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- position: relative; - top: -0.4em; - margin-left: 0.05em; - font-size: 11px; - font-weight: normal; - font-style: normal; - text-decoration: none; - white-space: nowrap; -} - -/* Transcriber's notes */ -.transnote { - background-color: #F5F5F5; - color: black; - font-size: 85%; - padding: 0.5em; - margin-bottom: 5em; - font-family: sans-serif, serif; -} - - </style> - </head> -<body> - - -<pre> - -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) - - - - - - -</pre> - - -<div class="transnote"> -<p><b><a id="Transcribers_notes"></a>Transcriber’s notes</b>:</p> - -<p>The text of this book has been preserved as in the original, apart -from a few obvious misspellings.</p> - -<p class="plhi">Corrected misspellings and redundancies include the following:<br /> -comparsion → comparison<br /> -dining → during<br /> -clamly → calmly<br /> -atronomer → astronomer<br /> -oi → of<br /> -the → (deleted)<br /> -a → (deleted)</p> - -<p>In this digital version a black dotted underline indicates a -hyperlink to a page or footnote (hyperlinks are also highlighted when -the mouse pointer hovers over them). Page numbers are shown in the -right margin and footnotes are at the end.</p> - -<p class="epubonly">An illustration in Chapter IX contains an HTML link -to a high-resolution image but this is not accessible with e-reader -devices.</p> - -<p>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.</p> - - - -</div> - - - -<div class="figcenter" style="width: 465px;"> -<a id="frontispiece"></a><img src="images/i_001.jpg" width="465" height="594" alt="" /> -<div><p class="tac">A WHIRLING SPIRAL NEBULA, TYPICAL OF THAT FROM WHICH THE SUN -AND PLANETS WERE PROBABLY EVOLVED</p> - -<p>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.</p></div> -</div> - - - -<h1><span class="t1">THE WAYS OF</span><br /> -<span class="t2">THE PLANETS</span></h1> - -<div class="tp1">BY</div> -<div class="tp2">MARTHA EVANS MARTIN, A.M.</div> -<div class="tp3">AUTHOR OF</div> -<div class="tp4">“THE FRIENDLY STARS”</div> - -<div class="figcenter" style="width: 85px;"> -<img src="images/logo.jpg" width="85" height="107" alt="" /> -</div> - - -<div class="tp5">NEW YORK AND LONDON</div> -<div class="tp6">HARPER & BROTHERS PUBLISHERS</div> -<div class="tp7">MCMXII</div> - - - - -<div class="tpv">COPYRIGHT, 1912, BY HARPER & BROTHERS</div> -<hr class="r10" /> -<div class="tpv">PRINTED IN THE UNITED STATES OF AMERICA<br /> -PUBLISHED OCTOBER, 1912</div> - - - -<hr class="chap" /> - -<h2>CONTENTS</h2> - - - - -<div class="center"> -<table border="0" cellpadding="3" cellspacing="0" summary="table of contents"> -<tr><td class="tal">CHAP.</td><td class="tal"></td><td class="tar">PAGE</td></tr> -<tr><td class="tal">I.</td><td class="tal pl2hi"><span class="smcap">On Making Acquaintance with the Planets</span></td><td class="tar"><a href="#Page_1">1</a></td></tr> -<tr><td class="tal">II.</td><td class="tal pl2hi"><span class="smcap">Our Relation to the Planets</span></td><td class="tar"><a href="#Page_11">11</a></td></tr> -<tr><td class="tal">III.</td><td class="tal pl2hi"><span class="smcap">What the Planets Are, and What They Appear to Be</span></td><td class="tar"><a href="#Page_17">17</a></td></tr> -<tr><td class="tal">IV.</td><td class="tal pl2hi"><span class="smcap">The Origin of the Planets</span></td><td class="tar"><a href="#Page_26">26</a></td></tr> -<tr><td class="tal">V.</td><td class="tal pl2hi"><span class="smcap">The Seven Great Planets</span></td><td class="tar"><a href="#Page_38">38</a></td></tr> -<tr><td class="tal">VI.</td><td class="tal pl2hi"><span class="smcap">The Movements of the Planets</span></td><td class="tar"><a href="#Page_46">46</a></td></tr> -<tr><td class="tal">VII.</td><td class="tal pl2hi"><span class="smcap">How the Inferior Planets Seem to Move</span></td><td class="tar"><a href="#Page_56">56</a></td></tr> -<tr><td class="tal">VIII.</td><td class="tal pl2hi"><span class="smcap">How the Superior Planets Seem to Move</span></td><td class="tar"><a href="#Page_65">65</a></td></tr> -<tr><td class="tal">IX.</td><td class="tal pl2hi"><span class="smcap">The Path of the Planets</span></td><td class="tar"><a href="#Page_71">71</a></td></tr> -<tr><td class="tal vat">X.</td><td class="tal pl2hi"><span class="smcap">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</span></td><td class="tar vab"><a href="#Page_93">93</a></td></tr> -<tr><td class="tal vat">XI.</td><td class="tal pl2hi"><span class="smcap">Venus—When and Where to See Venus—Distance -and Brilliancy—Venus’s -Likeness to the Earth—Atmosphere, -Day and Night, and Seasons—Transits</span></td><td class="tar vab"><a href="#Page_122">122</a></td></tr> -<tr><td class="tal vat">XII.</td><td class="tal pl2hi"><span class="smcap">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</span></td><td class="tar vab"><a href="#Page_151">151</a></td></tr> -<tr><td class="tal vat">XIII.</td><td class="tal pl2hi"><span class="smcap">Jupiter—Place in the Sky—Distance, -Light, and Heat—Day and Night, -Seasons, and Atmosphere—Surface -Features—System of Satellites</span></td><td class="tar vab"><a href="#Page_183">183</a></td></tr> -<tr><td class="tal vat">XIV.</td><td class="tal pl2hi"><span class="smcap">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</span></td><td class="tar vab"><a href="#Page_206">206</a></td></tr> -<tr><td class="tal">XV.</td><td class="tal pl2hi"><span class="smcap">Uranus</span></td><td class="tar"><a href="#Page_225">225</a></td></tr> -<tr><td class="tal">XVI.</td><td class="tal pl2hi"><span class="smcap">Neptune</span></td><td class="tar"><a href="#Page_234">234</a></td></tr> -<tr><td class="tal">XVII.</td><td class="tal pl2hi"><span class="smcap">The Little Planets, or the Asteroids</span></td><td class="tar"><a href="#Page_244">244</a></td></tr> -<tr><td class="tal">XVIII.</td><td class="tal pl2hi"><span class="smcap">Conclusion</span></td><td class="tar"><a href="#Page_258">258</a></td></tr> -<tr><td class="tal"></td><td class="tal pl2hi"><span class="smcap">Symbols Used in Almanacs</span></td><td class="tar"><a href="#Page_267">267</a></td></tr> -<tr><td class="tal"></td><td class="tal"><span class="smcap">Index</span></td><td class="tar"><a href="#Page_269">269</a></td></tr> -</table></div> - - - - -<h2>ILLUSTRATIONS</h2> - - - -<div class="center"> -<table border="0" cellpadding="3" cellspacing="0" summary="table of illustrations"> -<tr><td class="tal pl2hi"><span class="lowercase smcap">A WHIRLING SPIRAL NEBULA, TYPICAL OF THAT FROM WHICH THE SUN AND PLANETS WERE PROBABLY EVOLVED</span></td><td class="tar vab"><i><a href="#frontispiece">Frontispiece</a></i></td></tr> -<tr><td class="tal pl2hi"><span class="lowercase smcap">MAP SHOWING THE CONSTELLATIONS OF THE ZODIAC AND THE LINE OF THE ECLIPTIC RUNNING THROUGH THEM</span></td><td class="tar vab"><span class="ilb"><i>Facing p.</i> <a href="#Page_76">76</a></span></td></tr> -<tr><td class="tal pl2hi"><span class="lowercase smcap">THE LOVELY CRESCENT THAT VENUS SHOWS WHEN TO OUR VIEW SHE IS AT HER GREATEST BRILLIANCY</span></td><td class="tar vab">"  <a href="#Page_136">136</a></td></tr> -<tr><td class="tal pl2hi"><span class="lowercase smcap">RELATIVE APPARENT SIZE OF VENUS AT DIFFERENT PHASES OF ILLUMINATION</span></td><td class="tar vab"><i>Page</i> <a href="#Page_137">137</a></td></tr> -<tr><td class="tal pl2hi"><span class="lowercase smcap">THE TWO PHASES OF MARS</span></td><td class="tar vab"><span class="ilb"><i>Facing p.</i> <a href="#Page_152">152</a></span></td></tr> -<tr><td class="tal pl2hi"><span class="lowercase smcap">MARS: DIFFERENCE IN ITS APPARENT SIZE AT ITS NEAREST, MIDDLE, AND FARTHEST DISTANCE FROM THE EARTH</span></td><td class="tar vab"><i>Page</i> <a href="#Page_169">169</a></td></tr> -<tr><td class="tal pl2hi"><span class="lowercase smcap">JUPITER, THE MAMMOTH MEMBER OF THE SOLAR FAMILY—LARGER THAN ALL THE OTHER PLANETS PUT TOGETHER</span></td><td class="tar vab"><span class="ilb"> <i>Facing p.</i> <a href="#Page_184">184</a></span></td></tr> -<tr><td class="tal pl2hi"><span class="lowercase smcap">SATURN AND ITS RINGS</span></td><td class="tar vab">"  <a href="#Page_220">220</a></td></tr> -</table></div> - -<hr class="chap" /> - -<p><span class="pagenum" title="1"><a name="Page_1" id="Page_1"></a></span></p> - -<h2 class="fs240">THE<br /> -WAYS OF THE PLANETS</h2> - - - -<h2>I</h2> - -<h3>ON MAKING ACQUAINTANCE WITH THE PLANETS</h3> - - -<p>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.</p> - -<p>An endeavor is made also to so simplify<span class="pagenum" title="2"><a name="Page_2" id="Page_2"></a></span> -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.</p> - -<p>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,<span class="pagenum" title="3"><a name="Page_3" id="Page_3"></a></span> -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.</p> - -<p>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<span class="pagenum" title="4"><a name="Page_4" id="Page_4"></a></span> -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.</p> - -<p>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<span class="pagenum" title="5"><a name="Page_5" id="Page_5"></a></span> -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.</p> - -<p>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.</p> - -<p>But in addition to what, as a help in observation, -it may find to say regarding the<span class="pagenum" title="6"><a name="Page_6" id="Page_6"></a></span> -appearance and movements of the planets, -it will endeavor to give also ample information -concerning their character and constitution.</p> - -<p>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.</p> - -<p>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<span class="pagenum" title="7"><a name="Page_7" id="Page_7"></a></span> -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.</p> - -<p>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<span class="pagenum" title="8"><a name="Page_8" id="Page_8"></a></span> -courage to begin all over again, to sudden -inspirations, and sometimes to those lucky -discoveries that seem almost like miracles.</p> - -<p>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.</p> - -<p>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<span class="pagenum" title="9"><a name="Page_9" id="Page_9"></a></span> -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.</p> - -<p>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<span class="pagenum" title="10"><a name="Page_10" id="Page_10"></a></span> -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.</p> - -<hr class="chap" /> - -<p><span class="pagenum" title="11"><a name="Page_11" id="Page_11"></a></span></p> - - - - -<h2>II</h2> - -<h3>OUR RELATION TO THE PLANETS</h3> - - -<p>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.</p> - -<p>One evening last autumn I was coming -up Broadway, New York, with a friend, when -we encountered at Union Square a man with<span class="pagenum" title="12"><a name="Page_12" id="Page_12"></a></span> -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.</p> - -<p>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<span class="pagenum" title="13"><a name="Page_13" id="Page_13"></a></span> -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<span class="pagenum" title="14"><a name="Page_14" id="Page_14"></a></span> -more or less influenced by it. If anything -should happen to it, it might be a serious -matter to us.”</p> - -<p>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.</p> - -<p>Thus, while the sun controls the motions -of all of them, each pulls at the other, and,<span class="pagenum" title="15"><a name="Page_15" id="Page_15"></a></span> -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.</p> - -<p>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?</p> - -<p>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<span class="pagenum" title="16"><a name="Page_16" id="Page_16"></a></span> -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.</p> - -<p>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.</p> - -<hr class="chap" /> - -<p><span class="pagenum" title="17"><a name="Page_17" id="Page_17"></a></span></p> - - - - -<h2>III</h2> - -<h3>WHAT THE PLANETS ARE, AND WHAT -THEY APPEAR TO BE</h3> - - -<p>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.</p> - -<p>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 astronom<span class="pagenum" title="18"><a name="Page_18" id="Page_18"></a></span>ical -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.</p> - -<p>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<span class="pagenum" title="19"><a name="Page_19" id="Page_19"></a></span> -makes the journey in a little more than four -hours.</p> - -<p>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.</p> - -<p>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<span class="pagenum" title="20"><a name="Page_20" id="Page_20"></a></span> -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.</p> - -<p>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<span class="pagenum" title="21"><a name="Page_21" id="Page_21"></a></span> -the others in their splendor and size, and -in their importance as the centers of systems -of their own.</p> - -<p>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.</p> - -<p>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<span class="pagenum" title="22"><a name="Page_22" id="Page_22"></a></span> -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.</p> - -<p>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<span class="pagenum" title="23"><a name="Page_23" id="Page_23"></a></span> -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.</p> - -<p>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.</p> - -<p>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<span class="pagenum" title="24"><a name="Page_24" id="Page_24"></a></span> -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.</p> - -<p>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.</p> - -<p>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<span class="pagenum" title="25"><a name="Page_25" id="Page_25"></a></span> -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.</p> - -<p>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.</p> - -<hr class="chap" /> - -<p><span class="pagenum" title="26"><a name="Page_26" id="Page_26"></a></span></p> - - - - -<h2>IV</h2> - -<h3>THE ORIGIN OF THE PLANETS</h3> - - -<p>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.</p> - -<p>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<span class="pagenum" title="27"><a name="Page_27" id="Page_27"></a></span> -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.</p> - -<p>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.</p> - -<p><span class="pagenum" title="28"><a name="Page_28" id="Page_28"></a></span></p> - -<p>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.</p> - -<p>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.</p> - -<p>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<span class="pagenum" title="29"><a name="Page_29" id="Page_29"></a></span> -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.</p> - -<p>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.</p> - -<p>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<span class="pagenum" title="30"><a name="Page_30" id="Page_30"></a></span> -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.</p> - -<p>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.</p> - -<p><span class="pagenum" title="31"><a name="Page_31" id="Page_31"></a></span></p> - -<p>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.</p> - -<p>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<span class="pagenum" title="32"><a name="Page_32" id="Page_32"></a></span> -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.</p> - -<p>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.</p> - -<p>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<span class="pagenum" title="33"><a name="Page_33" id="Page_33"></a></span> -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.</p> - -<p>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<span class="pagenum" title="34"><a name="Page_34" id="Page_34"></a></span> -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.</p> - -<p>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<span class="pagenum" title="35"><a name="Page_35" id="Page_35"></a></span> -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.</p> - - -<p class="mt2em">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.</p> - -<p>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,<span class="pagenum" title="36"><a name="Page_36" id="Page_36"></a></span> -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.</p> - -<p>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.</p> - -<p>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<span class="pagenum" title="37"><a name="Page_37" id="Page_37"></a></span> -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.</p> - -<p>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.</p> - -<hr class="chap" /> - -<p><span class="pagenum" title="38"><a name="Page_38" id="Page_38"></a></span></p> - - - - -<h2>V</h2> - -<h3>THE SEVEN GREAT PLANETS</h3> - - -<p>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.</p> - -<p>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<span class="pagenum" title="39"><a name="Page_39" id="Page_39"></a></span> -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.</p> - -<p>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<span class="pagenum" title="40"><a name="Page_40" id="Page_40"></a></span> -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.</p> - -<p>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.</p> - -<p>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<span class="pagenum" title="41"><a name="Page_41" id="Page_41"></a></span> -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).</p> - -<p>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<span class="pagenum" title="42"><a name="Page_42" id="Page_42"></a></span> -other characteristics in common which the -planets of the other group do not have. The -two classes represent different stages of evolution.</p> - -<p>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.</p> - -<p>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.</p> - -<p>That there is at least one other planet -beyond the present boundary of our system<span class="pagenum" title="43"><a name="Page_43" id="Page_43"></a></span> -(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.</p> - -<p>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<span class="pagenum" title="44"><a name="Page_44" id="Page_44"></a></span> -smaller than Saturn or an ordinary first-magnitude -star does to us.</p> - -<p>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.</p> - -<p>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<span class="pagenum" title="45"><a name="Page_45" id="Page_45"></a></span> -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.</p> - -<p>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.</p> - -<hr class="chap" /> - -<p><span class="pagenum" title="46"><a name="Page_46" id="Page_46"></a></span></p> - - - - -<h2>VI</h2> - -<h3>THE MOVEMENTS OF THE PLANETS</h3> - - -<p>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<span class="pagenum" title="47"><a name="Page_47" id="Page_47"></a></span> -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.</p> - -<p>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<span class="pagenum" title="48"><a name="Page_48" id="Page_48"></a></span> -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.</p> - -<p>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.</p> - -<p>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 ex<span class="pagenum" title="49"><a name="Page_49" id="Page_49"></a></span>tent -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.</p> - -<p>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 <i>sidus</i>, 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.</p> - -<p>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<span class="pagenum" title="50"><a name="Page_50" id="Page_50"></a></span> -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.</p> - -<p>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.</p> - -<p>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<span class="pagenum" title="51"><a name="Page_51" id="Page_51"></a></span> -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.</p> - -<p>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 -<i>them</i>. 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.</p> - -<p>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<span class="pagenum" title="52"><a name="Page_52" id="Page_52"></a></span> -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.</p> - -<p>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<span class="pagenum" title="53"><a name="Page_53" id="Page_53"></a></span> -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.</p> - -<p>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.</p> - -<p>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<span class="pagenum" title="54"><a name="Page_54" id="Page_54"></a></span> -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<span class="pagenum" title="55"><a name="Page_55" id="Page_55"></a></span> -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.</p> - -<hr class="chap" /> - -<p><span class="pagenum" title="56"><a name="Page_56" id="Page_56"></a></span></p> - - - - -<h2>VII</h2> - -<h3>HOW THE INFERIOR PLANETS SEEM TO MOVE</h3> - - -<p>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<span class="pagenum" title="57"><a name="Page_57" id="Page_57"></a></span> -understanding of the apparent movements; -but it is only with the latter that, for ordinary -observation, we need to be particularly -acquainted.</p> - -<p>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 <i>among</i> the -fixed stars, and not <i>with</i> 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.</p> - -<p>The apparent motions of the inferior -planets, Mercury and Venus, always take -place near the sun. Venus never wanders<span class="pagenum" title="58"><a name="Page_58" id="Page_58"></a></span> -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 west<span class="pagenum" title="59"><a name="Page_59" id="Page_59"></a></span>ward -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.</p> - -<p>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<span class="pagenum" title="60"><a name="Page_60" id="Page_60"></a></span> -in the morning before dawn, occurs when -the planet is at its greatest apparent distance -west of the sun.</p> - -<p>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.</p> - -<p>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<span class="pagenum" title="61"><a name="Page_61" id="Page_61"></a></span> -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.</p> - -<p>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.</p> - -<p>As it starts back east again its distance -from the earth increases daily until it reaches<span class="pagenum" title="62"><a name="Page_62" id="Page_62"></a></span> -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.</p> - -<p>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<span class="pagenum" title="63"><a name="Page_63" id="Page_63"></a></span> -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.</p> - -<p>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,<span class="pagenum" title="64"><a name="Page_64" id="Page_64"></a></span> -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.</p> - -<hr class="chap" /> - -<p><span class="pagenum" title="65"><a name="Page_65" id="Page_65"></a></span></p> - - - - -<h2>VIII</h2> - -<h3>HOW THE SUPERIOR PLANETS SEEM TO MOVE</h3> - - -<p>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<span class="pagenum" title="66"><a name="Page_66" id="Page_66"></a></span> -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.</p> - -<p>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.</p> - -<p>Being, when in opposition, exactly oppo<span class="pagenum" title="67"><a name="Page_67" id="Page_67"></a></span>site -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.</p> - -<p>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.</p> - -<p>From conjunction to opposition the planet<span class="pagenum" title="68"><a name="Page_68" id="Page_68"></a></span> -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.</p> - -<p>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<span class="pagenum" title="69"><a name="Page_69" id="Page_69"></a></span> -opposition, the earlier in the evening it rises -and the longer it may be seen.</p> - -<p>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.</p> - -<p>In giving this rather rough outline of the<span class="pagenum" title="70"><a name="Page_70" id="Page_70"></a></span> -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.</p> - -<hr class="chap" /> - -<p><span class="pagenum" title="71"><a name="Page_71" id="Page_71"></a></span></p> - - - - -<h2>IX</h2> - -<h3>THE PATH OF THE PLANETS</h3> - - -<p>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.</p> - -<p>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<span class="pagenum" title="72"><a name="Page_72" id="Page_72"></a></span> -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.</p> - -<p>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 dis<span class="pagenum" title="73"><a name="Page_73" id="Page_73"></a></span>tance -of the moon from the ecliptic is about -one and a half degrees.</p> - -<p>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.</p> - -<p>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<span class="pagenum" title="74"><a name="Page_74" id="Page_74"></a></span> -that are just ninety degrees from the pole. -The word equinox is derived from <i>equus</i> -(equal) and <i>nox</i> (night), and when the sun -is at the equinoxes the days and nights are -of equal length.</p> - -<p>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.</p> - -<p>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<span class="pagenum" title="75"><a name="Page_75" id="Page_75"></a></span> -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.</p> - -<p>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.<a id="FNanchor_1" href="#Footnote_1" class="fnanchor">1</a></p> - -<p>The twelve constellations of the zodiac -are as follows:</p> - -<p><span class="pagenum" title="76"><a name="Page_76" id="Page_76"></a></span></p> - -<p class="ml2em"> -Pisces, the Fishes.<br /> -Aries, the Ram.<br /> -Taurus, the Bull.<br /> -Gemini, the Twins.<br /> -Cancer, the Crab.<br /> -Leo, the Lion.<br /> -Virgo, the Virgin.<br /> -Libra, the Scales or Balance.<br /> -Scorpio, the Scorpion.<br /> -Sagittarius, the Archer.<br /> -Capricornus, the Goat.<br /> -Aquarius, the Water-Carrier.<br /> -</p> - -<p>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.</p> - -<p>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<span class="pagenum" title="77"><a name="Page_77" id="Page_77"></a></span> -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.</p> - -<div class="figcenter" style="width: 600px;"> -<a href="images/i_085_086_full.jpg"><img src="images/i_085_086.jpg" width="600" height="162" alt="" title="Click to see hi-res image – enlarge window if necessary." /></a> -<div><p class="tac">MAP SHOWING THE CONSTELLATIONS OF THE ZODIAC AND -THE LINE OF THE ECLIPTIC RUNNING THROUGH THEM</p> - -<p class="tac">The paths of all the planets, save one, lie always within three -degrees of the ecliptic.</p></div> -</div> - -<p>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.</p> - -<p>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.</p> - - -<h3>ARIES</h3> - -<p>Aries is best seen in the autumn when the -sun is in the opposite side of the heavens. It<span class="pagenum" title="78"><a name="Page_78" id="Page_78"></a></span> -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.</p> - -<p>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,<span class="pagenum" title="79"><a name="Page_79" id="Page_79"></a></span> -and setting earlier each evening until the -sun blots it out. From this constellation -the ecliptic runs into Taurus, the third -zodiacal constellation.</p> - - -<h3>TAURUS</h3> - -<p>This constellation may be identified by -the brilliant first-magnitude star Aldebaran,<a id="FNanchor_2" href="#Footnote_2" class="fnanchor">2</a> -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.</p> - -<p>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<span class="pagenum" title="80"><a name="Page_80" id="Page_80"></a></span> -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.</p> - -<p>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<span class="pagenum" title="81"><a name="Page_81" id="Page_81"></a></span> -Taurus, and from there on into the fourth -constellation.</p> - - -<h3>GEMINI</h3> - -<p>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.<a id="FNanchor_3" href="#Footnote_3" class="fnanchor">3</a> 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<span class="pagenum" title="82"><a name="Page_82" id="Page_82"></a></span> -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.</p> - - -<h3>CANCER</h3> - -<p>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.</p> - - -<h3>LEO</h3> - -<p>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<span class="pagenum" title="83"><a name="Page_83" id="Page_83"></a></span> -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.</p> - -<p>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.</p> - -<p><span class="pagenum" title="84"><a name="Page_84" id="Page_84"></a></span></p> - - -<h3>VIRGO</h3> - -<p>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,<a id="FNanchor_4" href="#Footnote_4" class="fnanchor">4</a> 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.</p> - -<p>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.</p> - -<p>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<span class="pagenum" title="85"><a name="Page_85" id="Page_85"></a></span> -the evening. In October it sets at about -the same time as the sun.</p> - -<p>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.</p> - - -<h3>LIBRA</h3> - -<p>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.</p> - -<p>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.</p> - - -<h3>SCORPIO</h3> - -<p>It is a joy to know Scorpio, quite aside -from its connection with the path of the<span class="pagenum" title="86"><a name="Page_86" id="Page_86"></a></span> -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.</p> - -<p>Scorpio may be best identified by its brilliant -deep-red star Antares,<a id="FNanchor_5" href="#Footnote_5" class="fnanchor">5</a> 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.</p> - -<p>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<span class="pagenum" title="87"><a name="Page_87" id="Page_87"></a></span> -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.</p> - -<p>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.</p> - - -<h3>SAGITTARIUS</h3> - -<p>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 dis<span class="pagenum" title="88"><a name="Page_88" id="Page_88"></a></span>tinguished -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.</p> - - -<h3>CAPRICORNUS AND AQUARIUS</h3> - -<p>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.</p> - -<p>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<span class="pagenum" title="89"><a name="Page_89" id="Page_89"></a></span> -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.</p> - -<p>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.</p> - -<p>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.</p> - -<p>The <i>signs</i> of the zodiac are somewhat different -from the constellations. They are -simply twelve equal divisions of thirty de<span class="pagenum" title="90"><a name="Page_90" id="Page_90"></a></span>grees -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 -<i>signs</i> of the zodiac, and not with the <i>constellations</i>. -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 bear<span class="pagenum" title="91"><a name="Page_91" id="Page_91"></a></span>ing -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.</p> - -<p>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:</p> - - - -<div class="center"> -<table border="0" cellpadding="0" cellspacing="0" summary="signs of the zodiac"> -<tr><td class="tal"></td><td class="tar"></td><td class="tal"><span class="lowercase smcap">SIGN</span></td><td class="tar"></td><td class="tal"><span class="lowercase smcap ilb">CONSTELLATION</span></td></tr> -<tr><td class="tal pl2hi pt1" rowspan="3">Spring<br />signs</td><td class="tar pt1 vab" rowspan="3"> <img src="images/45x6bl.png" width="6" height="45" alt="" /></td><td class="tal pt1">Aries</td><td class="tar pt1">(♈)</td><td class="tal pl2 pt1">Pisces</td></tr> -<tr><td class="tal">Taurus</td><td class="tar">(♉)</td><td class="tal pl2">Aries</td></tr> -<tr><td class="tal">Gemini</td><td class="tar">(♊)</td><td class="tal pl2">Taurus</td></tr> -<tr><td class="tal pl2hi pt1" rowspan="3">Summer<br />signs</td><td class="tar pt1 vab" rowspan="3"> <img src="images/45x6bl.png" width="6" height="45" alt="" /></td><td class="tal pt1">Cancer</td><td class="tar pt1">(♋)</td><td class="tal pl2 pt1">Gemini</td></tr> -<tr><td class="tal">Leo</td><td class="tar">(♌)</td><td class="tal pl2">Cancer</td></tr> -<tr><td class="tal">Virgo</td><td class="tar">(♍)</td><td class="tal pl2">Leo</td></tr> -<tr><td class="tal pl2hi pt1" rowspan="3">Autumn<br />signs</td><td class="tar pt1 vab" rowspan="3"> <img src="images/45x6bl.png" width="6" height="45" alt="" /></td><td class="tal pt1">Libra</td><td class="tar pt1">(♎)</td><td class="tal pl2 pt1">Virgo</td></tr> -<tr><td class="tal">Scorpio</td><td class="tar">(♏)</td><td class="tal pl2">Libra</td></tr> -<tr><td class="tal">Sagittarius</td><td class="tar">(♐)</td><td class="tal pl2">Scorpio</td></tr> -<tr><td class="tal pl2hi pt1" rowspan="3">Winter<br />signs</td><td class="tar pt1 vab" rowspan="3"> <img src="images/45x6bl.png" width="6" height="45" alt="" /></td><td class="tal pt1">Capricornus</td><td class="tar pt1"> (♑)</td><td class="tal pl2 pt1">Sagittarius</td></tr> -<tr><td class="tal">Aquarius</td><td class="tar">(♒)</td><td class="tal pl2">Capricornus</td></tr> -<tr><td class="tal">Pisces</td><td class="tar">(♓)</td><td class="tal pl2">Aquarius<a id="FNanchor_6" href="#Footnote_6" class="fnanchor">6</a></td></tr> -</table></div> - - - -<p><span class="pagenum hide" title="92"><a name="Page_92" id="Page_92"></a></span></p> - -<hr class="chap" /> -<p><span class="pagenum" title="93"><a name="Page_93" id="Page_93"></a></span></p> - - - -<h2>X</h2> - -<h3>MERCURY</h3> - - -<p>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.</p> - -<p>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<span class="pagenum" title="94"><a name="Page_94" id="Page_94"></a></span> -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.</p> - -<p>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 tele<span class="pagenum" title="95"><a name="Page_95" id="Page_95"></a></span>scope, -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.</p> - - -<h3>WHEN AND WHERE TO FIND MERCURY</h3> - -<p>Mercury is never more than twenty-eight -degrees from the sun, and is bright<span class="pagenum" title="96"><a name="Page_96" id="Page_96"></a></span>est -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.</p> - -<p>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<span class="pagenum" title="97"><a name="Page_97" id="Page_97"></a></span> -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.</p> - -<p>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<span class="pagenum" title="98"><a name="Page_98" id="Page_98"></a></span> -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.</p> - -<p>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.</p> - -<p><span class="pagenum" title="99"><a name="Page_99" id="Page_99"></a></span></p> - -<p>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.</p> - -<p>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<span class="pagenum" title="100"><a name="Page_100" id="Page_100"></a></span> -before dawn during the first cool mornings -of autumn.</p> - -<p>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 <i>Nautical Almanac</i>. From -there it finds its way into all almanacs, so -it is easy of access to any one.</p> - -<p>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.</p> - -<p>The next seven eastern and western elongations -of Mercury occurring after the publication -of this book are as follows:</p> - - -<div class="center"> -<table border="0" cellpadding="0" cellspacing="0" summary=""> -<tr><td class="tar"></td><td class="tal">Eastern Elongation</td><td class="tar"></td><td class="tal">Western Elongation</td></tr> -<tr><td class="tar"></td><td class="tal"> (Evening Star).</td><td class="tar"></td><td class="tal"> (Morning Star).</td></tr> -<tr><td class="tar">18 </td><td class="tal">November, 1912.</td><td class="tar">27 </td><td class="tal">December, 1912.</td></tr> -<tr><td class="tar">10 </td><td class="tal">March, 1913.</td><td class="tar">24 </td><td class="tal">April, 1913.</td></tr> -<tr><td class="tar"></td><td class="tal">(Favorable for viewing.) </td></tr> -<tr><td class="tar">7 </td><td class="tal">July, 1913.</td><td class="tar">22 </td><td class="tal">August, 1913.</td></tr> -<tr><td class="tar"></td><td class="tal"></td><td class="tar"></td><td class="tal">(Favorable for viewing.)</td></tr> -<tr><td class="tar">1 </td><td class="tal">November, 1913.</td><td class="tar">10 </td><td class="tal">December, 1913.</td></tr> -<tr><td class="tar">22 </td><td class="tal">February, 1914.</td><td class="tar">6 </td><td class="tal">April, 1914.</td></tr> -<tr><td class="tar"></td><td class="tal">(Favorable for viewing.) </td></tr> -<tr><td class="tar">18 </td><td class="tal">June, 1914.</td><td class="tar">5 </td><td class="tal">August, 1914.</td></tr> -<tr><td class="tar"></td><td class="tal"></td><td class="tar"></td><td class="tal">(Favorable for viewing.)</td></tr> -<tr><td class="tar">15 </td><td class="tal">October, 1914.</td><td class="tar">23 </td><td class="tal">November, 1914.</td></tr> -</table></div> - -<p><span class="pagenum" title="101"><a name="Page_101" id="Page_101"></a></span></p> - - -<h3>DISTANCE AND BRIGHTNESS</h3> - -<p>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<span class="pagenum" title="102"><a name="Page_102" id="Page_102"></a></span> -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.</p> - -<p>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.</p> - -<p>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<span class="pagenum" title="103"><a name="Page_103" id="Page_103"></a></span> -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.</p> - -<p>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.</p> - -<p>These phases cannot be seen with the naked -eye, but it requires only a small telescope -to show them, and a very charming little<span class="pagenum" title="104"><a name="Page_104" id="Page_104"></a></span> -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.</p> - -<p>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.</p> - -<p>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<span class="pagenum" title="105"><a name="Page_105" id="Page_105"></a></span> -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.</p> - -<p>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.</p> - -<p><span class="pagenum" title="106"><a name="Page_106" id="Page_106"></a></span></p> - - -<h3>MERCURY’S SIZE AND THE CONSEQUENCES OF IT</h3> - -<p>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.</p> - -<p>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.</p> - -<p>The nucleus that was detached from the -great spiral, or the portion of nebula that was<span class="pagenum" title="107"><a name="Page_107" id="Page_107"></a></span> -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.</p> - -<p>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<span class="pagenum" title="108"><a name="Page_108" id="Page_108"></a></span> -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.</p> - -<p>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<span class="pagenum" title="109"><a name="Page_109" id="Page_109"></a></span> -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.</p> - -<p>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<span class="pagenum" title="110"><a name="Page_110" id="Page_110"></a></span> -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.</p> - -<p><span class="pagenum" title="111"><a name="Page_111" id="Page_111"></a></span></p> - - -<h3>WHAT THE SUN DOES FOR MERCURY</h3> - -<p>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.</p> - -<p>But, even if this is the predicament into -which Mercury has come, the planet is prob<span class="pagenum" title="112"><a name="Page_112" id="Page_112"></a></span>ably -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.</p> - -<p>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<span class="pagenum" title="113"><a name="Page_113" id="Page_113"></a></span> -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.</p> - -<p>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<span class="pagenum" title="114"><a name="Page_114" id="Page_114"></a></span> -three-eighths are always dark. It is this -dark, cold side that is turned toward us -when Mercury is nearest to us.</p> - -<p>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.</p> - -<p>Stiffened and frozen is what the dark side<span class="pagenum" title="115"><a name="Page_115" id="Page_115"></a></span> -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.</p> - -<p>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.</p> - -<p>Stability, at least, is a quality of the hot<span class="pagenum" title="116"><a name="Page_116" id="Page_116"></a></span> -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.</p> - -<p>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 sea<span class="pagenum" title="117"><a name="Page_117" id="Page_117"></a></span>sons -fulfils the expectation with little satisfaction.</p> - -<p>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.</p> - -<p>Moreover, it is believed that the axis on -which Mercury rotates stands perpendicular<span class="pagenum" title="118"><a name="Page_118" id="Page_118"></a></span> -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.</p> - -<p>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,<span class="pagenum" title="119"><a name="Page_119" id="Page_119"></a></span> -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.</p> - -<p>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?</p> - - -<h3>TRANSITS</h3> - -<p>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<span class="pagenum" title="120"><a name="Page_120" id="Page_120"></a></span> -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.</p> - -<p>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<span class="pagenum" title="121"><a name="Page_121" id="Page_121"></a></span> -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.</p> - -<p>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.</p> - -<hr class="chap" /> - -<p><span class="pagenum" title="122"><a name="Page_122" id="Page_122"></a></span></p> - - - - -<h2>XI</h2> - -<h3>VENUS</h3> - - -<p>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.</p> - -<p>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 suffi<span class="pagenum" title="123"><a name="Page_123" id="Page_123"></a></span>ciently -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.</p> - -<p>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.</p> - - -<h3>WHEN AND WHERE TO SEE VENUS</h3> - -<p>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<span class="pagenum" title="124"><a name="Page_124" id="Page_124"></a></span> -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.</p> - -<p>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<span class="pagenum" title="125"><a name="Page_125" id="Page_125"></a></span> -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.</p> - -<p>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<span class="pagenum" title="126"><a name="Page_126" id="Page_126"></a></span> -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.</p> - -<p>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.</p> - -<p>This is a brief outline of a typical journey -of Venus through one synodic revolution.<span class="pagenum" title="127"><a name="Page_127" id="Page_127"></a></span> -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.</p> - -<p>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<span class="pagenum" title="128"><a name="Page_128" id="Page_128"></a></span> -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.</p> - -<p>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.</p> - -<p>The synodic period of Venus is nearly five -hundred and eighty-four days, or a little -more than one year and seven months.<span class="pagenum" title="129"><a name="Page_129" id="Page_129"></a></span> -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.</p> - -<p>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.</p> - -<p><span class="pagenum" title="130"><a name="Page_130" id="Page_130"></a></span></p> - -<div class="blockquot2"> -<p class="tac mb03"> -1913—1921—1929—1937 -</p> - -<p>Greatest eastern elongation, February 12th. Inferior -conjunction, April 24th. Greatest western -elongation, July 3d.</p> - -<hr class="r40" /> - -<p class="tac mb03"> -1914—1922—1930—1938 -</p> - -<p>Superior conjunction, February 11th. Greatest -eastern elongation, September 17th. Inferior conjunction, -November 27th.</p> - -<hr class="r40" /> - -<p class="tac mb03"> -1915—1923—1931 -</p> - -<p>Greatest western elongation, February 8th. Superior -conjunction, September 14th.</p> - -<hr class="r40" /> - -<p class="tac mb03"> -1916—1924—1932 -</p> - -<p>Greatest eastern elongation, April 26th. Inferior -conjunction, July 5th. Greatest western elongation, -September 14th.</p> - -<hr class="r40" /> - -<p class="tac mb03"> -1917—1925—1933 -</p> - -<p>Superior conjunction, April 28th. Greatest eastern -elongation, December 2d.</p> - -<hr class="r40" /> - -<p class="tac mb03"> -1918—1926—1934 -</p> - -<p>Inferior conjunction, February 11th. Greatest eastern -elongation, April 22d. Superior conjunction, -November 25th.</p> - -<hr class="r40" /> - -<p class="tac mb03"> -1919—1927—1935 -</p> - -<p>Greatest eastern elongation, July 6th. Inferior -conjunction, September 14th. Greatest western elongation, -November 25th.</p> - -<hr class="r40" /> - -<p class="tac mb03"> -1920—1928 -</p> - -<p>Superior conjunction, July 5th.</p> - -<hr class="r40" /> - -</div> - -<p><span class="pagenum" title="131"><a name="Page_131" id="Page_131"></a></span></p> - -<p>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.</p> - - -<h3>DISTANCE AND BRILLIANCY</h3> - -<p>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<span class="pagenum" title="132"><a name="Page_132" id="Page_132"></a></span> -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.</p> - -<p>Usually at inferior conjunction Venus is a -little more than twenty-five million miles -from the earth. At her nearest possible ap<span class="pagenum" title="133"><a name="Page_133" id="Page_133"></a></span>proach -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.</p> - -<p>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 pre<span class="pagenum" title="134"><a name="Page_134" id="Page_134"></a></span>sents; -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.</p> - -<p>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<span class="pagenum" title="135"><a name="Page_135" id="Page_135"></a></span> -cause much more of a shadow than we ever -get from the light of Venus.</p> - -<p>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.</p> - -<p>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<span class="pagenum" title="136"><a name="Page_136" id="Page_136"></a></span> -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.</p> - -<div class="figcenter" style="width: 400px;"> -<img src="images/i_148.jpg" width="400" height="384" alt="" /> -<div><p class="tac">THE LOVELY CRESCENT THAT VENUS SHOWS WHEN TO -OUR VIEW SHE IS AT HER GREATEST BRILLIANCY</p> - -<p class="tac">This remarkable photograph was made at the Yerkes Observatory -by E. E. Barnard.</p></div> -</div> - -<p>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<span class="pagenum" title="137"><a name="Page_137" id="Page_137"></a></span> -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.</p> - -<div class="figcenter" style="width: 370px;"> -<img src="images/i_150.jpg" width="370" height="221" alt="" /> -<div><p class="tac">RELATIVE APPARENT SIZE OF VENUS AT DIFFERENT -PHASES OF ILLUMINATION</p> - -<p>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.</p></div> -</div> - -<p><span class="pagenum" title="138"><a name="Page_138" id="Page_138"></a></span></p> - - -<h3>VENUS’S LIKENESS TO THE EARTH</h3> - -<p>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 hun<span class="pagenum" title="139"><a name="Page_139" id="Page_139"></a></span>dred -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.</p> - -<p>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.</p> - -<p>The earth has a moon, and Venus has none;<span class="pagenum" title="140"><a name="Page_140" id="Page_140"></a></span> -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.</p> - - -<h3>ATMOSPHERE, DAY AND NIGHT, AND SEASONS</h3> - -<p>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,<span class="pagenum" title="141"><a name="Page_141" id="Page_141"></a></span> -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.</p> - -<p>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<span class="pagenum" title="142"><a name="Page_142" id="Page_142"></a></span> -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.</p> - -<p>The truth may be that, owing to the density -of her atmosphere, the surface of Venus<span class="pagenum" title="143"><a name="Page_143" id="Page_143"></a></span> -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.</p> - -<p class="mt2em">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-<span class="pagenum" title="144"><a name="Page_144" id="Page_144"></a></span>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).</p> - -<p>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<span class="pagenum" title="145"><a name="Page_145" id="Page_145"></a></span> -them some tempering effects on the climate, -as we know such currents do here on the -earth.</p> - -<p>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.</p> - -<p>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<span class="pagenum" title="146"><a name="Page_146" id="Page_146"></a></span> -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.</p> - -<p><span class="pagenum" title="147"><a name="Page_147" id="Page_147"></a></span></p> - -<p>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.</p> - - -<h3>TRANSITS</h3> - -<p>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<span class="pagenum" title="148"><a name="Page_148" id="Page_148"></a></span> -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.</p> - -<p>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.</p> - -<p>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 dis<span class="pagenum" title="149"><a name="Page_149" id="Page_149"></a></span>tance -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.</p> - -<p class="mt2em">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.</p> - -<p>The symbol of Venus is ♀, a figure which<span class="pagenum" title="150"><a name="Page_150" id="Page_150"></a></span> -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.</p> - -<hr class="chap" /> - -<p><span class="pagenum" title="151"><a name="Page_151" id="Page_151"></a></span></p> - - - - -<h2>XII</h2> - -<h3>MARS</h3> - - -<p>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.</p> - -<p>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<span class="pagenum" title="152"><a name="Page_152" id="Page_152"></a></span> -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.</p> - - -<h3>HOW TO IDENTIFY MARS</h3> - -<p>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<span class="pagenum" title="153"><a name="Page_153" id="Page_153"></a></span> -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.</p> - -<div class="figcenter" style="width: 675px;"> -<img src="images/i_166.jpg" width="675" height="400" alt="" /> -<div><p class="tac">THE TWO PHASES OF MARS</p> - -<p class="tac">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.</p></div> -</div> - -<p>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.<span class="pagenum" title="154"><a name="Page_154" id="Page_154"></a></span> -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.</p> - -<p>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.</p> - -<p>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.</p> - -<p><span class="pagenum" title="155"><a name="Page_155" id="Page_155"></a></span></p> - - -<h3>WHEN AND WHERE MARS MAY BE SEEN</h3> - -<p>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.</p> - -<p>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,<span class="pagenum" title="156"><a name="Page_156" id="Page_156"></a></span> -he can easily be observed at any time in the -night without any neck-breaking postures.</p> - -<p>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.</p> - -<p>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<span class="pagenum" title="157"><a name="Page_157" id="Page_157"></a></span> -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.</p> - -<p>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.</p> - -<p>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,<span class="pagenum" title="158"><a name="Page_158" id="Page_158"></a></span> -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.</p> - -<p>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<span class="pagenum" title="159"><a name="Page_159" id="Page_159"></a></span> -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.</p> - -<p>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<span class="pagenum" title="160"><a name="Page_160" id="Page_160"></a></span> -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.</p> - -<p>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.</p> - -<p>The next oppositions will take place the -last week in October, 1926, in December,<span class="pagenum" title="161"><a name="Page_161" id="Page_161"></a></span> -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.</p> - -<p>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.</p> - -<p>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<span class="pagenum" title="162"><a name="Page_162" id="Page_162"></a></span> -it, and which has been explained in the chapters -on the movements of the planets.</p> - - -<h3>SIZE, ATMOSPHERE, AND TEMPERATURE</h3> - -<p>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.</p> - -<p>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<span class="pagenum" title="163"><a name="Page_163" id="Page_163"></a></span> -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.</p> - -<p><span class="pagenum" title="164"><a name="Page_164" id="Page_164"></a></span></p> - -<p>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 tempera<span class="pagenum" title="165"><a name="Page_165" id="Page_165"></a></span>ture -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.</p> - -<p>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 varia<span class="pagenum" title="166"><a name="Page_166" id="Page_166"></a></span>tions. -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.</p> - - -<h3>DISTANCE AND BRILLIANCY</h3> - -<p>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.</p> - -<p><span class="pagenum" title="167"><a name="Page_167" id="Page_167"></a></span></p> - -<p>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.</p> - -<p>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 dis<span class="pagenum" title="168"><a name="Page_168" id="Page_168"></a></span>tant. -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.</p> - -<p>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.</p> - -<p>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<span class="pagenum" title="169"><a name="Page_169" id="Page_169"></a></span> -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<span class="pagenum" title="170"><a name="Page_170" id="Page_170"></a></span> -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.</p> - -<div class="figcenter" style="width: 330px;"> -<img src="images/i_184.jpg" width="330" height="147" alt="" /> -<div><p class="tac">MARS: DIFFERENCE IN ITS APPARENT SIZE AT ITS NEAREST, -MIDDLE, AND FARTHEST DISTANCE FROM THE EARTH</p> - -<p>Mars appears fifty times brighter when nearest than when farthest -away.</p></div> -</div> - - -<h3>DAY AND NIGHT, AND SEASONS</h3> - -<p>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.</p> - -<p>The axis of Mars is inclined to its orbit<span class="pagenum" title="171"><a name="Page_171" id="Page_171"></a></span> -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<span class="pagenum" title="172"><a name="Page_172" id="Page_172"></a></span> -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.</p> - - -<h3>SURFACE ASPECTS OF MARS</h3> - -<p>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<span class="pagenum" title="173"><a name="Page_173" id="Page_173"></a></span> -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.</p> - -<p>There are recorded observations made of -Mars as early as 272 <span class="lowercase smcap">B.C.</span>, 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 sur<span class="pagenum" title="174"><a name="Page_174" id="Page_174"></a></span>face -does, in fact, show first one and then -the other of them predominating.</p> - -<p>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 <i>canalli</i>, 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<span class="pagenum" title="175"><a name="Page_175" id="Page_175"></a></span> -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.</p> - -<p>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<span class="pagenum" title="176"><a name="Page_176" id="Page_176"></a></span> -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.</p> - -<p>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:</p> - -<p>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,<span class="pagenum" title="177"><a name="Page_177" id="Page_177"></a></span> -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.”</p> - -<p>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<span class="pagenum" title="178"><a name="Page_178" id="Page_178"></a></span> -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.</p> - -<p>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.</p> - -<p>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 view<span class="pagenum" title="179"><a name="Page_179" id="Page_179"></a></span>ing -“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.</p> - -<p>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<span class="pagenum" title="180"><a name="Page_180" id="Page_180"></a></span> -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.</p> - - -<h3>THE SATELLITES OF MARS</h3> - -<p>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<span class="pagenum" title="181"><a name="Page_181" id="Page_181"></a></span> -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.</p> - -<p>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<span class="pagenum" title="182"><a name="Page_182" id="Page_182"></a></span> -all parts of Mars, but gives very little light -to the planet—more than a thousand times -less than our moon gives us.</p> - -<p>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.</p> - -<hr class="chap" /> - -<p><span class="pagenum" title="183"><a name="Page_183" id="Page_183"></a></span></p> - - - - -<h2>XIII</h2> - -<h3>JUPITER</h3> - - -<p>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<span class="pagenum" title="184"><a name="Page_184" id="Page_184"></a></span> -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.</p> - -<p>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<span class="pagenum" title="185"><a name="Page_185" id="Page_185"></a></span> -plastic body to bulge at the equator, and -thus flatten at the poles.</p> - -<div class="figcenter" style="width: 420px;"> -<img src="images/i_200.jpg" width="420" height="583" alt="" /> -<div><p class="tac">JUPITER, THE MAMMOTH MEMBER OF THE SOLAR FAMILY—LARGER -THAN ALL THE OTHER PLANETS PUT TOGETHER</p> - -<p>This photograph shows the flattening at the poles and also the belts -encircling the planet. It was photographed at the Yerkes Observatory.</p></div> -</div> - -<p>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.</p> - -<p>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<span class="pagenum" title="186"><a name="Page_186" id="Page_186"></a></span> -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.</p> - - -<h3>JUPITER’S PLACE IN THE SKY</h3> - -<p>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<span class="pagenum" title="187"><a name="Page_187" id="Page_187"></a></span> -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.</p> - -<p>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<span class="pagenum" title="188"><a name="Page_188" id="Page_188"></a></span> -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.</p> - -<p>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.</p> - -<p>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;<span class="pagenum" title="189"><a name="Page_189" id="Page_189"></a></span> -and he will not be far from Antares in -1923.</p> - -<p>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.</p> - -<p>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.</p> - -<p>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<span class="pagenum" title="190"><a name="Page_190" id="Page_190"></a></span> -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.</p> - -<p>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.</p> - - -<h3>DISTANCE, LIGHT, AND HEAT</h3> - -<p>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<span class="pagenum" title="191"><a name="Page_191" id="Page_191"></a></span> -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.</p> - -<p>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<span class="pagenum" title="192"><a name="Page_192" id="Page_192"></a></span> -about forty-three minutes for light to pass -from the sun to Jupiter.</p> - -<p>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 atmos<span class="pagenum" title="193"><a name="Page_193" id="Page_193"></a></span>phere, -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.</p> - -<p>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.</p> - - -<h3>DAY AND NIGHT, SEASONS, AND ATMOSPHERE</h3> - -<p>Jupiter accomplishes one rotation in a -little less than ten hours; but, curiously -enough, all parts of the planet do not rotate<span class="pagenum" title="194"><a name="Page_194" id="Page_194"></a></span> -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.</p> - -<p>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 tem<span class="pagenum" title="195"><a name="Page_195" id="Page_195"></a></span>perature -between its perihelion and aphelion -positions.</p> - -<p>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.</p> - - -<h3>SURFACE FEATURES</h3> - -<p>Seen through a telescope, Jupiter shows -the loveliest variety of colors, with the red<span class="pagenum" title="196"><a name="Page_196" id="Page_196"></a></span>dish -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.</p> - -<p>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.</p> - -<p>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 com<span class="pagenum" title="197"><a name="Page_197" id="Page_197"></a></span>paratively -small telescope. Sometimes as -many as twenty or thirty belts have been -seen at one time. All of them are parallel -with the equator.</p> - -<p>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.</p> - -<p>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<span class="pagenum" title="198"><a name="Page_198" id="Page_198"></a></span> -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.</p> - -<p>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.</p> - -<p>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<span class="pagenum" title="199"><a name="Page_199" id="Page_199"></a></span> -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.</p> - - -<h3>JUPITER’S SYSTEM OF SATELLITES</h3> - -<p>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).</p> - -<p><span class="pagenum" title="200"><a name="Page_200" id="Page_200"></a></span></p> - -<p>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.</p> - -<p>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<span class="pagenum" title="201"><a name="Page_201" id="Page_201"></a></span> -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.</p> - -<p>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<span class="pagenum" title="202"><a name="Page_202" id="Page_202"></a></span> -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.”</p> - -<p>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<span class="pagenum" title="203"><a name="Page_203" id="Page_203"></a></span> -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.</p> - -<p>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.</p> - -<p>The eighth satellite, discovered in January, -1908, is certainly no larger, and is perhaps -still more tiny, than the sixth and the seventh,<span class="pagenum" title="204"><a name="Page_204" id="Page_204"></a></span> -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.</p> - -<p>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<span class="pagenum" title="205"><a name="Page_205" id="Page_205"></a></span> -in the <i>Nautical Almanac</i>, and it is through -observations of them that chronometers are -corrected at sea.</p> - -<p>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.</p> - -<p>The symbol of Jupiter is ♃, a hieroglyph -for the eagle, which was the bird of Jove.</p> - -<hr class="chap" /> - -<p><span class="pagenum" title="206"><a name="Page_206" id="Page_206"></a></span></p> - - - - -<h2>XIV</h2> - -<h3>SATURN</h3> - - -<p>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.</p> - -<p>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 some<span class="pagenum" title="207"><a name="Page_207" id="Page_207"></a></span>times -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.</p> - -<p>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.</p> - -<p>For nearly six months each year Saturn -shines as an evening star, and, returning each<span class="pagenum" title="208"><a name="Page_208" id="Page_208"></a></span> -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.</p> - -<p>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.</p> - -<p>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<span class="pagenum" title="209"><a name="Page_209" id="Page_209"></a></span> -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.</p> - - -<h3>AROUND ONE CIRCUIT OF THE SKIES WITH -SATURN</h3> - -<p>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.</p> - -<p><span class="pagenum" title="210"><a name="Page_210" id="Page_210"></a></span></p> - -<p>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.</p> - -<p>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.</p> - -<p>Saturn will then continue to move across<span class="pagenum" title="211"><a name="Page_211" id="Page_211"></a></span> -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.</p> - -<p>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 be<span class="pagenum" title="212"><a name="Page_212" id="Page_212"></a></span>tween -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.</p> - -<p>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.</p> - -<p>Saturn is brightest when he is in Taurus, not<span class="pagenum" title="213"><a name="Page_213" id="Page_213"></a></span> -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.</p> - - -<h3>DISTANCE AND SIZE</h3> - -<p>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.</p> - -<p>His average distance from the earth at -opposition is seven hundred and ninety-four -million miles, but at the most favorable<span class="pagenum" title="214"><a name="Page_214" id="Page_214"></a></span> -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.</p> - -<p>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.</p> - -<p>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<span class="pagenum" title="215"><a name="Page_215" id="Page_215"></a></span> -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.</p> - - -<h3>SURFACE ASPECTS AND CONSTITUTION</h3> - -<p>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.</p> - -<p>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<span class="pagenum" title="216"><a name="Page_216" id="Page_216"></a></span> -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.</p> - -<p>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.</p> - -<p>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<span class="pagenum" title="217"><a name="Page_217" id="Page_217"></a></span> -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.</p> - - -<h3>DAY AND NIGHT</h3> - -<p>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.</p> - -<p>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<span class="pagenum" title="218"><a name="Page_218" id="Page_218"></a></span> -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.</p> - - -<h3>THE RINGS AND MOONS OF SATURN</h3> - -<p>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, to<span class="pagenum" title="219"><a name="Page_219" id="Page_219"></a></span>gether -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.</p> - -<p>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 tele<span class="pagenum" title="220"><a name="Page_220" id="Page_220"></a></span>scope, -was that he was nestling in a concave -body of light—an appearance that is intensified -by his extreme flatness at the poles.</p> - -<p>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 <i>must</i> be composed -of separate bodies; the spectroscope -shows that they <i>are</i>; and it has recently been -thought that they have even been <i>seen</i> to be -so through a telescope.</p> - -<p>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<span class="pagenum" title="221"><a name="Page_221" id="Page_221"></a></span> -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.</p> - -<div class="figcenter" style="width: 668px;"> -<img src="images/i_238.jpg" width="668" height="400" alt="" /> -<div><p class="tac">SATURN AND ITS RINGS</p> - -<p class="tac">Photographed at Mt. Wilson by E. E. Barnard, the six exposures being made on one plate.</p></div> -</div> - -<p>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.</p> - -<p>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<span class="pagenum" title="222"><a name="Page_222" id="Page_222"></a></span> -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.</p> - -<p>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<span class="pagenum" title="223"><a name="Page_223" id="Page_223"></a></span> -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.</p> - -<p>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.</p> - -<p><span class="pagenum" title="224"><a name="Page_224" id="Page_224"></a></span></p> - - -<h3>SEASONS</h3> - -<p>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.</p> - -<p>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.</p> - -<hr class="chap" /> - -<p><span class="pagenum" title="225"><a name="Page_225" id="Page_225"></a></span></p> - - - - -<h2>XV</h2> - -<h3>URANUS</h3> - - -<p>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 <i>discovered</i>—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<span class="pagenum" title="226"><a name="Page_226" id="Page_226"></a></span> -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.</p> - -<p>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.</p> - -<p>George III. was then King of England, -and the loyal Herschel called the planet -<i>Georgium Sidus</i> in honor of that monarch.<span class="pagenum" title="227"><a name="Page_227" id="Page_227"></a></span> -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.</p> - -<p>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 ♅.</p> - -<p>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<span class="pagenum" title="228"><a name="Page_228" id="Page_228"></a></span> -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.</p> - -<p>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.</p> - -<p>Since Uranus was discovered he has made -one circuit of the skies, which he finished in<span class="pagenum" title="229"><a name="Page_229" id="Page_229"></a></span> -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.</p> - -<p>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.</p> - -<p>Uranus is twice as far from the sun as<span class="pagenum" title="230"><a name="Page_230" id="Page_230"></a></span> -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.</p> - -<p><span class="pagenum" title="231"><a name="Page_231" id="Page_231"></a></span></p> - -<p>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.</p> - -<p>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<span class="pagenum" title="232"><a name="Page_232" id="Page_232"></a></span> -stage of development than any of the terrestrial -planets.</p> - -<p>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.</p> - -<p>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<span class="pagenum" title="233"><a name="Page_233" id="Page_233"></a></span> -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.</p> - -<hr class="chap" /> - -<p><span class="pagenum" title="234"><a name="Page_234" id="Page_234"></a></span></p> - - - - -<h2>XVI</h2> - -<h3>NEPTUNE</h3> - - -<p>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.</p> - -<p>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<span class="pagenum" title="235"><a name="Page_235" id="Page_235"></a></span> -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.</p> - -<p>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.</p> - -<p>The discovery of Neptune in 1846 created -truly a sensation in astronomical circles. -And, unlike most sensational happenings, it<span class="pagenum" title="236"><a name="Page_236" id="Page_236"></a></span> -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.<a id="FNanchor_7" href="#Footnote_7" class="fnanchor">7</a> 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.</p> - -<p>The result of Adams’s work was announced -to the Astronomer Royal in England in the<span class="pagenum" title="237"><a name="Page_237" id="Page_237"></a></span> -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!</p> - -<p>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<span class="pagenum" title="238"><a name="Page_238" id="Page_238"></a></span> -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.</p> - -<p>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<span class="pagenum" title="239"><a name="Page_239" id="Page_239"></a></span> -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.</p> - -<p>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.</p> - -<p>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<span class="pagenum" title="240"><a name="Page_240" id="Page_240"></a></span> -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.</p> - -<p>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.</p> - -<p>Of the time of Neptune’s rotation on its -axis very little is known. That little, however, -indicates a slower rotation than the<span class="pagenum" title="241"><a name="Page_241" id="Page_241"></a></span> -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.</p> - -<p>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 posi<span class="pagenum" title="242"><a name="Page_242" id="Page_242"></a></span>tion -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.</p> - -<p>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.</p> - -<p>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<span class="pagenum" title="243"><a name="Page_243" id="Page_243"></a></span> -is so slight during any year that the change -of direction is hardly noticeable.</p> - -<p>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.</p> - -<hr class="chap" /> - -<p><span class="pagenum" title="244"><a name="Page_244" id="Page_244"></a></span></p> - - - - -<h2>XVII</h2> - -<h3>THE LITTLE PLANETS, OR THE ASTEROIDS</h3> - - -<p>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.</p> - -<p>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.</p> - -<p>It was early noted that, except in one instance, -the planets seemed to show in their<span class="pagenum" title="245"><a name="Page_245" id="Page_245"></a></span> -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:</p> - - -<div class="center"> -<table border="0" cellpadding="2" cellspacing="0" summary=""> -<tr><td class="tar pl1">0</td><td class="tar pl1">3</td><td class="tar pl1">6</td><td class="tar pl1">12</td><td class="tar pl1">24</td><td class="tar pl1">48</td><td class="tar pl1">96</td><td class="tar pl1">192</td><td class="tar pl1">384</td></tr> -<tr class="bb"><td class="tar pl1">4</td><td class="tar pl1">4</td><td class="tar pl1">4</td><td class="tar pl1">4</td><td class="tar pl1">4</td><td class="tar pl1">4</td><td class="tar pl1">4</td><td class="tar pl1">4</td><td class="tar pl1">4</td></tr> -<tr><td class="tar pl1">4</td><td class="tar pl1">7</td><td class="tar pl1">10</td><td class="tar pl1">16</td><td class="tar pl1">28</td><td class="tar pl1">52</td><td class="tar pl1">100</td><td class="tar pl1">196</td><td class="tar pl1">388</td></tr> -</table></div> - -<p>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<span class="pagenum" title="246"><a name="Page_246" id="Page_246"></a></span> -the law, thus leaving room for another planet -to occupy the allotted position and fill out -this very beautiful progression.</p> - -<p>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.</p> - -<p>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<span class="pagenum" title="247"><a name="Page_247" id="Page_247"></a></span> -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.</p> - -<p>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<span class="pagenum" title="248"><a name="Page_248" id="Page_248"></a></span> -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.</p> - -<p>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.</p> - -<p>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<span class="pagenum" title="249"><a name="Page_249" id="Page_249"></a></span> -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.</p> - -<p>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.</p> - -<p>In 1845 another period of discovery commenced, -and has ever since continued, until<span class="pagenum" title="250"><a name="Page_250" id="Page_250"></a></span> -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.</p> - -<p>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,<span class="pagenum" title="251"><a name="Page_251" id="Page_251"></a></span> -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.</p> - -<p>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<span class="pagenum" title="252"><a name="Page_252" id="Page_252"></a></span> -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.</p> - -<p>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.</p> - -<p><span class="pagenum" title="253"><a name="Page_253" id="Page_253"></a></span></p> - -<p>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.</p> - -<p>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,<span class="pagenum" title="254"><a name="Page_254" id="Page_254"></a></span> -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.</p> - -<p>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<span class="pagenum" title="255"><a name="Page_255" id="Page_255"></a></span> -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.</p> - -<p>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<span class="pagenum" title="256"><a name="Page_256" id="Page_256"></a></span> -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.</p> - -<p>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.</p> - -<p>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<span class="pagenum" title="257"><a name="Page_257" id="Page_257"></a></span> -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.</p> - -<hr class="chap" /> - -<p><span class="pagenum" title="258"><a name="Page_258" id="Page_258"></a></span></p> - - - - -<h2>XVIII</h2> - -<h3>CONCLUSION</h3> - - -<p>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.</p> - -<p>Having considered these brilliant bodies -individually and in detail, as we have, we -ought by this time to be able to identify<span class="pagenum" title="259"><a name="Page_259" id="Page_259"></a></span> -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.</p> - -<p>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.</p> - -<p>If the planet we seek to name is nearer to<span class="pagenum" title="260"><a name="Page_260" id="Page_260"></a></span> -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.</p> - -<p>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<span class="pagenum" title="261"><a name="Page_261" id="Page_261"></a></span> -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.</p> - -<p>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<span class="pagenum" title="262"><a name="Page_262" id="Page_262"></a></span> -not wholly scientific, cannot fail to stamp -them as in some sort individuals.</p> - -<p>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.<span class="pagenum" title="263"><a name="Page_263" id="Page_263"></a></span> -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.</p> - -<p>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.</p> - -<p>If there is life on any of these outer<span class="pagenum" title="264"><a name="Page_264" id="Page_264"></a></span> -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.</p> - -<p>Of the existence of life somewhat similar -to ours on the smaller, near-by planets we -may have something nearer a reasonable con<span class="pagenum" title="265"><a name="Page_265" id="Page_265"></a></span>ception, -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<span class="pagenum" title="266"><a name="Page_266" id="Page_266"></a></span> -at least understandable by us, even if not -wholly congenial.</p> - -<p>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.</p> - -<p><span class="pagenum hide" title="267"><a name="Page_267" id="Page_267"></a></span></p> -<hr class="chap" /> - -<h2>SYMBOLS USED IN ALMANACS</h2> - - -<div class="center"> -<table class="fs100" width="350" border="0" cellpadding="2" cellspacing="0" summary=""> -<tr><td class="tar">☿ =</td><td class="tal">Mercury.</td><td class="tar"><span class="ilb">   ⚫ =</span></td><td class="tal">New Moon.</td></tr> -<tr><td class="tar">♀ =</td><td class="tal">Venus.</td><td class="tar">   ☽ =</td><td class="tal">First Quarter.</td></tr> -<tr><td class="tar">⊕ =</td><td class="tal">Earth.</td><td class="tar">   ⚪ =</td><td class="tal">Full Moon.</td></tr> -<tr><td class="tar">♂ =</td><td class="tal">Mars.</td><td class="tar">   ☾ =</td><td class="tal">Last Quarter.</td></tr> -<tr><td class="tar">♃ =</td><td class="tal">Jupiter.</td><td class="tar">   ☉ =</td><td class="tal">Sun.</td></tr> -<tr><td class="tar">♄ =</td><td class="tal">Saturn.</td><td class="tar vat" rowspan="3">   ☌ =</td><td class="tal" rowspan="3">Conjunction with the sun; or, in the case of two planets or a planet and the moon, near together.</td></tr> -<tr><td class="tar"><span class="ilb">♅ or ⛢ =</span></td><td class="tal">Uranus.</td></tr> -<tr><td class="tar">♆ =</td><td class="tal">Neptune.</td></tr> -<tr><td class="tar"></td><td class="tar"></td><td class="tar"><span class="ilb">   ☍ =</span></td><td class="tal">Opposition.</td></tr> -<tr><td class="tar"></td><td class="tar"></td><td class="tar">   □ =</td><td class="tal">Quadrature.</td></tr> -</table></div> - - -<p class="ml20pc">Examples:</p> - - -<div class="center"> -<table class="fs100" border="0" cellpadding="2" cellspacing="0" summary=""> -<tr><td class="tar">☌ ♂ ♀</td><td class="tal">= Mars and Venus near together.</td></tr> -<tr><td class="tar">☍ ♃ ☉</td><td class="tal">= Jupiter in opposition.</td></tr> -<tr><td class="tar">☌ ♃ ☉</td><td class="tal">= Jupiter in conjunction.</td></tr> -<tr><td class="tar">☌ ☿ ☉</td><td class="tal" colspan="2">Inf. = Mercury in inferior conjunction.</td></tr> -<tr><td class="tar">☌ ☿ ☉</td><td class="tal" colspan="3">Sup. = Mercury in superior conjunction.</td></tr> -<tr><td class="tar">☌ ♀ ☽</td><td class="tal">= Venus and Moon near together.</td></tr> -</table></div> - -<hr class="chap" /> - -<p><span class="pagenum" title="269"><a name="Page_269" id="Page_269"></a></span></p> - - - - -<h2>INDEX</h2> - - -<p> -Adams, <a href="#Page_236">236–238</a>.<br /> -Alcor, star in Great Dipper, <a href="#Page_105">105</a>, <a href="#Page_236">236</a>.<br /> -Aldebaran, first-magnitude star, <a href="#Page_79">79–80</a>, <a href="#Page_153">153</a>, <a href="#Page_188">188</a>, <a href="#Page_210">210</a>.<br /> -Antares, star in Scorpio, <a href="#Page_86">86</a>, <a href="#Page_153">153</a>, <a href="#Page_160">160</a>, <a href="#Page_187">187</a>, <a href="#Page_189">189</a>, <a href="#Page_209">209</a>, <a href="#Page_212">212</a>.<br /> -Aquarius, constellation of the zodiac, <a href="#Page_76">76</a>, <a href="#Page_88">88–89</a>, <a href="#Page_91">91–92</a>, <a href="#Page_187">187</a>, <a href="#Page_212">212–213</a>, <a href="#Page_221">221</a>, <a href="#Page_242">242</a>.<br /> -Arcturus, <a href="#Page_24">24</a>, <a href="#Page_84">84</a>;<br /> - color of, <a href="#Page_102">102</a>.<br /> -Ariel, satellite of Uranus, <a href="#Page_232">232–233</a>.<br /> -Aries, constellation of the zodiac, <a href="#Page_76">76–78</a>, <a href="#Page_90">90–92</a>, <a href="#Page_212">212</a>, <a href="#Page_242">242</a>.<br /> -Asteroids, <a href="#Page_244">244–257</a>.<br /> -<br /> -Bee-hive, <a href="#Page_82">82</a>, <a href="#Page_211">211–212</a>.<br /> -Bode’s law, <a href="#Page_245">245–249</a>.<br /> -Boötes, star of first magnitude, <a href="#Page_102">102</a>.<br /> -<br /> -Callisto, satellite of Jupiter, <a href="#Page_200">200</a>, <a href="#Page_205">205</a>.<br /> -Cancer, constellation of zodiac, <a href="#Page_76">76</a>, <a href="#Page_82">82</a>, <a href="#Page_91">91–92</a>, <a href="#Page_188">188</a>, <a href="#Page_211">211–212</a>.<br /> -Capella, star of first magnitude, <a href="#Page_191">191</a>.<br /> -Capricornus, one of the twelve constellations of the zodiac, <a href="#Page_76">76</a>, <a href="#Page_88">88–89</a>, <a href="#Page_91">91–92</a>, <a href="#Page_187">187</a>, <a href="#Page_212">212</a>, <a href="#Page_229">229</a>.<br /> -Cassiopeia, constellation, <a href="#Page_77">77</a>.<br /> -Castor and Pollux, <a href="#Page_81">81</a>, <a href="#Page_188">188</a>, <a href="#Page_211">211</a>, <a href="#Page_242">242–243</a>.<br /> -Ceres, first planetoid discovered, <a href="#Page_251">251</a>, <a href="#Page_253">253</a>.<br /> -Constellations of the zodiac, <a href="#Page_75">75–92</a>.<br /> -<br /> -Deimos, satellite of Mars, <a href="#Page_180">180–181</a>.<br /> -Dione, satellite of Saturn, <a href="#Page_222">222</a>.<br /> -<br /> -Earth, relation to planets, <a href="#Page_11">11–15</a>, <a href="#Page_19">19</a>;<br /> - nearness to sun, <a href="#Page_19">19</a>;<br /> - terrestrial planet, <a href="#Page_41">41</a>;<br /> - movement of, <a href="#Page_51">51</a>;<br /> - position in regard to Mercury, <a href="#Page_120">120–121</a>;<br /> - likeness to Venus, <a href="#Page_138">138–140</a>.<br /> -Enceladus, satellite of Saturn, <a href="#Page_222">222</a>.<br /> -Encke’s comet, <a href="#Page_109">109</a>.<br /> -Equinox, derivation of word, <a href="#Page_74">74</a>.<br /> -Eros, small planet, <a href="#Page_255">255–256</a>.<br /> -Europa, satellite of Jupiter, <a href="#Page_200">200–201</a>.<span class="pagenum" title="270"><a name="Page_270" id="Page_270"></a></span><br /> -<br /> -Flagstaff, Arizona, observatory of, <a href="#Page_175">175–176</a>.<br /> -Fomalhaut, <a href="#Page_187">187</a>, <a href="#Page_209">209</a>, <a href="#Page_213">213</a>.<br /> -<br /> -Galileo, <a href="#Page_136">136</a>.<br /> -Ganymede, satellite of Jupiter, <a href="#Page_200">200–201</a>, <a href="#Page_205">205</a>.<br /> -Gemini, constellation of the zodiac, <a href="#Page_76">76</a>, <a href="#Page_81">81–82</a>, <a href="#Page_91">91–92</a>, <a href="#Page_188">188</a>, <a href="#Page_210">210–211</a>, <a href="#Page_213">213</a>.<br /> -George III., Uranus first called <i>Georgium Sidus</i> after, <a href="#Page_226">226</a>.<br /> -Great Dipper, <a href="#Page_73">73</a>, <a href="#Page_77">77</a>, <a href="#Page_84">84</a>, <a href="#Page_96">96</a>, <a href="#Page_104">104</a>, <a href="#Page_105">105</a>, <a href="#Page_186">186</a>, <a href="#Page_236">236</a>.<br /> -<br /> -Hamal, star in constellation of Aries, <a href="#Page_78">78</a>.<br /> -Herschel, discovery of Uranus by, <a href="#Page_226">226–227</a>, <a href="#Page_232">232</a>.<br /> -Hyades, the, <a href="#Page_79">79</a>.<br /> -Hyperion, satellite of Saturn, <a href="#Page_222">222</a>.<br /> -<br /> -Inferior planets, <a href="#Page_40">40</a>.<br /> -Io, satellite of Jupiter, <a href="#Page_200">200</a>, <a href="#Page_201">201</a>.<br /> -<br /> -Japetus, satellite of Saturn, <a href="#Page_222">222</a>.<br /> -Juno, planetoid, <a href="#Page_251">251</a>, <a href="#Page_253">253</a>.<br /> -Jupiter, color, <a href="#Page_5">5</a>;<br /> - attraction between Saturn and, <a href="#Page_15">15</a>;<br /> - distance from sun, <a href="#Page_19">19</a>;<br /> - size and importance of, <a href="#Page_20">20</a>;<br /> - movement, <a href="#Page_25">25</a>, <a href="#Page_65">65</a>;<br /> - satellites, <a href="#Page_34">34</a>, <a href="#Page_106">106</a>, <a href="#Page_199">199–205</a>;<br /> - long known, <a href="#Page_38">38</a>;<br /> - superior planet, <a href="#Page_41">41</a>;<br /> - space between Mars and, <a href="#Page_42">42</a>;<br /> - influence on comets, <a href="#Page_44">44</a>;<br /> - gibbous, <a href="#Page_66">66</a>;<br /> - distance from ecliptic, <a href="#Page_72">72</a>;<br /> - near Antares, <a href="#Page_86">86</a>;<br /> - in Scorpio, <a href="#Page_127">127</a>;<br /> - size and velocity, <a href="#Page_183">183–185</a>;<br /> - place in sky, <a href="#Page_186">186–190</a>;<br /> - distance, light, and heat, <a href="#Page_190">190–193</a>;<br /> - seasons and atmosphere, <a href="#Page_193">193–195</a>;<br /> - surface features, <a href="#Page_195">195–199</a>;<br /> - symbol, <a href="#Page_205">205</a>;<br /> - compared to Saturn, <a href="#Page_213">213–214</a>, <a href="#Page_215">215–218</a>;<br /> - nearness of asteroids to, <a href="#Page_244">244</a>;<br /> - how to recognize, <a href="#Page_259">259–264</a>.<br /> -<br /> -Laplace, nebulæ hypothesis of, <a href="#Page_28">28</a>, <a href="#Page_30">30</a>.<br /> -Leo, constellation of zodiac, <a href="#Page_76">76</a>, <a href="#Page_82">82–83</a>, <a href="#Page_91">91–92</a>, <a href="#Page_188">188</a>, <a href="#Page_211">211–212</a>, <a href="#Page_221">221</a>.<br /> -Leverrier, discovery of Neptune by, <a href="#Page_236">236–238</a>.<br /> -Libra, constellation of zodiac, <a href="#Page_76">76</a>, <a href="#Page_85">85</a>, <a href="#Page_91">91–92</a>, <a href="#Page_188">188</a>, <a href="#Page_212">212</a>.<br /> -Little Dipper of the Pleiades, <a href="#Page_79">79</a>.<br /> -Lyre, constellation of the, <a href="#Page_54">54</a>.<br /> -<br /> -Major planets, <a href="#Page_19">19</a>.<br /> -Mars, “eye” of, <a href="#Page_12">12</a>;<br /> - distance from sun, <a href="#Page_19">19</a>;<br /> - nearness to earth, <a href="#Page_20">20</a>;<br /> - movement of, <a href="#Page_25">25</a>, <a href="#Page_65">65</a>;<br /> - long known, <a href="#Page_38">38</a>;<br /> - superior planet, <a href="#Page_41">41</a>;<br /> - space between Jupiter and, <a href="#Page_42">42</a>;<br /> - speed, <a href="#Page_51">51</a>;<br /> - gibbous, <a href="#Page_66">66</a>;<br /> - distance from ecliptic, <a href="#Page_72">72</a>;<br /> - color, <a href="#Page_80">80</a>, <a href="#Page_86">86</a>, <a href="#Page_259">259</a>;<br /> - position in regard to Antares, <a href="#Page_87">87</a>;<br /> - density, <a href="#Page_110">110</a>;<br /> - nearness to Venus, <a href="#Page_128">128</a>;<br /> - variety in<span class="pagenum" title="271"><a name="Page_271" id="Page_271"></a></span> brightness, <a href="#Page_151">151–152</a>;<br /> - how and where to identify, <a href="#Page_152">152–162</a>, <a href="#Page_259">259–265</a>;<br /> - size, atmosphere, and temperature, <a href="#Page_162">162–165</a>;<br /> - distance and brilliancy, <a href="#Page_166">166–170</a>;<br /> - seasons, <a href="#Page_170">170–171</a>;<br /> - surface aspect, <a href="#Page_172">172–179</a>;<br /> - satellites, <a href="#Page_180">180–181</a>;<br /> - symbol of, <a href="#Page_182">182</a>;<br /> - nearness of asteroids to, <a href="#Page_244">244</a>;<br /> - Bode’s law and, <a href="#Page_245">245–246</a>, <a href="#Page_248">248–249</a>;<br /> - smallness, <a href="#Page_260">260</a>.<br /> -Mercury, <a href="#Page_18">18</a>;<br /> - nearest planet, <a href="#Page_19">19</a>;<br /> - unfavorable situation for observation, <a href="#Page_20">20</a>;<br /> - easily recognized, <a href="#Page_22">22</a>;<br /> - age of, <a href="#Page_34">34</a>;<br /> - dense matter of, <a href="#Page_37">37</a>;<br /> - long known, <a href="#Page_38">38</a>;<br /> - inferior planet, <a href="#Page_40">40</a>;<br /> - terrestrial planet, <a href="#Page_41">41</a>;<br /> - irregularities of, <a href="#Page_44">44–45</a>;<br /> - number of revolutions, <a href="#Page_47">47</a>;<br /> - orbit, <a href="#Page_48">48</a>;<br /> - apparent motions, <a href="#Page_57">57–58</a>;<br /> - transits, <a href="#Page_61">61</a>;<br /> - distance from ecliptic, <a href="#Page_72">72–73</a>;<br /> - color, <a href="#Page_80">80</a>, <a href="#Page_86">86</a>;<br /> - in Scorpio, <a href="#Page_87">87</a>;<br /> - elusiveness of, <a href="#Page_93">93–95</a>;<br /> - how to find, <a href="#Page_96">96–100</a>, <a href="#Page_259">259</a>;<br /> - distance and brightness of, <a href="#Page_101">101–105</a>;<br /> - size, <a href="#Page_106">106–110</a>;<br /> - relation to sun, <a href="#Page_111">111–118</a>;<br /> - transits, <a href="#Page_119">119–121</a>;<br /> - lack of atmosphere, <a href="#Page_144">144</a>, <a href="#Page_146">146</a>;<br /> - resemblance to Mars, <a href="#Page_153">153</a>;<br /> - Bode’s law and, <a href="#Page_245">245</a>.<br /> -Milky Way, <a href="#Page_87">87</a>, <a href="#Page_88">88</a>, <a href="#Page_89">89</a>.<br /> -Mimas, satellite of Saturn, <a href="#Page_222">222</a>.<br /> -Minor planets, <a href="#Page_19">19</a>.<br /> -Mizar, star in Great Dipper, <a href="#Page_105">105</a>, <a href="#Page_236">236</a>.<br /> -Moon, <a href="#Page_23">23</a>;<br /> - once called planet, <a href="#Page_39">39</a>;<br /> - distance from ecliptic, <a href="#Page_73">73</a>.<br /> -Moulton, Professor, <a href="#Page_178">178</a>.<br /> -<br /> -Neptune, discovery, <a href="#Page_15">15</a>;<br /> - distance from sun, <a href="#Page_19">19</a>, <a href="#Page_43">43</a>;<br /> - not visible to naked eye, <a href="#Page_20">20</a>;<br /> - age, <a href="#Page_34">34</a>;<br /> - diffuse matter of, <a href="#Page_37">37</a>;<br /> - unknown to ancients, <a href="#Page_40">40</a>;<br /> - superior planet, <a href="#Page_41">41</a>;<br /> - influence on comets, <a href="#Page_44">44</a>;<br /> - one revolution, <a href="#Page_47">47</a>;<br /> - orbit, <a href="#Page_48">48</a>;<br /> - movement of, <a href="#Page_65">65</a>;<br /> - distance from earth, <a href="#Page_234">234</a>;<br /> - discovery, <a href="#Page_235">235–237</a>, <a href="#Page_247">247</a>;<br /> - symbol, <a href="#Page_238">238</a>;<br /> - atmosphere, <a href="#Page_239">239–240</a>;<br /> - satellite, <a href="#Page_241">241</a>;<br /> - motion, <a href="#Page_242">242</a>;<br /> - brightness, <a href="#Page_243">243</a>.<br /> -<br /> -Oberon, satellite of Uranus, <a href="#Page_232">232–233</a>.<br /> -Orion, <a href="#Page_123">123</a>.<br /> -<br /> -Pallas, planetoid, <a href="#Page_251">251</a>.<br /> -Phecda, star in Great Dipper, <a href="#Page_104">104</a>.<br /> -Phobos, satellite of Mars, <a href="#Page_180">180–181</a>, <a href="#Page_202">202</a>.<br /> -Phœbe, satellite of Saturn, <a href="#Page_222">222–223</a>.<br /> -Pisces, constellation in zodiac, <a href="#Page_76">76–77</a>, <a href="#Page_90">90–92</a>, <a href="#Page_160">160</a>, <a href="#Page_187">187</a>, <a href="#Page_212">212</a>, <a href="#Page_242">242</a>.<br /> -Pleiades, <a href="#Page_79">79–80</a>, <a href="#Page_153">153</a>, <a href="#Page_188">188</a>, <a href="#Page_210">210</a>.<br /> -Præsepe, or the Bee-hive, <a href="#Page_82">82</a>, <a href="#Page_211">211–212</a>.<br /> -<br /> -Regulus, star in the constellation of Leo, <a href="#Page_83">83–84</a>, <a href="#Page_188">188</a>, <a href="#Page_212">212</a>.<span class="pagenum" title="272"><a name="Page_272" id="Page_272"></a></span><br /> -<br /> -Rhea, satellite of Saturn, <a href="#Page_222">222–223</a>.<br /> -<br /> -Sagittarius, constellation of zodiac, <a href="#Page_76">76</a>, <a href="#Page_87">87–88</a>, <a href="#Page_91">91–92</a>, <a href="#Page_186">186</a>, <a href="#Page_189">189</a>, <a href="#Page_209">209</a>, <a href="#Page_212">212</a>, <a href="#Page_229">229</a>.<br /> -Saturn, rings and moons of, <a href="#Page_12">12</a>, <a href="#Page_218">218–223</a>;<br /> - distance from sun, <a href="#Page_13">13</a>, <a href="#Page_19">19</a>;<br /> - attraction between Jupiter and, <a href="#Page_15">15</a>, <a href="#Page_185">185</a>;<br /> - size and importance, <a href="#Page_20">20</a>;<br /> - object-lesson from, <a href="#Page_29">29</a>;<br /> - long known, <a href="#Page_38">38</a>;<br /> - superior and outer planet, <a href="#Page_41">41–42</a>;<br /> - influence on comets, <a href="#Page_44">44</a>;<br /> - length of year on, <a href="#Page_47">47</a>;<br /> - movement, <a href="#Page_65">65</a>;<br /> - distance from ecliptic, <a href="#Page_72">72</a>;<br /> - satellites, <a href="#Page_106">106</a>;<br /> - color, <a href="#Page_206">206</a>, <a href="#Page_209">209</a>, <a href="#Page_259">259</a>;<br /> - as evening star, <a href="#Page_207">207</a>;<br /> - slight motion, <a href="#Page_208">208</a>;<br /> - circuit of skies, <a href="#Page_209">209–213</a>;<br /> - size and distance, <a href="#Page_213">213–215</a>;<br /> - surface aspects, <a href="#Page_215">215–216</a>;<br /> - day and night, <a href="#Page_217">217–218</a>;<br /> - seasons, <a href="#Page_224">224</a>;<br /> - symbol, <a href="#Page_224">224</a>;<br /> - Bode’s law and, <a href="#Page_245">245–246</a>;<br /> - how to recognize, <a href="#Page_260">260–264</a>.<br /> -Schiaparelli, <a href="#Page_174">174–175</a>.<br /> -Scorpio, constellation of zodiac, <a href="#Page_76">76</a>, <a href="#Page_85">85–88</a>, <a href="#Page_91">91–92</a>, <a href="#Page_127">127</a>, <a href="#Page_153">153</a>, <a href="#Page_186">186</a>, <a href="#Page_188">188</a>, <a href="#Page_212">212–213</a>.<br /> -Sidereal year, <a href="#Page_49">49–50</a>.<br /> -Sirius, the dog-star, <a href="#Page_123">123</a>.<br /> -Spica, <a href="#Page_84">84–85</a>, <a href="#Page_188">188</a>.<br /> -Sun, controls planets, <a href="#Page_14">14</a>, <a href="#Page_17">17</a>;<br /> - distance from earth, <a href="#Page_18">18</a>;<br /> - center of planet system, <a href="#Page_27">27</a>;<br /> - probable formation of, <a href="#Page_36">36</a>;<br /> - once called planet, <a href="#Page_39">39</a>;<br /> - situation in orbit, <a href="#Page_52">52</a>;<br /> - vernal equinox, <a href="#Page_76">76</a>;<br /> - relation to Mercury, <a href="#Page_111">111–118</a>;<br /> - relation to Mars, <a href="#Page_166">166–167</a>;<br /> - relation to Jupiter, <a href="#Page_183">183–185</a>.<br /> -Superior planets, <a href="#Page_41">41</a>, <a href="#Page_65">65–70</a>.<br /> -Symbols in almanacs, <a href="#Page_267">267</a>.<br /> -Synodic year, <a href="#Page_50">50</a>, <a href="#Page_52">52</a>.<br /> -<br /> -Taurus, constellation in zodiac, <a href="#Page_76">76</a>, <a href="#Page_79">79–80</a>, <a href="#Page_90">90–92</a>, <a href="#Page_188">188</a>, <a href="#Page_210">210</a>, <a href="#Page_212">212</a>, <a href="#Page_242">242</a>.<br /> -Tethys, satellite of Saturn, <a href="#Page_222">222</a>.<br /> -Themis, satellite of Saturn, <a href="#Page_222">222–223</a>.<br /> -Titan, satellite of Saturn, <a href="#Page_222">222–223</a>.<br /> -Titania, satellite of Uranus, <a href="#Page_232">232</a>.<br /> -Triangulum, <a href="#Page_78">78</a>.<br /> -<br /> -Umbriel, satellite of Uranus, <a href="#Page_232">232–233</a>.<br /> -Uranus, gravitational influence on, <a href="#Page_15">15</a>;<br /> - distance from sun, <a href="#Page_19">19</a>, <a href="#Page_229">229–230</a>;<br /> - unknown to ancients, <a href="#Page_40">40</a>;<br /> - superior planet, <a href="#Page_41">41</a>;<br /> - influence on Neptune, <a href="#Page_43">43</a>;<br /> - influence on comets, <a href="#Page_44">44</a>;<br /> - movement, <a href="#Page_65">65</a>;<br /> - nearness to ecliptic, <a href="#Page_72">72</a>;<br /> - discovery, <a href="#Page_225">225–226</a>, <a href="#Page_246">246</a>;<br /> - symbol, <a href="#Page_227">227</a>;<br /> - time of revolution, <a href="#Page_228">228</a>;<br /> - size, <a href="#Page_231">231</a>;<br /> - satellites, <a href="#Page_232">232–233</a>;<br /> - irregularity of, <a href="#Page_236">236</a>.<br /> -<br /> -Vega, in constellation of the Lyre, <a href="#Page_54">54</a>, <a href="#Page_191">191</a>, <a href="#Page_266">266</a>.<br /> -Venus, the planet, <a href="#Page_2">2</a>, <a href="#Page_4">4</a>, <a href="#Page_5">5</a>;<br /> - nearness to sun, <a href="#Page_19">19</a>;<br /> - nearness to earth, <a href="#Page_20">20</a>, <a href="#Page_256">256</a>;<br /> - movement of, <a href="#Page_25">25</a>;<br /> - long known, <a href="#Page_38">38</a>;<br /> - early names of, <a href="#Page_39">39</a>;<br /> - inferior planet, <a href="#Page_40">40</a>;<br /> - terrestrial planet, <a href="#Page_41">41</a>;<br /> - brightest planet, <a href="#Page_42">42</a>;<br /> - apparent motions, <a href="#Page_57">57–58</a>;<br /> - transits, <a href="#Page_61">61</a>;<br /> - distance from ecliptic, <a href="#Page_72">72</a>;<br /> - seen from Mercury, <a href="#Page_105">105</a>;<br /> - density, <a href="#Page_110">110</a>;<br /> - beauty, <a href="#Page_122">122</a>;<br /> - how and when to see, <a href="#Page_123">123–131</a>;<br /> - distance and brightness, <a href="#Page_132">132–137</a>;<br /> - likeness to earth, <a href="#Page_138">138–140</a>;<br /> - atmosphere and seasons, <a href="#Page_141">141–147</a>;<br /> - transits, <a href="#Page_147">147–149</a>;<br /> - sign of, <a href="#Page_150">150</a>;<br /> - Bode’s law and, <a href="#Page_245">245</a>;<br /> - how to know, <a href="#Page_259">259–264</a>.<br /> -Vesta, planetoid, <a href="#Page_251">251</a>, <a href="#Page_253">253</a>, <a href="#Page_254">254</a>, <a href="#Page_257">257</a>.<br /> -Virgo, constellation of the zodiac, <a href="#Page_76">76</a>, <a href="#Page_84">84–85</a>, <a href="#Page_188">188</a>, <a href="#Page_212">212</a>.<br /> -<br /> -Zodiac, the, <a href="#Page_71">71–92</a>.<br /> -</p> - - -<p>THE END</p> - - -<div class="footnotes"><h3>FOOTNOTES:</h3> - -<div class="footnote"> - -<p><a id="Footnote_1" href="#FNanchor_1" class="label">1</a> -The reader will find fuller descriptions of the stars in the -zodiac in <i>The Friendly Stars</i>, by the author of this book.</p></div> - -<div class="footnote"> - -<p><a id="Footnote_2" href="#FNanchor_2" class="label">2</a> -See “Aldebaran” in <i>The Friendly Stars</i>.</p></div> - -<div class="footnote"> - -<p><a id="Footnote_3" href="#FNanchor_3" class="label">3</a> -See “The Heavenly Twins” in <i>The Friendly Stars</i>.</p></div> - -<div class="footnote"> - -<p><a id="Footnote_4" href="#FNanchor_4" class="label">4</a> -See “Spica” in <i>The Friendly Stars</i>.</p></div> - -<div class="footnote"> - -<p><a id="Footnote_5" href="#FNanchor_5" class="label">5</a> -See “Antares” in <i>The Friendly Stars</i>.</p></div> - -<div class="footnote"> - -<p><a id="Footnote_6" href="#FNanchor_6" class="label">6</a> -For those who find rhymes an aid to memory, the following -list may prove useful: -</p> - -<p> -This is the way the spring begins:<br /> -First Aries, then Taurus, then the Heavenly Twins.<br /> -The first summer sign is the one we call Cancer;<br /> -The next two to Leo and Virgo will answer.<br /> -Then autumn brings Libra and bright Scorpio,<br /> -And next Sagittarius, with his strong bow.<br /> -Capricornus then ushers the winter in,<br /> -And near old Aquarius the year we begin.<br /> -Pisces comes next, and then winter is done;<br /> -And with Aries’s approach, a new spring is begun.<br /> -These are the <i>signs</i>; but bear this well in mind:<br /> -The sun lags in one constellation behind.<br /> -When his place is Aries, we’ll find him in Pisces;<br /> -When in Taurus he should be, in Aries he stays.<br /> -If Gemini’s his place, and to find him our wish is,<br /> -We must look back in Taurus to see his bright rays.<br /> -And so through the year, whatever his place is,<br /> -The bright group behind is the one that he graces. -</p></div> - -<div class="footnote"> - -<p><a id="Footnote_7" href="#FNanchor_7" class="label">7</a> -See, in <i>The Friendly Stars</i>, “The Seven Stars of the -Dipper.”</p></div></div> - - - - - - - - - -<pre> - - - - - -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-h.htm or 51284-h.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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