulæ hypothesis of, 28, 30. - - Leo, constellation of zodiac, 76, 82–83, 91–92, 188, 211–212, 221. - - Leverrier, discovery of Neptune by, 236–238. - - Libra, constellation of zodiac, 76, 85, 91–92, 188, 212. - - Little Dipper of the Pleiades, 79. - - Lyre, constellation of the, 54. - - - Major planets, 19. - - Mars, “eye” of, 12; - distance from sun, 19; - nearness to earth, 20; - movement of, 25, 65; - long known, 38; - superior planet, 41; - space between Jupiter and, 42; - speed, 51; - gibbous, 66; - distance from ecliptic, 72; - color, 80, 86, 259; - position in regard to Antares, 87; - density, 110; - nearness to Venus, 128; - variety in brightness, 151–152; - how and where to identify, 152–162, 259–265; - size, atmosphere, and temperature, 162–165; - distance and brilliancy, 166–170; - seasons, 170–171; - surface aspect, 172–179; - satellites, 180–181; - symbol of, 182; - nearness of asteroids to, 244; - Bode’s law and, 245–246, 248–249; - smallness, 260. - - Mercury, 18; - nearest planet, 19; - unfavorable situation for observation, 20; - easily recognized, 22; - age of, 34; - dense matter of, 37; - long known, 38; - inferior planet, 40; - terrestrial planet, 41; - irregularities of, 44–45; - number of revolutions, 47; - orbit, 48; - apparent motions, 57–58; - transits, 61; - distance from ecliptic, 72–73; - color, 80, 86; - in Scorpio, 87; - elusiveness of, 93–95; - how to find, 96–100, 259; - distance and brightness of, 101–105; - size, 106–110; - relation to sun, 111–118; - transits, 119–121; - lack of atmosphere, 144, 146; - resemblance to Mars, 153; - Bode’s law and, 245. - - Milky Way, 87, 88, 89. - - Mimas, satellite of Saturn, 222. - - Minor planets, 19. - - Mizar, star in Great Dipper, 105, 236. - - Moon, 23; - once called planet, 39; - distance from ecliptic, 73. - - Moulton, Professor, 178. - - - Neptune, discovery, 15; - distance from sun, 19, 43; - not visible to naked eye, 20; - age, 34; - diffuse matter of, 37; - unknown to ancients, 40; - superior planet, 41; - influence on comets, 44; - one revolution, 47; - orbit, 48; - movement of, 65; - distance from earth, 234; - discovery, 235–237, 247; - symbol, 238; - atmosphere, 239–240; - satellite, 241; - motion, 242; - brightness, 243. - - - Oberon, satellite of Uranus, 232–233. - - Orion, 123. - - - Pallas, planetoid, 251. - - Phecda, star in Great Dipper, 104. - - Phobos, satellite of Mars, 180–181, 202. - - Phœbe, satellite of Saturn, 222–223. - - Pisces, constellation in zodiac, 76–77, 90–92, 160, 187, 212, 242. - - Pleiades, 79–80, 153, 188, 210. - - Præsepe, or the Bee-hive, 82, 211–212. - - - Regulus, star in the constellation of Leo, 83–84, 188, 212. - - Rhea, satellite of Saturn, 222–223. - - - Sagittarius, constellation of zodiac, 76, 87–88, 91–92, 186, 189, - 209, 212, 229. - - Saturn, rings and moons of, 12, 218–223; - distance from sun, 13, 19; - attraction between Jupiter and, 15, 185; - size and importance, 20; - object-lesson from, 29; - long known, 38; - superior and outer planet, 41–42; - influence on comets, 44; - length of year on, 47; - movement, 65; - distance from ecliptic, 72; - satellites, 106; - color, 206, 209, 259; - as evening star, 207; - slight motion, 208; - circuit of skies, 209–213; - size and distance, 213–215; - surface aspects, 215–216; - day and night, 217–218; - seasons, 224; - symbol, 224; - Bode’s law and, 245–246; - how to recognize, 260–264. - - Schiaparelli, 174–175. - - Scorpio, constellation of zodiac, 76, 85–88, 91–92, 127, 153, 186, - 188, 212–213. - - Sidereal year, 49–50. - - Sirius, the dog-star, 123. - - Spica, 84–85, 188. - - Sun, controls planets, 14, 17; - distance from earth, 18; - center of planet system, 27; - probable formation of, 36; - once called planet, 39; - situation in orbit, 52; - vernal equinox, 76; - relation to Mercury, 111–118; - relation to Mars, 166–167; - relation to Jupiter, 183–185. - - Superior planets, 41, 65–70. - - Symbols in almanacs, 267. - - Synodic year, 50, 52. - - - Taurus, constellation in zodiac, 76, 79–80, 90–92, 188, 210, 212, - 242. - - Tethys, satellite of Saturn, 222. - - Themis, satellite of Saturn, 222–223. - - Titan, satellite of Saturn, 222–223. - - Titania, satellite of Uranus, 232. - - Triangulum, 78. - - - Umbriel, satellite of Uranus, 232–233. - - Uranus, gravitational influence on, 15; - distance from sun, 19, 229–230; - unknown to ancients, 40; - superior planet, 41; - influence on Neptune, 43; - influence on comets, 44; - movement, 65; - nearness to ecliptic, 72; - discovery, 225–226, 246; - symbol, 227; - time of revolution, 228; - size, 231; - satellites, 232–233; - irregularity of, 236. - - - Vega, in constellation of the Lyre, 54, 191, 266. - - Venus, the planet, 2, 4, 5; - nearness to sun, 19; - nearness to earth, 20, 256; - movement of, 25; - long known, 38; - early names of, 39; - inferior planet, 40; - terrestrial planet, 41; - brightest planet, 42; - apparent motions, 57–58; - transits, 61; - distance from ecliptic, 72; - seen from Mercury, 105; - density, 110; - beauty, 122; - how and when to see, 123–131; - distance and brightness, 132–137; - likeness to earth, 138–140; - atmosphere and seasons, 141–147; - transits, 147–149; - sign of, 150; - Bode’s law and, 245; - how to know, 259–264. - - Vesta, planetoid, 251, 253, 254, 257. - - Virgo, constellation of the zodiac, 76, 84–85, 188, 212. - - - Zodiac, the, 71–92. - - -THE END - - - - - -End of Project Gutenberg's The Ways of the Planets, by Martha Evans Martin - -*** END OF THIS PROJECT GUTENBERG EBOOK THE WAYS OF THE PLANETS *** - -***** This file should be named 51284-0.txt or 51284-0.zip ***** -This and all associated files of various formats will be found in: - http://www.gutenberg.org/5/1/2/8/51284/ - -Produced by Shaun Pinder, Thiers Halliwell and the Online -Distributed Proofreading Team at http://www.pgdp.net (This -file was produced from images generously made available -by The Internet Archive) - -Updated editions will replace the previous one--the old editions will -be renamed. - -Creating the works from print editions not protected by U.S. copyright -law means that no one owns a United States copyright in these works, -so the Foundation (and you!) can copy and distribute it in the United -States without permission and without paying copyright -royalties. 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