Transcriber’s notes:

The text of this book has been preserved as in the original, apart from a few obvious misspellings.

Corrected misspellings and redundancies include the following:
comparsion → comparison
dining → during
clamly → calmly
atronomer → astronomer
oi → of
the → (deleted)
a → (deleted)

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A WHIRLING SPIRAL NEBULA, TYPICAL OF THAT FROM WHICH THE SUN AND PLANETS WERE PROBABLY EVOLVED

In the process of evolution the dense center becomes the controlling sun and the smaller spots of condensation form the planets. This particular nebula lies just under the end of the handle of the Big Dipper. It was photographed at Mt. Wilson Observatory.

THE WAYS OF
THE PLANETS

BY

MARTHA EVANS MARTIN, A.M.

AUTHOR OF

“THE FRIENDLY STARS”

NEW YORK AND LONDON

HARPER & BROTHERS PUBLISHERS

MCMXII

COPYRIGHT, 1912, BY HARPER & BROTHERS

PRINTED IN THE UNITED STATES OF AMERICA
PUBLISHED OCTOBER, 1912

CONTENTS

ILLUSTRATIONS

THE
WAYS OF THE PLANETS

I

ON MAKING ACQUAINTANCE WITH THE PLANETS

It is sought in the following pages to give a simple account of what may now be said to be known of the character of the planets, and to describe with as little technicality as possible their movements and aspects and relations. An endeavor is made to impart concerning each one of them not, surely, profound learning, but just a good, every-day, practical notion, so that the mere name will call up a definite object, with its own attributes, appearance, and behavior, entirely distinct from any other planet or from any other object in the skies.

An endeavor is made also to so simplify and direct the observation that any one, after a little practice, will know almost without hesitation, on seeing a planet in the sky, that it is a planet, and not a fixed star, and exactly what planet it is. The situation and aspect of it will then as quickly and clearly pronounce it to be the individual planet that it is as the sight of a person of one’s acquaintance proclaims him to be that person, and no other. The very name of Venus, for example, and still more the sight of Venus, will call up a conception of Venus, with the particular atmosphere and light and movements and wanderings which make her what she is. On looking at her the observer will at once know why she occupies the special position in the sky in which he sees her, why she is not so bright as she was when she was last in view, or is so much brighter than she was then, about how long she is likely to remain where she is, and when she goes what will become of her.

For far off and truly mysterious as the planets are, it still is with them as with most other objects in nature: a very little knowledge of their aspects and their ways begets a sense about them that makes the most casual observation of them interesting and, as far as it goes, intelligent. The slightest glance at them betrays some shape, or position, or light, or other quality, which at once makes recognition of them unmistakable. They disclose themselves oftentimes, one can scarcely say how, just as persons with whom we are intimate do by some half-caught outline, motion, or posture; or just as the trees do to an observer who knows, for example, an oak-tree from an elm, whether they are covered with their own peculiar verdure, or whether they stand with bare branches stretched out and colored in their own peculiar way.

This instant recognition of the planets is, of course, not to be had by simply reading about them. Such practical familiarity with them is attained only by seeking them out over and over again and looking at them with attention, with eagerness, and with all one’s faculty. With them, as with other natural objects, it requires observation truly to know them. But then, observation, when one gets a little started in it, is a great deal more interesting, a great deal more absorbing, than any reading about them can ever be. It is also a very easy thing to begin, for, after all, it is not much more than looking and then looking again. In doing this one can hardly tell just when an object ceases to be strange, and then becomes familiar, and finally is so much a part of every-day knowledge that one knows it at a glance. But this is what happens in the case of any natural object when we observe it often and with true attention.

In the case of the planets, if one is interested at all, every stage in the cultivation of such an acquaintance is full of pleasure. Even to one who regards them only as a part of the general aspect of the sky, they are the most beautiful objects in it and always the first to attract special attention. Nine times out of ten, when any one asks what a certain star is, it proves to be one of the planets. When one of them is visible a person can hardly glance at the heavens without noticing it, even if he does not stop to think about it. But if he does stop to think about it and notices that it is far larger than any star he has noted before, that it hangs low in the western sky early in the evening, and shines with a brilliant silvery light, and if he then learns that it is Venus, will he not always have a pleasant thrill of recognition when he again sees such a star in such a position and knows it as Venus, among the planets as surpassing in beauty as the goddess of that name was among the immortals? Or, if in the east, at the same time in the evening, he sees a brilliant, solid-looking, unblinking star shining with a white light, but pinkish white, not silvery, and finds it to be Jupiter, will not such a star in such a situation be to him ever after a pleasant acquaintance that he can call by name? Not that Jupiter and Venus are always in these positions, or shine in just this way at all times. These are their places and aspects at certain times, frequently recurring, and at such times always unmistakably distinguish them.

It is, then, merely the matter of a little more and yet a little more observation, in order to come to know any one of the visible planets in all its varying aspects and situations. Of course, at the start some guidance is necessary, but only a little; and that little, if it is of the right sort, should not be irksome. To provide such guidance is one of the aims of this book. That is, indeed, its main aim.

But in addition to what, as a help in observation, it may find to say regarding the appearance and movements of the planets, it will endeavor to give also ample information concerning their character and constitution.

It is hoped that this may be done without weighting the narrative with figures, though some of the peculiarities of the planets must be expressed by means of numbers. Certainly no mathematical problems will be presented. But it will be profitable to remember that every one of the intimate things we know about the planets has come to us through the long and laborious mathematical work of astronomers. To them we owe the extinguishable debt that we owe to all special workers who put us in possession of the facts that interpret life to us.

For the astrology and poetry and romance of the planets one must go elsewhere. Nearly every book on the subject of the planets—and there are many of them—has some information about these things; and properly, too, for every genuine emotion and every real fancy has its value. But neither curious lore of the planets nor the sentiment and emotion they have produced in others is what the author of this book is striving to set forth. It is something much more vital than this. What we wish to contemplate here are the plain facts, the knowledge of which enlivens and enriches one’s mind and nature. If the contemplation of them kindles one’s fancy or excites one’s emotions, these results at least will not be second-hand. If the bare facts, simply and plainly told, and the view of the planets themselves as they wander through their courses in the sky, do not awaken one’s understanding and imagination, no amount of poetry or romance that other people have built up around the planets will arouse anything more than a factitious interest in them. It is when our own faculties are at work and our own fancy plays over a subject that we become genuinely and lastingly interested in it.

The facts themselves are in the main quite simple, and will not be given here as anything else than that. They have been fairly wrested from that mysterious thing called space by the mighty power of mind and unceasing labor. Our knowledge of them is due to long nights of watching and long days of calculating; to long and careful testing and considering of theories, only to find that something else must be tried; to courage to begin all over again, to sudden inspirations, and sometimes to those lucky discoveries that seem almost like miracles.

The subject of the planets has in some respects a greater interest even than that of the stars, because we know, after all, more about them. We sometimes have a feeling, though, that we know more of the stars, although the stars are so much farther off. Why we have this feeling it is easy to explain. Knowing them to be so far removed from us, we really approach the stars with a different expectation. The few things that we have learned about them have in themselves such a magnitude that it makes them seem a greater body of knowledge than they truly are. The stars are indeed so far away, and what we know of them has to be expressed in such large terms, that the mind does not demand in that information the minute exactness that it seeks for in the case of nearer objects.

In the case of the stars, we seek mainly to know their distances, the direction of their motions, the speed with which they travel, and their probable connection with each other. The fact that in computing the distance of a single star, many trillions of miles away, the result may be a little less than exact does not keep us from learning what ones are sufficiently near for their distances to be measured at all and what ones are immeasurably remote. Whether they travel at the rate of exactly three or three hundred miles a second, we can learn that some are traveling at somewhat the same rate of speed as our sun travels, and some incredibly faster; that certain groups are going in one direction and certain groups in another; that some are approaching us and some are receding from us. And thus we can classify them and learn the significance of these facts, and, little by little, gain a definite understanding of the construction and meaning of the entire universe. Their very remoteness gives a certain compactness to the information we have about the stars, by making it necessary to generalize more than we would if they were near enough to yield more details; and we are in a way satisfied with this more general sort of knowledge of them.

But the very fact of our knowing so much about the planets extends our curiosity concerning them and makes us feel that we ought to know more. The mind is provoked into more minute speculations about them, and we demand more exactness of information and knowledge of a more specific or intimate sort than would satisfy us in regard to the stars. Atmosphere, habitability, exact size, seasons, and day and night, are the kind of things we most seek to know in reference to the planets. These are such definite things that conclusions concerning them are subject to close criticism, and differences of opinion in regard to them thus sometimes occur which tend to give one a more or less confused notion of what is really known. As a matter of fact, our information about the planets is much fuller than our knowledge of the stars, as we would naturally expect it to be. Much of what we seek to know about the stars has long been common knowledge about the planets.

II

OUR RELATION TO THE PLANETS

To know about the planets is to know about ourselves. The earth is one of them. Whatever their origin, the earth’s is the same. It and they are formed from the same nebula, controlled by the same central body, subject to the same laws, and destined for the same fate in the end. In this, the stars and the planets are not alike. They all shine upon us with the same sweet friendliness, and commonly we make no difference between them in our feeling for them. But the stars are bright and beautiful acquaintances living far away in their own domain. The planets are members of our own family, bone of our bone and flesh of our flesh, living comparatively near to us, within the domain of our common source of life, the sun.

One evening last autumn I was coming up Broadway, New York, with a friend, when we encountered at Union Square a man with a six-inch telescope directed toward the eastern sky. He was soliciting those who passed to stop and look at Mars and Saturn. Both of these planets were then very bright. They were also fairly near together, and so low in the east that one could scarcely help seeing them. But the people passed back and forth with hardly so much as a glance at the man and his telescope, and for the most part never even raised their eyes to the sky with a passing curiosity to see what it might be that he wanted to show them. My friend and I stopped and took each a view first of Mars and then of Saturn. While we were looking at the planets, a few of the passers-by began to loiter about, half smiling at us for so playing in public, slightly curious to see how we were faring at it, but for the most part apparently indifferent to what we were seeing. We had a fine view of Saturn lightly resting in his nest of rings, and an equally good view of the comical “eye” of Mars.

After we had finished, one or two others, evidently prompted by our example, followed us at the telescope. One or two inquired of us what the stars were that had so interested us, and one, pointing to Mars, wanted to know if it was Venus. As the crowd grew larger a few more ventured to take a look, much as they might venture to take their chance at hitting the bull’s-eye in some shooting-gallery. With the telescope pointed at Saturn, the man droningly chanted: “This planet is 887,000,000 miles from the sun. The ring you see is 170,000 miles in diameter,” and so on. These, to be sure, were the facts—and most marvelous facts, too—but without much meaning to one who knows nothing much about the planets; and the manner of their recital certainly did not make them alluring. I could not myself help feeling that the people there were missing a valuable opportunity, and that it would be only fair to them for some one fairly to cry out: “Come here and look at this planet. It is different from anything else you have ever seen or ever will see. It was at one time a part of the same nebulous mass that we were a part of. It is in the same system of worlds with us. It was formed in the same way that this world was formed. It is in itself the most wonderful thing you ever saw, and it is bound, as we are, to the sun by the ever-drawing tie of gravitation. The very position of our own world in space is more or less influenced by it. If anything should happen to it, it might be a serious matter to us.”

For it is true that we are thus closely bound to the planets. The family tie among us is of far more force and significance than in any ordinary case of common origin. Human family ties wear, as we know, often into the merest threads, or even become no ties at all. But that between the earth and the planets remains apparently as close and strong as ever it was. The law of gravity, under which the earth draws toward its center every atom of matter surrounding it, and thus holds together all the atoms composing it, is not solely terrestrial in its application. It is probably universal. It certainly applies to every part of our little family of worlds. Every particle in the solar system attracts toward it every other particle in that system with a force determined by its mass and its distance. The sun, by reason of its immense size, compels the earth and all the other planets forever to circle around it. But the planets themselves have just as much power of attraction as the sun, atom for atom.

Thus, while the sun controls the motions of all of them, each pulls at the other, and, according to its power, determines how much the path of each shall vary from the course around the sun it otherwise would make. In the case of the smaller planets, this gravitational influence is, of course, very slight, and so subtle that we here on earth are not even conscious of it. But it is, nevertheless, real and continuous. It is greatest between the two largest planets, Jupiter and Saturn; but it was enough in the case of Uranus and Neptune to lead, by its mere manifestation on the earth, to the discovery of Neptune, the farthest planet.

Being thus of the same origin with the planets, having the same life history, being bound to them in space by a tie that is perhaps eternal, how can we fail to have the most intimate interest in their nature and all that concerns them?

But in addition to their close relationship to us there is, to make them of peculiar interest, the fact that, after the sun and the moon, they are for our eyes the most splendid objects in all the brilliant panorama of the sky. Such of them as we can see at all with the naked eye are most of the time much brighter than any first-magnitude star. As they wander from constellation to constellation the soft light of their placid faces gives a beauty and variety to the spectacle that endears them to us, and at the same time enhances by contrast their own charm and that of the glittering, unchanging stars.

There is nothing that gives one such a sense of sweet familiarity with the heavens as a really recognizing acquaintance with the planets. They are not, like the stars, associated with particular seasons. They come sometimes with the gay company of stars that dance their way across the cold winter skies, and sometimes with those that shine during the soft summer nights. Often in the spring and autumn we see some one of them before the sun is fairly down, and, before the light of an ordinary star can yet be seen, hanging in lone brilliancy as the evening star; and often an early riser has the reward of seeing one as a morning star glowing almost in the rays of the rising sun. Thus they are, one and another, with us at all times and seasons, and it accords with the fact of the relation being a family one that we have in their coming and going a sense of frequency and informality which we cannot have in the more regular and seasonal coming and going of the stars.

III

WHAT THE PLANETS ARE, AND WHAT THEY APPEAR TO BE

The planets are dark, opaque bodies which revolve at varying distances and at varying rates of speed in orbits more or less circular around the sun as a center. They have no light of their own, as the stars have, but shine wholly by reflected light received from the sun, which itself is a star. The amount of light they show to us depends upon their size, their distance, and their power of reflecting the light they receive.

In comparison with the stars, the planets are very near to us. Our sun, which is in constitution a star, but very widely separated from any other star in the universe, holds all his family of planets by the tether of gravitation, and so keeps them circling about him in a very small space, as astronomical space is measured. To all of the planets except Mercury, we ourselves are nearer than the sun is. To be sure, this distance between us and the planets, as measured by any terrestrial measure, is not exactly small. It is only by comparison that we can be said to have anything like a cozy relation to them. For merely earthly affairs we use terrestrial measures. In solar affairs we measure by an astronomical unit, which is the sun’s distance from the earth, ninety-three millions of miles. When we say a planet’s distance from the sun is thirty astronomical units, we mean it is thirty times farther than the earth is from the sun.

For matters outside of the solar system, the unit of measure is the number of miles that light travels in a year. The speed of light is a little more than 186,000 miles in a second. This is equal to about six trillions of miles in a year, or about sixty-three thousand times the distance of the sun from the earth, our family measuring-stick. From the nearest star it takes light more than four years to come to us. From the nearest planet light comes in less than three minutes, and from the farthest one it makes the journey in a little more than four hours.

As compared with other heavenly bodies, therefore, the sun and the planets are very near together, occupying a very small space in the immensity of the universe, immeasurably isolated from all the other systems and, so far as we know, immeasurably smaller as a system than most of them.

The whole body of the planets is divided according to size into two classes, the major and the minor planets. When we refer generally to the planets, the major planets only are meant. The minor planets are usually called the asteroids, or planetoids. There are many hundreds of them, and only one—and that barely—can be seen with the naked eye. The other planets are eight in number, including the earth, which is, after all, nothing but one of the smaller of the major planets. They are, in the order of their distances from the sun: Mercury, the nearest, Venus, the Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Of these only five—Mercury, Venus, Mars, Jupiter, and Saturn—can be seen from the earth without optical aid. Occasionally, when Uranus is very favorably situated, a person with an exceptionally good eye, who knows exactly where to look for the planet, can see it. Neptune is about equal to an eighth-magnitude star in brightness, and can never be seen without the aid of a telescope. Mercury, while quite bright enough to be seen, is not often situated favorably for observation. It is very near the sun, and is generally obscured either by the light of the sun when the sun and the planet are above the horizon, or by the haziness of the atmosphere when the sun is below the horizon and the planet a little above it. In regions of considerable altitude with a clear, rare atmosphere, Mercury is more often seen; but never for very long at a time.

The only planets, therefore, that are a part of our evening spectacle in the skies are Venus, Mars, Jupiter, and Saturn. These four happen to be not only the ones we oftenest see, but also the most interesting of all the planets from various points of view. Venus and Mars are the nearest to the earth, and most resemble it, and hence are the most inviting for speculations which have a human interest, such as habitability, the presence of life, and kindred ideas. Jupiter and Saturn are interesting above all the others in their splendor and size, and in their importance as the centers of systems of their own.

As seen by us, the planets are similar to the stars, but with very distinct differences in appearance, which, when once familiar, mark them so unmistakably as planets, and not fixed stars, that we need never get the two confused. The first and easiest distinguishing mark to notice is that they do not twinkle, as the stars do, but shine with a steady light similar to that of the moon. This is an invariable difference between stars and planets, and one needs only to stop and truly look at them in order to detect it. And once it has become familiar, it discloses itself at a glance.

This difference between stars and planets is due almost solely to difference of distance, though the twinkling is caused by our own atmosphere. The stars are too far away to send us anything but a mere point of light, and the unequal density of the waves of air sweeping over this point of light keeps it dancing before our eyes, causing the phenomenon that we call twinkling. But the planets, being nearer to us, show a disc, from every point of which comes a line of light, making the total light of some volume; and these inequalities of the air are too small to interfere with it to any extent. Sometimes, when the atmosphere is particularly unsteady, it happens that the light of a planet is somewhat affected by it when the planet is just rising or setting and is, consequently, near the horizon, and that it then seems to twinkle a little. But this departure from the rule is always slight and of short duration, in the case of the four planets most seen. Mercury, never being seen anywhere except near the horizon, often seems to twinkle; but then he is seldom seen at all, and, when visible, is in other ways so well marked that one cannot fail to recognize him.

So the steady light may justly be said to be invariable, because the unusual conditions are easily detected. When the atmosphere is such as to cause even the planets to blink a little, it has an effect also on the stars. At such a time they will appear to be fairly dancing. This effect is apt to occur on the clear nights of winter, the atmosphere being more unsteady then. Such nights, because of the extreme liveliness and brilliancy that they lend to the stars, are attractive times for amateur observations. For the astronomer, however, they are not so favorable. For the seeing of small details such as he seeks, the steadiest atmosphere is necessary.

Though the planets are near enough to show a disc, they are not sufficiently near to show to the naked eye as sharp an outline as the moon’s. Usually the edge is more or less rayed like that of a fixed star, which adds somewhat to the difficulty of distinguishing them from the stars until their aspect has become familiar to us. The fact that we are looking at a disc is plainly shown when an occultation by the moon occurs. When the moon occults a fixed star, it passes between us and the star. At such times the star disappears behind the edge of the moon instantly, as a mere point naturally would. When a planet is occulted by the moon, it disappears gradually as the moon covers more and more of its disc, thus showing unmistakably the nature of it.

After steadiness of shining, the next most obvious mark of difference between a planet and a star, from our point of view, is the movement of the planets. A star remains always in one place with relation to the other stars, while the planets move about from constellation to constellation, seeming to travel sometimes toward the east and sometimes toward the west.

This difference also is due solely to a difference of distance. The stars as well as the planets are constantly in motion. Most of them, in truth, move at a rate which would make the rate of motion of a planet a mere snail’s pace in comparison. Arcturus, for instance, is supposed to be moving at the rate of two or three hundred miles a second, and there are other fixed stars with an equally rapid motion. The swiftest moving of the planets does not achieve much more than twenty-nine miles a second, while the slowest swings along at a rate of but little more than three miles in the same length of time.

These are the real rates of speed of the stars and planets; but they are not at all what they seem to us. The difference in distance is so great that for centuries and centuries the flying stars have seemed to men to remain in the same place in the skies, and so we call them fixed. The planets, so slow-journeying as they are in comparison, seem to us to be moving among the constellations at rates varying from more than a degree a day in the swiftest to between two and three degrees a year in the slowest.

Hence, if through lack of practice in observation a person is not at once able to distinguish the difference between the stars and the planets in the character of their light—that is, whether they twinkle or shine steadily—he can, by taking a little longer time, at most only a few days, determine whether the object he sees is a star or a planet by noticing whether it has any motion among the other stars. Venus and Mars will show some movement in one evening. Jupiter and Saturn may require a little more time to disclose their motion.

IV

THE ORIGIN OF THE PLANETS

Different as the planets are as individuals, they have too many characteristics in common to admit any question of their common origin. They are not simply stars of one sort and another that happen to lie nearer to us than the great body of stars that spangle the heavens, but are, without doubt, all of one family with us in their origin, as well as in their situation. How they originated, and exactly what has been their course of evolution, has long been an engrossing problem among philosophers; and it is not yet solved.

In the sense that the human race is all of one family, the planets are but a part of the great universe that lies about us and is in part visible to us. The forms in which we know matter as existing in the universe, outside of the solar system and of the minor forms in our own world, are those of stars and nebulæ. It seems as if either of these could, and in fact does, form out of the other. We do not at all know how in the beginning matter took the form of either, or which came first. But it is believed that a star is formed by the condensation of a nebula, and that a nebula is often formed by the collision or near approach of two stars and the consequent disintegration of their particles.

The sun is a star not very different from most of the other stars, as we believe them to be, except that it is smaller than most of them. It is the center around which we and all the planets revolve, and it is believed that we were all once a part of the very body of it. For astronomers are substantially agreed that the whole solar family, including the sun and all the planets, has been evolved from a great nebula which, in one form or another, at one time filled practically the whole of the immense space from the sun to the outermost planet of the system. While this cannot be said to have been exactly proved, yet it accords with all the known facts of the solar system. As to how this nebula originated, and what its shape was, and in just what way the planets were formed from it, there is more diversity of opinion.

Up to the middle of the eighteenth century no really scientific theory of the evolution of the solar system was formulated, and it was not until the very last years of that century that any theory of the origin of the planets was published which received anything like universal acceptance.

This was the case, however, with the famous nebular hypothesis of Laplace, which was published in 1796, and for a time seemed so nearly to account for the various phenomena of the motions and relations of the planets that it was not only accepted in the scientific world, but became almost as much a part of universal knowledge as that the earth is round. But even this theory has not completely stood the test of time, which inevitably brings that close scientific investigation that any theory must undergo when it is used as a working basis to which all facts and secondary theories must be correlated.

The original nebular hypothesis supposed this vast nebula to be in rotation on its axis. As it condensed, the falling-in of the particles caused its rotation to become more rapid, until finally, under the strain of this, a ring of matter was “thrown off” from the outer edge. Or, as was sometimes said, the inner part condensed and left a detached ring of matter. This ring, continuing to rotate in the direction given it by the rotation of the central mass, finally condensed into a planet, rotating on its axis and revolving about the central sun in the same direction as the ring had revolved. The satellites of the planets were thought to have been formed by the same process from the planets while these were still in a plastic state. Saturn, with its wonderful system of rings and satellites, was thought to be a minute object-lesson of a planet in course of evolution, and this we have often heard said.

I am sorry it is not so. I had much enthusiasm in my youth over this beautiful and orderly arrangement of things: first, the splendid hypothesis, the achievement of a noble mind; then the little model showing the work in its progress; and, finally, the beautifully finished system, the rings all rolled up into planets, traveling unceasingly in paths which eternally marked the size of the central body, or sun, at the time of the separation.

But it is now pretty certain that this cannot be the way it all happened. Closer investigation shows that there are mechanical difficulties which were not at first fully recognized. A series of rings could not have been left off by a body so wholly gaseous. The particles composing them would not be sufficiently coherent to permit of separation in any such compact, uniform, and decisive manner. Then, even if such a ring were thrown off, it is not at all certain that it could condense into a planet. Its tendency, indeed, would be to disintegrate rather than to condense. In a body so tenuous the mutual gravitation of its particles would be too feeble to complete the work. Besides, in conflict with the theory is the fact that a few of the satellites of the planets revolve in a direction contrary to that of the planet. And there are other minor, but still important, details in the mechanism of the solar system which cannot be accounted for by the ring theory.

And so, while astronomers are still agreed that the whole solar system, which includes the planets, was evolved from a primeval nebula, the theory of leaving off rings which condensed into planets is not found tenable, and the search for some more acceptable theory or some modification of the Laplace theory is now occupying a number of eminent astronomers and philosophers.

The result of all this is that no theory of the manner of the evolution of the planets is definitely accepted by the body of astronomers. Much hard labor and ingenious reasoning have been expended in endeavoring to formulate some hypothesis by means of which we may account for observed phenomena. The astronomers with whom these theories have originated are, naturally, more or less ardent in setting them forth. Thus one occasionally sees a decisive and authoritative statement of a theory of the evolution of the planets that seems at first view to account for everything. But no one of these has yet been entirely accepted by astronomers, who are as a class cautious and conservative, and are necessarily critical of any theory, because the value of much of their future work depends upon its accuracy and sufficiency for all details.

The theory which at present seems more nearly than any other to offer a reasonable explanation of most planetary phenomena is based upon the supposition that the nebula from which the sun and planets were evolved was in the shape of a spiral, and not the gaseous mass that the original nebular hypothesis supposed. The fact that among the many thousands of nebulæ that have been discovered and observed a very large proportion of them are in this form, aside from any other consideration, suggests a great probability that the one from which the solar system was evolved was a spiral.

The spiral nebulæ seem to be of a somewhat different constitution from the other nebulæ, and show on observation spots of condensation here and there, which at least suggest the formation of systems of planets. This indicates that ours may be only one of many such systems in process of evolution; but it is certainly among the smallest of them, for most of the spiral nebulæ are immensely greater in size than the one required to form our little system. Its few trillions of miles of diameter, though it seems so vast to us, is quite insignificant in comparison with a large proportion of the spiral nebulæ in the universe.

A spiral nebula is in the form of a disc somewhat resembling that familiar form of fireworks known as a pinwheel. The typical form of it has two arms projecting from opposite sides of the whirling figure. It is much denser toward the center, where the spiral would naturally be more tightly wound, and has smaller spots of condensation scattered like knots here and there along the fiery arms. In the process of evolution the denser center becomes the controlling sun, and the smaller spots of condensation form the planets, which are eventually detached from the revolving mass, but continue to revolve about the center as they were doing from the beginning. According to the mass it has in the beginning, the planet gathers up by gravitative attraction all the material in its region, gaseous or more or less condensed, and grows by this accretion. If the nucleus happened to be a large one before it separated from the parent body, it will have sufficient force of gravitation to gather in large quantities of material and greatly increase its size, and thus become a large planet. If it is only a small nucleus, it has less power of attraction, and gathers in less material.

When these condensations of matter which are the nuclei of the planets break away from the parent body, they sometimes carry with them still smaller nuclei, which, if they are not too near the original center, or sun, are destined to remain under the control of the planets and become their satellites. The number and size of the satellites a planet has depends upon the size, and hence the controlling force, of the nucleus which is its foundation, and also upon the number of spots of condensation that chanced to be formed in its neighborhood sufficiently near to come under the gravitational control of the planet. If by any chance the nucleus which was to form the largest satellite of Jupiter had been in the situation of Mercury, for instance, it might well have given its allegiance to the sun, instead of to Jupiter, and thus have become a planet.

Under the ring theory the outermost planet, Neptune, would be the oldest of the planet family, and the one nearest the sun, Mercury, would be the latest born and youngest. But the physical development of these planets seems to indicate, in truth, exactly the opposite of this, as we shall see later on. Under the spiral-nebula theory the planets may be nearly of the same age, their different states of development being due mainly to difference in size and to some peculiarities of situation. If the nucleus happened to be near the outer edge of the spiral, it would be formed from the lighter matter composing the outer part of the nebula, and this seems to be the case with the outer planets. If it were near the dense center of the nebula, it would be composed of denser material, and this seems to be so in the case of the inner planets.

A nebula, it is thought, is formed by the collision or the near approach of two of the many stars, or suns, that we know are traveling about at high velocities as vagrants here and there through space. If the two bodies come together centrally, the force of the impact will generate heat sufficient to convert them into a nebula; but this will not necessarily be spiral in form. If they come together obliquely, the chances are that they will form into a rapidly rotating spiral disc.

But in order to form a spiral, it is not necessary that there should be an actual collision. Because of the force of gravitation the near approach of two stars would subject them to an enormous strain from their pull upon each other, and there is a limit within which they cannot approach without being literally torn to pieces from the effect of this tidal force. Even if they do not approach within this fatal limit, which is a little less than two and one-half times the radius of the body, they may come so near as to change their character entirely, and, through their tidal influence on each other, form into a rotating spiral nebula with two arms projecting from opposite sides of the spiral.

It now seems probable that it was after this manner that the sun and its family of planets were formed. The matter which is contained in them may have been in the form of a dark, solid body pursuing some sort of course in space. In its journeying it came near another body and was awakened into a life of activity in the form of a flat, spiral nebula which was left spinning around in a pyrotechnic manner, the matter composing it much diffused at the outer edges and densest in the center. Scattered through it were the more or less condensed spots which were the embryonic forms destined to come forth from the parent body as the individual planets.

When the separation was completed, each planet fed and grew upon all the matter that it had the force to draw to it, and it swept clean the space that lay within the limits of its power. If the particles thus gathered in were small and slow of motion, they became a part of the body of the planet. If they were large and swift, they became members of the planet’s family as satellites. In whatever area of the nebula each planet came into a separate existence, it fed upon the matter which that area afforded. In the case of Neptune, at the outer edge of the system, it was very diffuse matter; in Mercury’s region, nearer the center, it was more dense.

Thus in our family of planets, though its members were born of the same parent and developed under the same guiding laws, each has a distinct individuality arising from its inherent qualities and its environment during the early stages of its existence. The spiral-nebula theory seems to offer a better explanation of these individual qualities than any other that has been advanced thus far, and in its main features it is pretty generally accepted. But one must keep in mind that the details of any theory of the beginning and growth of the planets are more or less speculative, or, at least, have not yet been proved with finality.

V

THE SEVEN GREAT PLANETS

So far as we know, five of the planets—Mercury, Venus, Mars, Jupiter, and Saturn—have been known from time immemorial. There are existing records of them made thousands of years ago. There is no reason why they should not have been thus known, since they have always been as they are now, visible to the naked eye, and all of them save Mercury are as easily seen as the sun or the moon. They do not, of course, exact the instant attention that those great luminaries do, because, being smaller, they are less isolated from the great body of the stars; but they are in their seasons plainly visible, and can then always be seen if one looks at them.

In ancient times, when people lived more out-of-doors than is the habit now, they did look at them. The same primitive shepherds that, while tending their flocks at night on the hills, named the constellations according to the fanciful shapes that the unchanging stars seemed to outline, watched also the five wandering stars, more wonderful to them than any of the others. They observed how mysteriously these stars came at certain seasons and silently threaded their way across the shining heavens, and then as mysteriously disappeared. They saw them not only differing from the other stars in glory, but changing in their own brilliancy from one time to another, until, in some cases, they failed to recognize them as the same stars under varying aspects. Venus, for instance, they called Phosphorus, or Lucifer, when they saw her as a morning star, and Hesperus, or Vesper, when she shone in the evening.

The sun and the moon, they noted, also moved from place to place among the fixed stars, and they called all these errant bodies planets, which means “wanderers.” These are the “seven planets” referred to in the earlier literatures and in all early books on astronomy or astrology. This is sometimes a little confusing, because, though the sun and the moon are no longer called planets, we still (omitting the earth) have seven. But Neptune and Uranus, not being visible to the naked eye, were not known to the ancients. They were discovered by means of the telescope, and that only within the last century and a half. So, owing to these comparatively new-found members of the solar family, we have yet the magic number of planets, seven.

These seven are the major planets and the ones with which mainly it will be our endeavor here to promote and strengthen an acquaintance. With Uranus and Neptune the acquaintance will necessarily be less intimate than with the others, because we cannot see them in the same free way; but they are not on this account much less interesting than the others, and a little knowledge of them is pleasant family history. They simply do not live within sight.

The planets that are nearer to the sun than we are, and hence lie between us and the sun, are called the inferior, or sometimes interior, planets. Those that lie outside the orbit of the earth are called the superior, or the exterior, planets. In so grouping them the earth is the dividing-point, and is not itself in either class. Mercury and Venus are the inferior planets. The superior planets are Mars, Jupiter, Saturn, Uranus, and Neptune. The distinction has importance, especially when we are discussing the planets with relation to their movements, as seen from the earth, because the planets with orbits between us and the sun (the inferior planets) have very different phases and apparent motions from those whose orbits are beyond us from the sun (the superior planets).

When considered in regard to size, constitution, development, and their likeness to each other, the planets are sometimes distinguished as the terrestrial planets and the major planets. This need occasion no confusion with the general division of them into major and minor planets, because, as has been said, when simply “the planets” are mentioned, these seven large planets are always the ones that are meant, the others being usually called asteroids, or planetoids. The terrestrial planets are Mercury, Venus, Earth, and Mars. As the name implies, they are so called because they are in some respects similar to the earth. The major planets are Jupiter, Saturn, Uranus, and Neptune. They are all larger than the terrestrial planets, and, in addition, have some other characteristics in common which the planets of the other group do not have. The two classes represent different stages of evolution.

The four planets forming the terrestrial group are sometimes called the inner planets, and the four major planets are then known as the outer planets. The point of division in mind then is the space between Mars and Jupiter. This is so vast in comparison with the spaces between the other planets from the sun out to Mars that it becomes a convenient dividing-line, particularly as the groups divided by it are in some respects essentially different from each other.

Of the four planets which have an especial interest to us because of their being the ones most easily seen, two are terrestrial, or inner, planets, Mars and Venus, and two are major, or outer, planets, Jupiter and Saturn. The differences between the two classes are solely matters of constitution and situation, and have nothing to do with their appearance to us. Venus, the brightest of them all, belongs to one group; Jupiter, the second in brilliancy, belongs to the other.

That there is at least one other planet beyond the present boundary of our system (which is the orbit of Neptune) seems to be quite probable. Some astronomers think there may be several others. There are certain perturbations, or irregularities, in the movements of Neptune which the influence of Uranus does not account for, and they seem to indicate that there is some disturbing body even beyond the orbit of that farthest known planet.

Several astronomers are working on the problem of locating this undiscovered body. At various times it has been announced that such a planet would probably be found in a certain position in the skies at a specified date; but as yet no one has been able to get a view of it. Recently the orbit of a far-off hypothetical planet has been calculated, and its place predicted for 1914. Perhaps it may be found then. Of course it could never be seen through any but the most powerful telescopes. Its calculated distance from the sun is one hundred and five times that of the earth. This would be more than nine billions of miles, or more than three times farther than Neptune is from the sun. It would require fourteen hours for light to pass from the sun to a planet at that distance, and the sun would appear to it smaller than Saturn or an ordinary first-magnitude star does to us.

A further reason for suspecting the existence of such a planet is suggested by the orbits of certain comets. These erratic bodies, when they chance to come within the bounds of the solar system, are sometimes forced to remain because of the powerful influence of one of the planets near which their path has taken them. Jupiter holds as many as thirty of them in this way, Saturn and Uranus have two or three, and Neptune has captured as many as six. But there are still others that return to us in regular periods, but which go sufficiently far beyond Neptune to escape entirely if there were not some still more distant watch-dog to turn them back. So there seems good reason to believe that Neptune is not really the outermost of the planets.

There has also been much said about the possibility of a planet nearer to the sun than Mercury. When Mercury is at perihelion, or nearest to the sun, there are certain irregularities in his movements which might be explained by the presence of another planet between Mercury and the sun. In 1859 it was thought that such a planet had been observed. Its time of revolution and its distance from the sun were estimated, and it was named Vulcan. In some of the books of astronomy published about that time, and even in some published as many as fifteen years later, Vulcan is mentioned as a reality. But now it is believed that the observation was a mistake, and no such body is known to exist.

In 1878 it was again thought that two bodies nearer to the sun than Mercury had been discovered during an eclipse. These observations have never been explained or confirmed; but it is thought that the objects seen were probably stars which were mistaken for planets by the observers. If a body so situated does exist, it is so near the sun that it probably can never be seen except during an eclipse, and the time of observation is then so short and mistakes are so easily made that it is difficult to verify the observation. The continued search for the cause of the perturbations of Mercury may finally lead to the discovery of something between it and the sun. But if it is a single body, this seems a much less promising task than the search for a planet, or planets, on the outer edge of the solar system.

VI

THE MOVEMENTS OF THE PLANETS

In considering the movements of the planets, we have to regard their actual motion in space and that motion as it appears to us. They all have two principal motions in space. They revolve about the sun in their orbits, and they rotate on their axes. The manner in which they accomplish the rotation on their axes determines the length of their days and nights, or whether, indeed, they shall have any such grateful alternations of light and darkness. Those planets which, like the earth, turn on their axes in less time than they make their journey around the sun have one day and one night every time they make a complete rotation. Those that turn on their axes in the same time that they revolve around the sun, of which sort there seems to be at least one, face always toward the sun, and have no alternations of day and night. On one side it is always day; on the other it is always night. The number of days a planet has during each revolution around the sun depends upon how much time it requires to make a revolution, and how fast it spins on its axis. In one year here on the earth we have three hundred and sixty-five days and nights. Saturn, in its year, has more than twenty-three thousand days and nights.

The manner in which the revolution of the planets in their orbits takes place determines the length and character of their year; the nearer a planet is to the sun, the shorter its orbit is, and the faster the rate of speed at which the sun compels it to move, and hence the shorter its year. The nearest of the planets, Mercury, makes more than five hundred revolutions around the sun, while the farthest, Neptune, makes one. Three times in a year—that is, a terrestrial year—the nearest planet speeds around its orbit and back to the starting-place with seventeen days to spare. One hundred and sixty-five terrestrial years are necessary for the farthest planet to make one circuit of its orbit. The first goes at the average rate of nearly thirty miles a second over a path more than two hundred million miles long. The second travels a path more than seventeen billion miles in length, at the average rate of three and four-tenths miles a second. Between these two extremes the other planets have orbits and rates of speed varying with their distances from the sun. The farther they are from the sun, the larger the orbit and the slower the speed.

To get something like a picture of the sun and the planets as they actually lie and as they move in space, one should have in mind an immense flat, circular disc five and a half billions of miles in diameter passing through the sun, which is in the center of it. Around the edge of the disc is the orbit through which Neptune moves. At varying distances inside of it are the orbits of the other planets, each growing smaller and smaller as one comes nearer and nearer to the sun, until the orbit of Mercury, the planet nearest to the sun, is reached.

Since it is not a hard metal disc that we are considering, but only an imaginary one in space, there may be a little latitude allowed for the orbits to tip somewhat out of the exact plane of the disc without materially altering the figure in mind. And this they do, very slightly—most of them to the extent only of from one to two degrees, though one of them falls outside of the common plane about seven degrees. In these orbits all the planets, as seen from the sun, are going around from west to east. At the same time they are turning on their axes in the same direction, some standing almost erect, as it were, in their orbits and whirling like a dancing dervish as they skim along, and others more or less inclined like a traveling top.

The time a planet requires to make one circuit of its orbit constitutes, as with the earth, its year. But we who are on the earth have, in our study of another planet, to regard it as having in a sense two years. First, there is the time it takes, starting from a given point in its orbit, to circle around the sun and return to that point. This is known as its sidereal period, or year, and is so called from sidus, meaning a star, because the only way to mark any point in space is by a fixed star, and, as viewed from the sun, one revolution of a planet would be from a given star back again to that star.

Then there is the time a planet takes, starting when it is in a straight line with the earth and the sun in space, to return to the place where the three bodies will be again in the same relative position. This is known as its synodic period, or year. Synodic is from our word synod, meaning a meeting or assembly, and the synodic year is the time between two successive and similar meetings of these three bodies. The sidereal year concerns the planet in its relation to the sun; the synodic year, in its relation to the earth. The synodic year is the only one that much concerns us while regarding the planets as a part of the spectacle of the sky. It is the one that we know from observation, while the sidereal year is mathematically computed.

The two periods, or years, are not of the same length, because the sun with reference to the planet is always stationary, and the motion resulting in the sidereal year is that of the planet only, while the synodic year is the result of the movements of both the earth and the planet, each, in its own orbit, being always in motion.

An inferior planet, situated as it is nearer to the sun than the earth is, and so having a shorter orbit than the earth’s, will, when it finishes its sidereal year and comes around to the point from which it started, find the earth advanced from that position and will, therefore, have to travel farther on in order to overtake it and come into the same relative position from which they started, which makes the time of its circuit with reference to the earth obviously longer than with reference to the sun.

With the superior planets the case is just reversed. The earth is the inside planet, or the one nearest the sun, and it must overtake them. With one exception, they are all so far away from the sun and move so slowly that it takes us but little more than one of our years to overtake them and bring them into the same relative position with us that they had when we started, while it requires many of our years for any one of them to make a single circuit of the sun. Hence their circuit with reference to the earth is shorter than with reference to the sun.

With Mars, the exception referred to, we have a more hardly fought race. That planet is not so far from us as are the other superior planets. It makes its revolution around the sun in a little less than two of our years. We travel eighteen miles a second, and it travels fifteen miles in the same length of time. If we are in line with it at the beginning of our journey, we glide off swiftly, and easily leave it far behind. When, however, we come back to the starting-point, it has not loitered, and is many millions of miles ahead of us, and it remains ahead until more than seven weeks after we have returned to the starting-point a second time. Fifty days after we have begun to make our third round we overtake it, and are again in a direct line with the planet and the sun. This makes its period with reference to the earth ninety-three days longer than its own year, and fifty days longer than two of ours. This is the longest synodic period among the planets.

The orbits in which the planets move all have the form of an ellipse—that is, of a circle more or less flattened. This flattening, or the extent to which an orbit departs from the form of a true circle, is called its eccentricity. The sun is never at the exact center of an orbit, but is always situated a little to one side of the center—that is, it is at one of the foci of the ellipse. Consequently, the planet, as it travels in its orbit, is not always at the same distance from the sun, the amount of the variation in distance depending upon the eccentricity of the orbit. The point in the orbit where the planet is nearest to the sun is its perihelion, and the point at which it is farthest is its aphelion. It is necessary to keep these elementary facts in mind in order fully to understand the changes in the motions and brightness of the planets.

The influence of one body over another that is circling around it is to make it move faster or more slowly according to its distance from the central body. Since a planet varies in its distance from the sun in the different parts of its orbit, it is forced to move fastest when it is in that part of the orbit which is nearest to the sun, and slowest when it is in the part farthest away. In other words, the motion of a planet is more rapid at perihelion than at aphelion. The earth is in perihelion, or nearest to the sun, in winter—that is, winter in the northern latitudes—and in consequence it moves faster in winter than in summer, and the northern winters are, for this reason, a little shorter than the summers.

These two simple movements of the planets—that around the sun and that on their axes—are their principal real movements, and are such as they would show to be if seen from the sun, which is the center of them. There are also certain minor real movements arising from various causes, one being the influence that the planets exercise on one another; but for the ordinary observer these have no particular significance. Then, the planets all share the one grand movement which the sun itself is known to be making through limitless space to a destination of which we are in utter ignorance, over even a path which we know nothing of save that it leads toward the bright star Vega, in the constellation of the Lyre. As the sun moves on in that direction at the rate of eleven miles a second he takes with him all his family of planets and planetoids, with their satellites, and whatever other bodies have their abode in his domain. Thus they travel as a body, each individual spinning on its axis, from the sun itself down to the smallest planetoid, the satellites circling around the planets, and the planets in their turn around the sun. And in all these movements the earth takes part as one of the planets. The sun itself is following a comparatively straight line in space, and, so far as we know, in allegiance to no other body. It is, though, just possible that this comparatively straight line may be the arc of a circle so vast that we have not yet had time to discover its curvature, and that the sun itself may be pursuing its own circuit around some still more powerful body.

VII

HOW THE INFERIOR PLANETS SEEM TO MOVE

Of the real movements of the planets, as described in the last chapter, we get here on the earth only a very fragmentary view. Without the aid of the telescope none of them is visible to us except the movements in their orbits, and these, to our view, are somewhat different from the simple, circling course apparent to an observer on the sun. The difference is due to the fact that the earth itself is always in movement in just the same way that the other planets are, and we, being never at any time at the center of the orbits, do not see the movements of the planets as they truly take place, but only as they are outlined against the sky. So the appearances and disappearances and visible travels among the stars by which we know the planets are only as we see them. Some knowledge of the real movements is necessary to a proper understanding of the apparent movements; but it is only with the latter that, for ordinary observation, we need to be particularly acquainted.

The rotation of the earth on its axis, as we know, causes the familiar daily apparent rising, passing, and setting of all the heavenly bodies. In this apparent motion the planets share as well as the sun, moon, and stars. But it is their movement among the fixed stars, and not with them, that distinguishes them as planets, and this it is necessary to know in order to keep track of them and be able to recognize them in their varying places and guises. For they sometimes shine in their greatest glory in one season, and sometimes in another, and at the recurrence of the same season they are sometimes in one part of the sky and sometimes in another, so that their ways of coming and going border almost on the mysterious, until one learns the manner of this apparent vagrancy. Happily, this knowledge is easily attained, and then the matter is simple enough.

The apparent motions of the inferior planets, Mercury and Venus, always take place near the sun. Venus never wanders more than forty-eight degrees from it, and Mercury never more than twenty-eight. Most of the time they are much nearer than this. Since we cannot see either of them except when the sun is below the horizon, the consequence of their being always thus near to him is that they are in view for only a short time after the sun has set or before he has risen. If they are in the evening sky, and hence east of the sun, they soon follow him when he sinks below the western horizon. If they are west of the sun, and, consequently, are the first to rise in the morning, it is not long before his brilliant rays flood with light the eastern sky and blot the planets from our view. Venus can be seen sometimes for three hours at a time, Mercury for never more than one. Within this limited region of the sky they appear to journey evening by evening away from the sun, somewhat obliquely, but toward the zenith, until they have reached the end of their tether. Then they journey back and pass to the other side of the sun. There they climb their path toward the zenith, moving westward and, as we see them, obliquely upward. Morning by morning they get farther from the sun until their westward limit of freedom is reached, when they again draw in toward the sun, pass it, appear in the evening sky, and pull off up the sky toward the east again. Thus they swing from east to west of the sun, and back again, in unceasing repetition.

As they pass the sun going from east to west—that is, from the evening to the morning sky—the inferior planets go between us and the sun; and when they swing back from west to east, or from the morning to the evening sky, they pass on the side of the sun farthest away from us. When they are in a direct line with the earth and the sun they are said to be in conjunction. If at this point they are between us and the sun, it is inferior conjunction. If they are on the other side of the sun, they are said to be in superior conjunction. When the planet, as seen in the evening, has traveled toward the east as far from the sun as it will go during that particular revolution, it is said to be at its greatest eastern elongation. Elongation means simply apparent distance from the sun; hence, greatest eastern elongation is the greatest distance possible east of the sun from our point of view. Greatest western elongation, which we see in the morning before dawn, occurs when the planet is at its greatest apparent distance west of the sun.

While apparently drawing near and then away from the sun, traveling obliquely up and down the evening and the morning sky, the planet has all the time been moving in one direction around the sun; but we could see the motion only as it appeared on the background of the sky. The planet is in reality just as far from the sun when it is in conjunction as at elongation. The difference is that we see it at a different angle, or from a different point of view. But it has not been at all times equally near to the earth.

When an inferior planet is at greatest eastern elongation, it is, of course, east of the sun, and can be seen above the sun in the evening after sunset, and is an evening star. As it moves westward nearer and nearer to the sun, it is above the horizon a proportionately shorter time each evening, and is more and more obscured by the sun’s rays until it reaches inferior conjunction, when it is exactly between us and the sun, and hence at the point nearest to us. Here it becomes invisible, largely because it has its dark side toward us, but partly because the dazzling light of the sun entirely obscures it. Once in a while our relative positions are such that we see it pass like a black dot directly over the bright face of the sun. This is called a transit. But a transit does not occur at every inferior conjunction. It would so occur if the planet’s orbit and the earth’s were in exactly the same plane. But the small tilt that they have is sufficient to throw the planet, when it is passing the sun, into such an angle that it does not pass directly between the disc of the sun and us, but a little above or below. Thus transits are rather rare, though they occur periodically in the case of both Venus and Mercury, and will be spoken of elsewhere.

When the planet has passed inferior conjunction, it is then west of the sun, and rises in the morning before the sun is up, and is a morning star. For a few days it can be seen either not at all or with difficulty. Then, as it works its way out of the rays of the sun and on toward the west, it rises earlier each morning until it reaches its farthest point west.

As it starts back east again its distance from the earth increases daily until it reaches its greatest distance from us at superior conjunction. It is then the whole diameter of its orbit farther from us than when it was at inferior conjunction, and it is again invisible. The illuminated side of it is toward us; but it is at its smallest, because it is at its greatest distance from us, and even when it is not directly behind the sun the light of that luminary is too great for successful competition. After it has passed superior conjunction it is again in the evening sky, apparently moving farther from the sun each day. It is at the same time actually coming nearer to us each day, and these two facts cause a daily increase in its brightness.

But an inferior planet is not, like the superior planets and the stars, brightest when it is nearest to us. It is, in fact, darkest when it is nearest—that is, when it is at inferior conjunction—and we cannot see it at all. This is because an inferior planet passes through phases, like the moon, changing gradually during its rounds from full to crescent, and back again. Its full face is toward us when it is on the opposite side of the sun and farthest from us. The proportion of the face that is illuminated grows smaller as the planet approaches its eastern elongation. But the planet grows brighter because it is coming nearer to us and is getting out of the dazzling rays of the sun. One-half of its surface is illuminated when it is at greatest elongation; but it is brightest a few days later, when less than half of its face is illuminated, because it is enough nearer to compensate for the slight diminution in the proportion of light on its disc. It is brightest in the morning a short time before its western elongation, for the same reason.

This in a general way describes the motion of an inferior planet, and this is all that we need to know in order to understand its ordinary visible movements. If we watch it carefully, however, we may detect that shortly before inferior conjunction it pauses in its onward sweep and seems for a time to be stationary, and then to retrace its way among the stars until a short time after inferior conjunction, when it again pauses and appears stationary, and finally starts off again in its original direction on its way toward greatest western elongation. During this capricious sort of progress the planet usually describes more or less of a loop, sometimes almost a flourish, in its path. The appearance is wholly due to the planet’s overtaking and passing us in our journey around the sun. For a time it travels behind us, then beside us, and then beyond us; and, since we are both in motion, the effect is much the same as when one train passes another while they are both traveling in the same direction. The orbits of the earth and the planet are not exactly in the same plane, and, both bodies being in motion, we are not in a position to see the planet at the same angle more than once as it seems to pass back and forth, and so we get the effect of its making a flourish or loop. But this effect, while interesting, takes place only when the planet is so near the sun that to the ordinary observer it itself does not count for much. We can see but little of the inferior planets at that time, anyway, though it is important for us to know where they are, in order to keep track of them and to be ready for them when they are to be seen.

VIII

HOW THE SUPERIOR PLANETS SEEM TO MOVE

The movements of the superior planets, Mars, Jupiter, Saturn, Uranus, and Neptune, as they appear to us, are different from those of the inferior planets in some important respects. Instead of swinging back and forth east and west of the sun, and never appearing very far away from it, as the inferior planets do, the superior planets make an entire circuit of the heavens, and it is possible to see them at any distance from the sun, and at any time during the night. Sometimes they are, with relation to the earth, in that part of the sky exactly opposite to the sun, and hence in line with it and the earth. At such times they can be seen all night. They are then said to be in opposition, and are in the best position for our observation. The earth being, when in this situation, in a direct line between them and the sun, we have the sun at our backs, as it were, shedding its full rays on the disc of the planet under observation, which is then at its nearest to us, and also at its brightest. For, since the orbits of all the superior planets are outside of ours, the planets never get between us and the sun, and, in consequence, never turn a dark side toward us. Their entire discs are practically always illuminated, and their changes in brightness depend largely upon their changes in distance, which, as we have seen, is not the case with the inferior planets.

Mars, the nearest of them, is at times somewhat gibbous (that is, shows a little less than a full face, as the moon does when just beginning to wane), and, in less degree, Jupiter also. But in neither case is this departure from fullness sufficient to have any appreciable effect on the planet’s brightness, and, moreover, it does not occur when the planet is in the most favorable position for us to see it. At opposition, therefore, we always have the full face of the planet presented to us; and being, as we then are, on the same side of the sun with it, we are ninety-three millions of miles (our distance from the sun) nearer to it than the sun is.

Being, when in opposition, exactly opposite the sun, the planet rises just as the sun sets. After opposition it rises a little earlier each evening, and is higher up in the sky at each succeeding sunset. When we find it just half-way between the eastern and the western horizon at sunset, it is at quadrature. After quadrature it appears nearer and nearer the western horizon each evening at sunset, until it finally is too near the sun to be visible. It is then traveling in that part of its orbit which is beyond the sun from us. From opposition to this situation it has been an evening star.

When a superior planet is in line with the sun and the earth, and is on the far side of the sun from us, it is said to be in conjunction, and we are then one hundred and eighty-six millions of miles, or twice our distance from the sun, farther from it than we are when it is in opposition. But besides being placed at so much greater distance from it, we have in this situation the bright sun excluding the planet from our view. It will be readily seen, therefore, why the superior planets are in so much better position for us to see them in opposition than at conjunction.

From conjunction to opposition the planet is west of the sun, and will be below the horizon at sunset, and will rise some time during the night. At first it will appear just before sunrise as a morning star, but will gradually rise earlier each night until, when it reaches opposition again, it will rise just as the sun sets. Half-way between conjunction and opposition it is again at quadrature.

From opposition to conjunction the planet will be east of the sun and above the horizon at sunset. When a planet is in conjunction with the sun, it passes the meridian, or the point half-way between rising and setting, about noon, and is above the horizon with the sun during the day. When it is in opposition it passes the meridian about midnight, and is above the horizon during the night. When it is at quadrature and moving toward conjunction, it passes the meridian about six o’clock in the evening, and may be seen in the western half of the sky during the early evening, and will set before midnight. When it is at quadrature and moving toward opposition, it will rise some time between midnight and sunset, and will be in view in the east during a part of the first half of the night. The nearer it is to opposition, the earlier in the evening it rises and the longer it may be seen.

The main movement of the superior planets among the stars is from west to east, and this is known as their direct motion. But not far from opposition they seem to hesitate, then move more slowly, then finally stop, remain stationary for a time, turn back on their tracks, and start off in the opposite direction. This is their retrograde motion. They do not continue in it as long as in the direct motion; but after a comparatively short time they again hesitate, go more slowly, stop, remain stationary, then turn back and swing off in the original direction, and continue to move in this direction until they are again approaching opposition. It is exactly in the middle of this sweep toward the west that the planet is in opposition. Close observation will show that the superior planets also make something of the same sort of a loop in their path among the stars that the inferior planets make, and for the same reason. The only difference is that when a superior planet is retrograding we are passing it, and when an inferior planet retrogrades it is passing us.

In giving this rather rough outline of the way the planets in general move among the stars, reaching in their wanderings these various positions with relation to the sun and the earth, the intention is only to fix some definite situations from which to consider the movements of the individual planets. When we come to know each planet as an individual, and to follow it as it comes and goes in the heavens, and to watch its ever-wonderful changes in brilliancy, these situations will have a much more definite meaning to us and a relatively greater interest and importance. The planets as they appear to us all move along pretty much the same path; but each has its own way of gracing this path, and each its particular manner of changing in aspect.

IX

THE PATH OF THE PLANETS

Though the planets are called wanderers, they are not by any means the vagrants that the name might imply. They have a fixed course among the stars from which they never deviate, and the ways of all of them, and also of the sun and the moon, are confined to a comparatively narrow strip in the sky.

That strip is called the zodiac. It is only sixteen degrees wide, and extends like a narrow band all the way around the heavens. It lies so that it is always easy to observe; and, being so limited, very little observation is necessary to become familiar with every part of it. Within its limits all the movements of the sun, the moon, and the planets take place. Through the center of it is the ecliptic, the great circle that marks the annual apparent path of the sun through the heavens. It is the standard circle from which we measure the paths of the moon and the planets. Whatever degree their courses vary from the ecliptic is what we call the inclination of their orbits. If the plane of the orbit of a planet is tilted away from the ecliptic, the planet will travel half the time on one side of it, and half the time on the other.

The orbits are, in fact, very little inclined to the ecliptic, and all but one of the planets may always be found within three degrees of it, most of them nearer than this. The one exception is Mercury, which is sometimes as much as seven degrees from this central line of the zodiac, but ordinarily it is not so far as this. Uranus is so nearly on the ecliptic that an ordinary observer would not notice the deviation, and particularly as Uranus can rarely be detected with the naked eye, and can never be thus followed. Of the four planets which are the ones we ordinarily see, Mars and Jupiter are never as much as two degrees from the ecliptic, Saturn never more than two and a half degrees, and Venus never more than about three degrees. They are all usually nearer than these outside limits. The greatest distance of the moon from the ecliptic is about one and a half degrees.

Hence, with the exception of Mercury, all the planets and the sun and the moon travel in a path six degrees wide, which is only one degree wider than the distance between the pointers as we see them in the Great Dipper. The fact that the zodiac is sixteen degrees wide, or eight degrees on each side of the ecliptic, is due only to a very generous allowance for the vagaries of Mercury, which he really does not quite need. For Mercury is always as much as twice the breadth of the moon, or one degree, inside of the zodiac, and usually more than that.

Because the earth is tilted on its axis twenty-three and a half degrees from the perpendicular, the ecliptic runs through the heavens in an oblique circle, crossing the line of the equator at two points called the vernal and autumnal equinoxes. The equator in the heavens is the great circle extending around the celestial sphere half-way between the north and south poles. It is always practically ninety degrees from the north star, and the points at which the ecliptic intersects it are called the equinoxes. These are the only two points on the ecliptic that are just ninety degrees from the pole. The word equinox is derived from equus (equal) and nox (night), and when the sun is at the equinoxes the days and nights are of equal length.

From the vernal to the autumnal equinox the line of the ecliptic is north of the equator, and hence high in the sky, reaching its highest point midway between the equinoxes. It then crosses the equator again and runs obliquely south to the lowest point in its path, and then curves northerly back to the vernal equinox. The vernal equinox is the point at which the sun arrives when spring begins. This results in the sun’s being north of the equator from spring until autumn, and south of it from autumn to spring.

As the part of the zodiac that we can see best at night is that opposite where the sun is, so in summer, when the sun is high, we see best the part of the zodiac which is low in the southern skies in the evening; and in the winter, when the sun is in the southern half of his journey, the part of the zodiac best seen by us is high in the heavens. No part of it, however, is ever as high as the zenith, or directly overhead, and no planet is ever seen as far north as the zenith in any place whose latitude is more than twenty-three and one-half degrees from the equator.

To know the paths of the planets it is necessary to know only twelve constellations out of the seventy or more in the entire heavens; but it is difficult to imagine any one’s learning these twelve without becoming interested in and more or less acquainted with many of the splendid stars and constellations that lie on each side of them. The larger one’s acquaintance is with the appearance of the skies as a whole, the easier, naturally, it will be to distinguish the planets from the stars, and to follow their courses. But the planets themselves may be intimately known quite apart from any but the twelve constellations forming the zodiac. Happily, among them we shall find some of the most beautiful constellations in the heavens, and some of the most splendidly brilliant first-magnitude stars.[1]

The twelve constellations of the zodiac are as follows:

Pisces, the Fishes.
Aries, the Ram.
Taurus, the Bull.
Gemini, the Twins.
Cancer, the Crab.
Leo, the Lion.
Virgo, the Virgin.
Libra, the Scales or Balance.
Scorpio, the Scorpion.
Sagittarius, the Archer.
Capricornus, the Goat.
Aquarius, the Water-Carrier.

We shall begin at the point of the vernal equinox to trace the line of the ecliptic through these constellations, and that line will mark for us the path of the sun, the moon, and all the planets. It is convenient to begin at this point, because it is where the sun crosses the equator in the spring, and hence it is at the beginning of that part of the ecliptic which lies north of the equator.

The point of the vernal equinox is now situated in the constellation Pisces. It is not marked by any bright star, but is not very difficult to find. It marks the point on the eastern horizon where the sun rises about March 21st, and about the 21st of September it is on the eastern horizon exactly opposite that point in the western sky where the sun sets. It is always ninety degrees from the pole, and if one chances to know the constellation Cassiopeia, which is shaped like a chair and is on the opposite side of the pole from the Big Dipper, one can locate the vernal equinox by drawing a line from the pole-star through the star which marks the lower part of the front of the chair, and extending it until it is ninety degrees long. The ninety degrees can be estimated by using the distance between the pointers in the Dipper (which is five degrees) as a measure. The star mentioned in Cassiopeia is about thirty-two degrees from the north star.

MAP SHOWING THE CONSTELLATIONS OF THE ZODIAC AND THE LINE OF THE ECLIPTIC RUNNING THROUGH THEM

The paths of all the planets, save one, lie always within three degrees of the ecliptic.

Having once learned the constellations of the zodiac and, approximately, the line of the ecliptic, it is not necessary for the ordinary observer to keep in mind the exact location of the vernal equinox. It is, however, an important point for the student of mathematical astronomy.

Beginning at this point, the ecliptic runs through Pisces in a northeasterly direction for about thirty degrees to Aries, the second constellation of the zodiac.

ARIES

Aries is best seen in the autumn when the sun is in the opposite side of the heavens. It is marked by a small acute-angled triangle, with the apex toward the north and the brightest star of the three at the apex. This star is called Hamal, and, while not a first-magnitude star, is a rather bright one of the second magnitude; and the triangle itself is very distinctly marked. It is the only group of stars by which to distinguish Aries, and it is sometimes confused with the little constellation called Triangulum, which lies just west of it, or above it, as it rises. With this in mind, Triangulum may be made to serve as an identifying mark. They both rise just a trifle north of the exact east early in the evenings of late September and October. Triangulum rises first, with its apex toward the south. In less than an hour the triangle of Aries arrives with its apex pointed north. The ecliptic runs about five degrees below this triangle, and its path across Aries is about twenty-eight degrees long. When one sees any very bright star in Aries, one may be sure it is a planet. The sun is in Aries from April 16th to May 13th.

During the summer this constellation is not visible in the early evening; but it may be seen every evening from September to April, drawing all the time nearer to the sun, and setting earlier each evening until the sun blots it out. From this constellation the ecliptic runs into Taurus, the third zodiacal constellation.

TAURUS

This constellation may be identified by the brilliant first-magnitude star Aldebaran,[2] and the misty Little Dipper of the Pleiades. It is a very beautiful and large constellation. About an hour and a half after the triangle of Aries has risen, the soft-twinkling cluster of tiny stars which form the Pleiades comes above the eastern horizon, and about an hour later a V-shaped cluster of brighter stars, with a very bright-red one at the end of the lower half of the V, appears. This last cluster is the Hyades, and the bright star is Aldebaran.

By these two clusters we may know the constellation. The ecliptic passes across Taurus about four degrees east of the Pleiades, and about seven degrees west of Aldebaran. The planets in passing through this region often come very close to the Pleiades, and parts of the group are sometimes occulted by the moon. Taurus is conspicuous in the eastern evening sky from September until nearly January. From that time on until May it may be seen in the evening, high up in the sky, a little farther west each evening, until it disappears in May. Among the four planets that we most see Mars is the only one that resembles Aldebaran in color. They are both reddish, but Mars is always west of Aldebaran near the line of the ecliptic, and also it does not have the same twinkling face that Aldebaran shows; hence the star and the planet need never be confused. Mercury, it is true, is reddish and twinkles, but so seldom needs to be taken into account that it will not be troublesome. The other planets when in Taurus will proclaim themselves by their color and size. There is no very bright star in Taurus except Aldebaran, which has been described. Any bright star north of it in the constellation is sure to be a planet.

Through Taurus the line of the ecliptic runs in a northeasterly direction, and about fifteen degrees east from Aldebaran it passes about half-way between two fairly bright stars which mark the tips of the horns of Taurus, and from there on into the fourth constellation.

GEMINI

Gemini lies northeast of Taurus, and is outlined by a box-shaped figure something more than twenty degrees long and about five degrees wide. The two stars marking the end of it farthest from Taurus are the famous twins, Castor and Pollux.[3] Pollux is a first-magnitude star, and Castor is very little less bright. They are both very charming stars, and too conspicuous to escape easy identification. Castor is greenish in tint, and rises between an hour and a half and two hours later than Aldebaran. About fifteen minutes after he appears, Pollux, with a yellow-tinted face, comes up over the eastern horizon. They rise about thirty degrees north of the exact east. The ecliptic has reached its highest point north just after passing through the horns of Taurus. It then runs through Gemini in a southeasterly direction, curving diagonally across the main figure and passing five or six degrees below Pollux. Gemini can be seen from October to early June. It is particularly charming in May in the northwest just after sundown, and when any of the planets are going along this part of their path at that season, they are sure to win one’s interest and admiration.

CANCER

After leaving Gemini the ecliptic passes through the small constellation Cancer. Its way runs southeasterly for about twenty degrees, passing just south of a charming little cluster of stars which can be dimly seen with the unaided eye, but comes out brilliantly with an opera-glass. It is called Præsepe, or the Bee-hive, and is the only object to attract attention in Cancer. Fortunately, it is so situated as to mark the line of the ecliptic through the constellation. The Bee-hive rests almost exactly on the ecliptic.

LEO

Leaving Cancer, the sun enters Leo, a large, well-marked constellation known to many persons by the conspicuous figure in it of a sickle. At the end of the handle of the Sickle is Regulus, one of the bright first-magnitude stars. A little more than fifteen degrees east of the Sickle the rest of the constellation is marked by a large triangle formed by three rather bright stars. Both of these figures are well marked and easily seen, making Leo one of the easiest of the constellations to find. The sun crosses it in a southeasterly direction which leads straight across Regulus. The star is often occulted by the moon, and by the sun also, though that we cannot see on account of the blinding light of the sun.

Leo is visible nearly eight months in the year. It is in the eastern sky early in the evening in the winter, and shines all night from late in December until April. In May and June it is traveling westerly, but high up in the sky. In July it is in the western sky in the evening. The sun passes through it from August 7th to September 14th. Regulus is a white star, and twinkles violently, so that it is easily distinguished from any planet that is passing near it. In the other part of the constellation the path of the planets runs about ten degrees below the triangle.

VIRGO

When the sun has passed Leo it enters the largest of all the constellations, Virgo, and passes through it in forty-five days, from September 14th to October 29th. The constellation is far from rich in bright stars; but one may find the ecliptic, or path of the sun, by following a curved southeasterly line from Regulus about sixty-five degrees until it reaches Spica,[4] a very bright first-magnitude star in this comparatively starless region. If there is any doubt about Spica, it may be found by following the curve of the handle of the Big Dipper about thirty degrees, which brings one to the splendid Arcturus, and then about thirty degrees farther on, which points one to Spica.

Eight or nine days after entering Virgo the sun crosses the equator at the autumnal equinox, and the rest of the ecliptic lies farther south. Spica is about ten degrees south of the equator.

Spica is in the east during the early evenings in April and May; throughout June and July it may be seen in the south during the evening. In October it sets at about the same time as the sun.

The autumnal equinox, or the point where the ecliptic crosses to the south of the equator, is in Virgo, and lies about fifteen degrees northeast of Spica.

LIBRA

Libra is the next zodiacal constellation, and it is a small one. The sun passes through it in about twenty-three days. It may be known by four fairly bright stars which form a more or less imperfect square. The ecliptic passes along the southern edge of this figure.

During the summer and early autumn, Libra is best seen. It is then passing across the southern sky, drawing nearer the west each evening. A planet passing across this constellation would always be easy to identify, since it would always be so much brighter than any star in this region. The sun enters Libra about October 29th, and it is not visible in the evening during the rest of the year.

SCORPIO

It is a joy to know Scorpio, quite aside from its connection with the path of the planets. It is a brilliant constellation, best seen during the summer and autumn, as it passes across the southern sky. It is the most southerly of any of the constellations of the zodiac; but the ecliptic passes through only a very small portion of the northern part of it, so the sun does not reach the most southerly point in its path while it is in this constellation.

Scorpio may be best identified by its brilliant deep-red star Antares,[5] which is supposed to lie in the heart of the Scorpion. The whole figure makes a splendid serpent-like sweep toward the southern horizon, and is one of the most conspicuous objects just west of the Milky Way in the south in summer.

The line of the ecliptic runs about three degrees north of Antares; hence the planets in their course sometimes pass very near it. Jupiter has been in that region all this year (1912), and will not be far from there the early part of 1913. Mercury and Mars both have something the color of Antares; but this is not likely to result in any confusion. The star is always there, and in the same relative situation with reference to the other stars. When Mars is there, it will always be above the star. Mercury can seldom be seen when he is in Scorpio. If he is in greatest elongation while there, he will still be near the sun, and the sun, as seen from the middle latitudes, is so far south and so near the horizon when in that part of the ecliptic that the situation will not be favorable for seeing the planet. Farther south, and particularly in high altitudes, Mercury could be well seen in Scorpio, but if the position of Antares is kept in mind, Mercury will easily be recognized as a stranger in the constellation.

The sun enters Scorpio about November 21st, and the constellation then ceases to be visible in the evening sky until the following May. It is in its greatest glory during the summer and early autumn.

SAGITTARIUS

When the sun leaves Scorpio it crosses the Milky Way into Sagittarius, and there reaches the lowest point in its path, twenty-three and one-half degrees south of the equator. This constellation is best distinguished by the little “milk dipper,” which is easily seen turned upside down just at the eastern edge of the Milky Way. The line of the ecliptic runs a little north of it. The constellation may be best seen during about the same months that Scorpio is visible. The sun enters it, and it passes out of view about the middle of December.

CAPRICORNUS AND AQUARIUS

From Sagittarius the ecliptic runs in a northeasterly direction through a region in which there are no very bright stars, nor any very distinct outlines of figures. The two constellations through which it passes are Capricornus and Aquarius. It then runs a few degrees into Pisces, and there reaches the vernal equinox, where we began to trace its course.

Although one cannot trace the line of the ecliptic with the same definiteness in this region as in one where there are bright stars to mark the way, yet when a planet is in this part of its path it is perhaps more conspicuous and more easily recognized than when it appears in any other part of the sky, because of the very absence of other bright bodies. These constellations comprise all that region running from the Milky Way east to the vernal equinox. It is a part of the heavens easily seen during the pleasant evenings of summer and autumn, and if a planet is crossing it during those seasons it is particularly well placed for observation.

The two brightest stars in Capricornus are of the third magnitude, and lie about twenty degrees northeast of the “milk dipper.” The ecliptic runs just under them. Through Aquarius it runs six or seven degrees above a waving line of faint stars, which are supposed to represent the water that Aquarius is pouring from his urn.

If one will take the trouble to trace the line of the ecliptic through the sky, and remember that it lies exactly in the center of the zodiac, and that the planets are, therefore, within a very few degrees of it, one will have no trouble in keeping track of them. The mere knowing of these constellations is in most cases sufficient, since the planets will disclose their identity in other ways than by position merely.

The signs of the zodiac are somewhat different from the constellations. They are simply twelve equal divisions of thirty degrees each, making in all three hundred and sixty degrees, which is the whole number of degrees in any circle. They are so divided for convenience in scientific observation and reckoning. About two thousand years ago the signs and the constellations in the main coincided, and they still bear the same names. The point of the vernal equinox was then at the beginning of the sign and the constellation Aries. But, owing to certain motions of the earth, this point shifts backward, or toward the west, about one degree every seventy-two years. In two thousand years it has shifted about twenty-eight degrees, until now the sign Aries, with the vernal equinox at its western boundary, lies almost wholly in the constellation Pisces, the sign Taurus corresponds approximately to the constellation Aries, and so on around the circle. It is important to know this in following the planets, because all almanacs and scientific publications deal mainly with the signs of the zodiac, and not with the constellations. When a planet’s place is said to be in Aries, Taurus, or Gemini, one will find it in Pisces, Aries, or Taurus, respectively. And so it is with all the other signs; they are each one constellation behind the one bearing the same name. And this is why, beginning with the vernal equinox, Pisces is the first constellation in the zodiac, while Aries is the first sign.

The following is a list of the signs of the zodiac, with the corresponding constellations. The symbols given in parenthesis are the ones used for these signs in all almanacs:

		SIGN		CONSTELLATION
Spring signs		Aries	(♈)	Pisces
		Taurus	(♉)	Aries
		Gemini	(♊)	Taurus
Summer signs		Cancer	(♋)	Gemini
		Leo	(♌)	Cancer
		Virgo	(♍)	Leo
Autumn signs		Libra	(♎)	Virgo
		Scorpio	(♏)	Libra
		Sagittarius	(♐)	Scorpio
Winter signs		Capricornus	(♑)	Sagittarius
		Aquarius	(♒)	Capricornus
		Pisces	(♓)	Aquarius[6]

X

MERCURY

While Mercury is one of the five planets that can be seen with the naked eye, it must be confessed that he does not play any important part in the great spectacle of nature as we see it in the skies. But in a certain way this only adds to our interest in him. The very rarity of his appearances and the difficulty of finding him give a zest to the search, and a sense of achievement, when it is successful, that one does not have with regard to the other planets. It is something akin to the feeling one has when, after a long tramp to some secluded recess in the woods in search of the shy pink lady’s slipper, a splendid specimen of that lovely flower suddenly comes into view hanging gaily on its stalk, ready for the use of whatever fairy foot may tread its shady groves.

Then, too, the spring o’ the year is the most likely time to see Mercury in the evening sky. He comes into his best position for this view of him just when the evenings are growing longer and milder and one begins to hunger for outdoor things, so that the quest of him at that time has the gladness that goes with our first excursions into the open after a winter’s housing, whether it be in search of flowers, or birds, or stars, or simply the general loveliness of everything that belongs to the beginning of the outdoor season.

The reason Mercury is so elusive is that he is always very near the sun, and in consequence his light is dimmed by the brighter light shed by that luminary until it is well below the horizon; and after the sun has set, the planet is so involved in the usual haziness of the atmosphere near the horizon that the conditions must be very favorable in order to see him. Though there are recorded observations of Mercury as far back as nearly three hundred years before Christ, yet some of the older of the modern astronomers, before the days of the perfected telescope, are said not to have seen him at all; and the most important observations of the planet nowadays are made in broad daylight, when it is higher up in the skies and free from the mists of the horizon. This can be done by means of a powerful telescope, because it is possible in this way to shut off the light of surrounding bodies; but, of course, the conditions are not as favorable as if midnight observations could be made. Still, if one knows just when and where to look, Mercury can be seen with the naked eye at least once or twice a year, and sometimes oftener than this, especially if one chances to live in one of the Western States, where the air is very clear and the situation in latitude and altitude more favorable than, say, in New England, or in the middle Atlantic States. In our Northern States, and in the whole of England, this planet is more difficult to see, because of the longer twilight in northern latitudes, and also because the line of the ecliptic, over which it passes, seems there lower down in the skies, while in the far South, say in Cuba or Porto Rico, the twilight is shorter, the ecliptic runs high in the sky, and the situation is favorable for a good view even though the atmosphere is no clearer than it is farther north.

WHEN AND WHERE TO FIND MERCURY

Mercury is never more than twenty-eight degrees from the sun, and is brightest when the distance between them is somewhere near twenty-two degrees, or about four times the distance between the pointers in the Big Dipper. The direction in which to search for him must always be along the line of the ecliptic obliquely above the sun. Since his orbit is inclined seven degrees to the ecliptic, he will be some place within seven degrees of this line, on one side or the other. Within this narrow strip in the sky, fourteen degrees wide and twenty-eight degrees long, Mercury will be found whenever he is visible at all. And this strip may be further shortened by at least twelve degrees; for when the planet is nearer than that to the sun it is futile to attempt to see him with the naked eye, save in very exceptional conditions. The five degrees between the pointers will serve as an aid in measuring these distances.

We can never see Mercury with the naked eye except when he is near one elongation or the other; and even then he is visible only about an hour after the sun is down in the evening or about an hour before it rises in the morning. Three times each year he appears in the evening for more or less than a week, according to the situation of the observer, and three times a year he is visible in the morning for about the same length of time. But, owing to his position with relation to us, the evening exhibit that comes in the spring is the most favorable one for a good view of him, and the morning appearance that is most favorable is the one that comes in the autumn.

The mean synodic period of Mercury is about one hundred and sixteen days, or a little less than four months. That is, he returns to greatest eastern elongation and can be seen in the evening sky about every one hundred and sixteen days, and the same length of time elapses between his appearances in the morning sky at greatest western elongation. But this mean synodic period is made up of synodic periods varying in different revolutions from one hundred and five to one hundred and thirty-four days. So, though one may mark the dates at which the various positions of the planet occurred during any one revolution, one cannot so easily determine the exact time at which he will be found in the same positions at the next revolution; that is, whether the revolution will take place in less or more than one hundred and sixteen days. The earth and the planet are each traveling at varying rates of speed, according as they are near the sun or farther from it, and obviously it is a situation that requires careful mathematical work to compute. The almanac must be referred to for the exact date.

But, lacking an almanac, one will generally find that Mercury will return to the same position relative to the earth and the sun within a few days of his mean synodic period. Three periods, however much they may vary individually, are almost always equal to three hundred and forty-eight days, or three times the mean period. This is seventeen days less than a year. Hence, if one is lucky enough to have seen Mercury at eastern elongation one spring, and will look the next year about seventeen days earlier, the planet will be found a little to the east (about fifteen degrees) of where he was when first seen the year before. He is there in the same position with relation to us and the sun that he had the preceding spring, but in a slightly different relation to us and the stars, because the sun lacks seventeen days of having completed its apparent yearly journey around the zodiac. It must still go through about one half of a constellation.

When Mercury shows himself at eastern elongation, he may be seen in the west as an evening star for somewhere near a week, each evening drawing nearer to the sun. When he disappears from view he passes between us and the sun, and about four weeks later appears in the morning sky before the sun rises. Under favorable conditions he is again visible for a week or more; and then, again approaching the sun, he can be seen no more for about ten weeks, during which time he passes through superior conjunction on the other side of the sun from us and comes back to eastern elongation.

Thus we can get, under very favorable conditions, six short views of Mercury during the year—three in the evening and three in the morning. So many views, however, are rarely secured by any but the professional observer. The circumstances may well be considered felicitous if one succeeds in getting a glimpse of him once or twice a year—at his favorable situation in the evening in the spring and the morning in the autumn. The sight of him, though, is truly worth a little inconvenience—even to the extent of facing a cold evening wind in the very early spring or getting out of a comfortable bed before dawn during the first cool mornings of autumn.

It is hardly possible to say exactly where one can find Mercury at all times during a long succession of revolutions. Moreover, it is not necessary. These computations are made anew each year by experts in the employ of the government, and the result is published in the Nautical Almanac. From there it finds its way into all almanacs, so it is easy of access to any one.

In the almanacs Mercury is represented by the sign (☿). It is a conventionalized form of the caduceus, or wand, carried by the god Mercury as a symbol of his power.

The next seven eastern and western elongations of Mercury occurring after the publication of this book are as follows:

DISTANCE AND BRIGHTNESS

Of all the planets Mercury is nearest the sun. His average distance is thirty-six million miles. He is nearly eighty times nearer than Neptune, the outermost planet, and more than two and one-half times nearer than we are. But his orbit departs so far from being a circle that his distance from the sun varies as much as fifteen million miles. When he is nearest the sun, or in perihelion, he is only twenty-eight million miles from it; when he is farthest, or in aphelion, his distance is forty-three million miles. There is even greater variation in his distance from us. The difference between his least possible and his greatest possible distance from us is as much as eighty-nine millions of miles. For the earth has an elliptical orbit as well as Mercury, and when we are at perihelion, which occurs in the winter, we are three millions of miles nearer to the sun than we are in mid-summer. If Mercury chances to be then at his greatest distance from the sun, and also at inferior conjunction, or between us and the sun, he is only forty-seven millions of miles from us. If, when we are farthest from the sun, he also is at his greatest distance from it, and is in superior conjunction, or on the other side of the sun from us, he is one hundred and thirty-six millions of miles from us.

These changes in distance from the earth have much to do with Mercury’s changes in apparent brightness to us. At his brightest, when he appears at greatest elongation and we can see him without a telescope, he is brighter than Arcturus, the brilliant first-magnitude star in Boötes, that swings over us nightly from early spring to late autumn. When seen with the naked eye, he is also red in color, somewhat like Arcturus; but through a telescope he is dull silver, like the moon, or even more ashy in his paleness. As he goes farther and farther from us he becomes dimmer and dimmer and can be followed only with a telescope until, even with this aid to vision, he is lost in the rays of the sun at superior conjunction. His apparent diameter as mathematically measured varies from five seconds, when he is farthest away, to thirteen seconds, when he is nearest.

When he is at his nearest possible distance from us, light travels from Mercury to us in a little more than four minutes. At his greatest possible distance we could not receive the waves of light that he sends out in less than twelve minutes. As a matter of fact, we do not receive them at all, for, as we have seen, he is invisible when at his greatest possible distance from us, being then on the far side of the sun.

Another cause of Mercury’s apparent change in brightness is due to the fact that, in common with Venus, he goes through phases from crescent to full like the moon. This is, as we have seen, a result of his shining only by reflected light and of his orbit’s being between ours and the sun. If he shone by his own light, he would be at his nearest approach to us a very brilliant body indeed. As it is, his dark side is turned toward us when he is nearest, and when his full face is illuminated he is on the far side of the sun. We see half of his face when he is at greatest elongation; but he is brightest when we see less than half, because he is then nearer to us, and the difference in distance more than compensates for the difference in illumination.

These phases cannot be seen with the naked eye, but it requires only a small telescope to show them, and a very charming little moon-like body Mercury is when we see them. His horns point toward the east when he is coming toward us and nearing inferior conjunction, and when he is backing away from us and going toward greatest western elongation they point toward the west. It was through the blunting of one of these horns when the planet was in certain positions that a mountainous surface was suspected, so great is the significance of small details in observations.

As a mere place from which to view the other bright bodies Mercury would be far superior to the earth. He not only has the sun nearly seven times larger in appearance at its mean distance than we see it, but, being himself nearest the sun, all the other planets are outer planets in relation to him, and all have their discs fully illuminated.

The earth and the moon, as seen from Mercury, would show as a splendid pair of stars circling about each other, the earth more brilliant than any first-magnitude star, and the moon of the third magnitude, or about as bright as Phecda, the star at the bottom of the bowl of the Big Dipper, just under the beginning of the handle. The earth would show a disc of about twenty seconds, and the moon one of about eight seconds, with a distance between them of about 871 seconds. Some idea of what this distance is may be had if one knows Mizar, the star at the bend of the handle of the Dipper, and its tiny shining attendant, Alcor. These two stars are 708 seconds apart. The distance between them is about equal to one-third of the diameter of the moon as measured from the earth. It does not appear to be nearly so much as that, and some persons have difficulty in separating the two stars; but the moon is not only inconstant but deceptive, and owing to its brilliancy seems always proportionately larger than it really measures.

Venus would appear from Mercury as much as four times as large as she seems to us—a veritable little moon, and always full, her size varying slightly as Mercury speeded back and forth from the farthest to the nearest point in his orbit, changing the extreme of the distance between them from one hundred and ten million to less than twenty-four million miles. If Mercury needed a moon, he could well find some consolation for his lack of it in the presence of the lovely Venus in his sky.

MERCURY’S SIZE AND THE CONSEQUENCES OF IT

Mercury is the smallest of all the major planets. His diameter is about three thousand miles. It is only about nine hundred miles greater than that of our moon. The surface of Mercury is only one-seventh that of the earth, and his volume only one-twentieth. Jupiter and Saturn each have a satellite that is considerably larger.

Mercury would make a splendid satellite or a giant asteroid, but as a planet seems hardly to have had a fair chance in life. For being a small planet means something more than being constructed on smaller lines than some others are. It means a difference in physical development. It means less power to hold the gases that compose an atmosphere, which is the cover that shields the planets from the too burning rays of the sun and keeps their internal heat from radiating too quickly into space. It means less power to resist the tidal friction that the parent body uses as a brake to retard rotation. It means a shorter time of activity in life, and a long, dull, monotonous old age.

The nucleus that was detached from the great spiral, or the portion of nebula that was separated in whatever way from the parent body, to form Mercury chanced to be a small one. Being small, it was unable to add materially to its mass by attracting other particles to it through the power of gravitation, as a larger planet might do, and thus Mercury was doomed to develop with the limitations that nature’s law has decreed as inevitable in the small bodies of our solar system, be they planets, satellites, or asteroids. Of these limitations the first and most far-reaching in its effect is the feebleness of its force of gravity, or power to attract other bodies.

Mercury’s force of gravity is small. It is smaller than that of any of the other planets. It is a little less than one-quarter that of the earth. The same weight of feathers that would compose a pillow here would make a whole feather bed on Mercury. Any object weighing one hundred pounds here would weigh only twenty-four there. The materials composing our earth and all the planets are held together only by the force of gravity. The air we breathe would dart off into space with almost incredible fleetness if the earth had not sufficient gravitative force to hold it. Its particles are struggling all the time to get beyond this power. The lightest of them do get beyond it and are lost, and the less power we have to hold them the sooner they leave us. The greater the mass of a body, the rarer the gases it can hold in its atmosphere, for this mysterious force which pulls everything toward the center of a planet depends upon its mass, or the quantity of material in it. The planet may be very large because it is very much expanded. It may be gaseous even, and its mass would then be very small in proportion to that of a solid body of the same size. As it condenses, the particles draw closer and closer together, the density increases; but the mass is the same. It is only the size that diminishes.

So a planet with a small mass starts out in life with a disadvantage. It not only has little power to grow by drawing in particles from its environment, but also has little power to hold such as by their nature are volatile and swift of motion, as the molecules of gases are. The mass of Mercury is not exactly known. The only way we have of measuring the masses of the planets is by their influence through gravitation on other bodies near them. When a planet has satellites, the movements of the satellites tell the story, and by mathematical calculation the amount of material in the planet can be determined. But Mercury has no satellite, and the only way to determine his mass is by observation of his influence on Venus, and on an occasional comet which passes near enough to be disturbed by the planet. The particular comet which has been useful in determining the mass of Mercury is Encke’s. On passing near the sun it comes sometimes near Mercury, and the pull it has repeatedly received from that little planet on such occasions is thought to be largely responsible for the comet’s having become a part of the solar system. The changes in its orbit caused by these encounters show the power of Mercury, and hence the mass.

In these ways the mass of Mercury has been found, with reasonable belief in its accuracy, to be about three one-hundredths that of the earth. Yet there are, indeed, considerable differences regarding it among astronomers. The exact figures are not important to any but the close student. It is certain that the mass of Mercury is very small—so small that the planet probably never had much atmosphere, and almost undoubtedly has none to speak of now. The planet could not hold any molecule moving faster than two and forty-five one-hundredths miles a second, and few gases move as slowly as this. The proportion of light that Mercury reflects to that which he receives also points to a probable scarcity of atmosphere. If he had an atmosphere, it would have clouds. Clouds have a very high reflecting power, giving out about seventy-two per cent. of the light that falls upon them. Mercury reflects only fourteen per cent. of the light he receives, which shows at least a lack of clouds, and something more. It indicates a hard, dark, almost metallic surface, and a very considerable density. Density, however, is the only quality in the possession of which Mercury seems to occupy a middle ground among the planets, being slightly less dense than either Venus, or Mars, or the earth. The earth is the densest of all the planets, and it is about one-third more dense than Mercury. Density is simply the closeness with which the particles composing a body are packed together. A piece of gold, for example, is denser than a piece of iron of the same size.

WHAT THE SUN DOES FOR MERCURY

It is probable that Mercury has no alternations of light and darkness, causing day and night such as we know them. That is, the planet does not rotate on its axis in such a way as to turn first one side and then the other toward the sun as the earth does. In this, as in some other things, Mercury must accept the fate that overtakes many other small bodies which revolve around large ones—that of our moon, for instance, and the satellites of some of the other planets. Working under the law of gravitation, which gives such power to the large bodies, the sun has so retarded the rotation of Mercury that the planet now makes but one rotation on its axis during one circuit around that central body, and so keeps always the same face toward the sun. Some astronomers do not regard this as having been wholly proved; but all the later observations of Mercury strongly indicate that it is the fact, and it is coming to be more and more regarded as established.

But, even if this is the predicament into which Mercury has come, the planet is probably not in so bad a plight as many another body to which the same sort of thing has happened. The extreme eccentricity of his orbit, which has given him the true mercurial temperament, resulting in sprightliness, agility, and changeableness, is accountable for some mitigating circumstances. The sun may hold him so that he cannot turn his face away from that luminary; but it cannot keep him from rotating on his axis at a uniform rate of speed, and from this, combined with the vagaries caused by his eccentric orbit, come some interesting things.

Since Mercury is less than two-thirds as far from the sun at perihelion as he is at aphelion, there is a corresponding variation in his rate of speed. When he is nearest the sun, at perihelion, he darts along at the rate of thirty-five miles a second; at aphelion, when he is farthest from the sun, he travels only twenty-three miles a second. Twenty-three miles in one second is not exactly a snail’s pace, terrestrially considered, and it is faster than the earth moves at any time; but the planet was named Mercury because of his swiftness, and we would not expect much lagging even when he is moving at his slowest gait. This difference in speed in different parts of his orbit causes what is called the librations of Mercury. When he is traveling at his swiftest pace he gets a little ahead of his rotation, the speed of which is uniform, and thus throws the sunlight somewhat farther around on one side. When his speed decreases, he falls behind his time of rotation, and thus gets a little more sunlight on the other side. Thus, during each revolution he juggles the sunlight a little farther around him than he could if he were a more steady-going planet.

These librations result in there being two strips on the surface of Mercury—one on each side—which undoubtedly have a day and night, varying in length in the different parts of the strips. The part that lies nearest the illuminated side of the planet has alternate periods of sunlight and darkness, each of considerable duration, while that part nearest the dark side has merely a glimmer of sunlight every eighty-eight days, which is Mercury’s sidereal year, or the time required for him to make one revolution around the sun. These two strips on which the light varies comprise about one-eighth of the surface of Mercury. One half of his entire surface is always light, and of the other three-eighths are always dark. It is this dark, cold side that is turned toward us when Mercury is nearest to us.

It is possible that on those parts of Mercury where the sunlight and darkness are unstable there may be something resembling a tolerable temperature. They are something more than a thousand miles in breadth, and perhaps near the center of them the sun may give heat sufficient to enliven and yet not burn. More than likely, they are alternately scorched and frozen. For it takes more than the mere presence of sunlight to make a climate tolerable. Atmosphere is what is necessary, and we have seen that Mercury has probably lost practically all his atmosphere long, long ago. An atmosphere absorbs much of the radiant energy that comes from the sun before it reaches the more solid parts of a planet, and it also acts as a blanket in preventing the too rapid escape of such heat as the planet may have acquired. Thus it has the doubly beneficent office of tempering the rays that would otherwise be scorching and of hindering a radiation that would leave the planet stiffened and frozen.

Stiffened and frozen is what the dark side of Mercury undoubtedly is. The sun has never shone upon it since Mercury became a solid body. All the inherent heat it had has long since passed off into space, and its temperature must be somewhere near the absolute zero. The absolute zero is the point in temperature where all known substances become solid. It is more than 450° below the Fahrenheit zero, or more than 350° lower than any temperature recorded in our arctic regions—a degree of cold unthinkable to any but the scientist.

On the other side of Mercury the heat is beyond anything we have any notion of. With an equal atmosphere it would receive from the sun six thousand times as much light and heat as Neptune on an equal space, and, on an average, seven times as much as the earth. At Mercury’s distance from the sun his hot side would be more than 300° above zero, if there were absolutely no atmospheric protection. Even though tempered by a thin atmosphere, as it may be, the heat on this side is still probably enough to boil away any water that might be there and to change some other substances from what we regard as their normal state.

Stability, at least, is a quality of the hot and the cold side of Mercury. Scorched and seared and desolate of life, as we know it, the one side lies under a blazing, dazzling sun. Cold and hard and bleak, and no less desolate, the other side turns its face toward the darkness of space. Thus they will remain until the end of time. And let us hope that, when the final catastrophe occurs and a new nebula is formed, the matter composing Mercury may find a place in a larger mass, and in its new incarnation have a fuller and larger life.

It is the atmosphere also which causes twilight, as well as the gradual changing from heat to cold. With no atmosphere, we would drop from full daylight to the darkness of starlight at the setting of the sun. So, with the thin air that Mercury probably has (if he has any), the two zones which are alternately light and dark, and hot and cold, are not much better off than the parts which are permanently either light or dark. They are plunged alternately from the temperature and light of the hot side of Mercury to the temperature of the cold side, with few gradations to prepare them for such extremes. Thus the only part of the planet that might be expected to have any variations of seasons fulfils the expectation with little satisfaction.

The only changes in climate which may have an appreciable effect are mainly those caused by the eccentricity of Mercury’s orbit, which carries him so near the sun at certain times and so comparatively far away at others. When he is nearest the sun he receives more than twice as much heat and light as when he is farthest away. At aphelion he receives four times as much heat and light as the earth. At perihelion the amount of heat and light is increased to more than nine times that of the earth. Since it takes Mercury a little more than twelve weeks to make one revolution around the sun, he passes from nearest distance to farthest, or the reverse, every six weeks. And thus, as viewed from the planet, the sun expands gradually for six weeks until it has increased its diameter two and one-half times, and the next six weeks it diminishes in the same proportion. At such times, of course, the amount of heat is more or less according to the planet’s distance from the sun; but all the time it is very great.

Moreover, it is believed that the axis on which Mercury rotates stands perpendicular to his orbit. This being the case, there would be on Mercury no change of seasons such as the earth has. The earth’s axis is inclined a little more than twenty-three degrees to its orbit, and from this we get the sun’s rays in a great variety of directions and different degrees of obliquity, causing the seasons, as we know them, in grateful variation. With the axis perpendicular, as it probably is in the case of Mercury, the sun’s rays fall on the face of the planet always with the same degree of directness, the only relief from their greatest heat being when the planet backs away from the sun every six weeks, and when in his librations he turns first one sun-burned cheek and then the other toward the coolness of space.

Thus we must regard the smallest of our family of planets, Mercury, as always the dwarf among us, with never a fair chance to develop a rich and luscious life according to our ideas of such a life. Beaten by the sun’s hard rays, and with no sufficient atmospheric protection; pulling always at his tether, but held firmly with his face to the center; circling at times with mercurial swiftness and thus cheating the sun into sending its rays farther toward the dark, cold side of him than it otherwise would, and with all his defects from a human point of view, we may still regard him as a right merry, roguish little planet, after all. He may be prematurely aged, he may have missed many experiences that the larger planets are having, he may have a long time to wait for the final change that will reunite us all; but he is not lying in sluggish inactivity until it comes.

In view of the fact that he is the only planet that twinkles, may it not suggest, when we see his ruddy face peering through the thick atmospheric mists near our horizon, that the impish little body is winking at us, and that it may be with planets as it is with people: they may not always be in an unfortunate plight because their fate is different from ours?

TRANSITS

Occasionally Mercury passes at inferior conjunction between us and the disc of the sun, appearing like a black spot against the sun, and thus makes what we call a transit. Because the planet is so small, his transit across the sun cannot be seen with the naked eye; but it is an interesting phenomenon to those who can view it with a telescope, though, apparently, astronomers do not regard it as having any great scientific importance. It is during a transit, however, that we watch for confirmation of the theories concerning Mercury’s atmosphere, which, if it were a reality, would show a diffused light about the planet; and until this question is settled beyond any dispute it will always come up at the time of a transit of Mercury. At nearly every transit some observer sees these indications of an atmosphere; but the better the telescope, the less they seem to be seen. Hence it is probable that there is an illusion somewhere either of eye, or instrument, or mind, and that the majority opinion, which accords to Mercury practically no atmosphere, is about the correct one.

These transits occur at intervals of seven, thirteen, or forty-six years, according to the position of the earth. They would occur every time that Mercury passed inferior conjunction if the earth’s orbit and that of Mercury were in exactly the same plane. But the orbit of Mercury, we have seen, is tilted out of the plane of the ecliptic, which marks our orbit, seven degrees, so that the only time the earth and the planet are anywhere nearly in the same plane is when they are at or near the points where their orbits cross each other.

The earth is near the two points where Mercury crosses the ecliptic about May 8th and November 9th, so that transits can occur only near these dates. Mercury passes these points four times every year, or once in each revolution around the sun. But the earth is not always there at the same time, and it is because of this that transits occur only in periods of seven, thirteen, or forty-six years. They occur more frequently in November than in May. The last transit was in November, 1907. The next will be on November 7, 1914, and there will not be another in November until 1927, an interval of thirteen years. But at the point where the May transits occur there will be one on May 7, 1924.

XI

VENUS

Of all the planets lovely Venus is the one that is best known and most admired. It far exceeds all the other planets in brilliancy and beauty when as an evening star it hangs in gracious silvery softness above the sun, which has just passed below the horizon; and it is not less surpassing in loveliness when as a morning star it comes into view shortly before the sun rises, its glowing face still silvery and bright, but yet tinged with the rosy flush of the eastern morning sky.

In either position it never twinkles as Mercury sometimes does, but shines so steadily and softly that at times its disc can almost be seen with the naked eye, and it has such brilliancy that its light can often be seen in the daytime, if one knows when and how to look for the planet. At its brightest it frequently throws a light sufficiently strong to cast a shadow, as one may easily prove by holding a book or some other opaque object between Venus and a white background, such as the wall of a white house. It is six times as bright as the brightest of all the fixed stars, Sirius, the beautiful dog-star, which we see in winter chasing across the southern skies after Orion.

Venus’s superior brilliancy is due in part to the fact that it comes nearer to the earth than any other planet; but it is also intrinsically brighter than any of the others. From equal areas it reflects almost four times as much light as Mercury and three times as much as Mars.

WHEN AND WHERE TO SEE VENUS

When Venus appears in the sky she is not often mistaken for any other planet. Among all the planets she is the most readily recognized and the easiest to find. This is due largely to her extreme brilliancy and a peculiar silvery appearance that none of the other planets have; but also, in part, to her limited range in the sky, and her favorable situation for observation. Unlike Mercury, she is far enough away from the sun to be seen above the horizon for as much as three hours after sunset, and is then sufficiently high in the heavens to be seen free from the vapors of the atmosphere at the horizon. Yet, being one of the inferior planets, with her orbit smaller and nearer the sun than that of the earth, she can never get so far from the sun as to be at any uncomfortable height for viewing, and hence, when she can be seen at all, is always an obvious bit of brilliancy and a joy to the beholder. She is never higher in the sky than forty-five degrees, which is half-way between the horizon and the zenith, and is never farther away from the sun than forty-eight degrees. One frequently sees a bright planet higher up in the heavens than this; but it is never Venus nor Mercury.

We first begin to notice Venus in the evening sky about six weeks after she has passed superior conjunction. She is then very near the sun, and sets a little less than half an hour after sundown. Evening by evening she grows gradually brighter, mounts higher and higher in the sky and, consequently, sets correspondingly later, until in a little more than seven months after superior conjunction, and about six months after we have begun to watch her, she reaches her greatest elongation east from the sun. At that time she is usually somewhere near forty-five degrees above the sun, and is a very lovely and conspicuous object in the evening sky, setting a little more than three hours after sundown.

From this point she begins to travel back toward the sun, still becoming brighter each evening, because she is really coming nearer to us; and in about four or five weeks she attains the greatest brilliancy that she will have as an evening star during the particular revolution she is making. About twelve days after her brightest she will reach the point where she seems to be stationary for a time. This is when she is about to overtake us in our journey around the sun. After a short pause she will move on gradually, her course among the stars then being retrograde or westward; but what we most notice is that she is drawing nearer to the sun, setting earlier each evening, and becoming more and more difficult to see. At the end of about three weeks she is in inferior conjunction, on a line between us and the sun, and invisible. She has run her course as an evening star for nine and a half months, and has been visible anywhere from seven to eight months, the time of her invisibility depending upon the eye of the observer and the conditions of situation and atmosphere.

A week or two later we shall find her a splendid morning star, rising nearly an hour earlier than the sun. About three weeks thereafter she will be at her brightest as a morning star, and will continue to be very brilliant for some weeks. In about five more weeks she will have reached her greatest elongation west of the sun, and will rise about three hours and a half before dawn. Then she will begin to retrace her path, moving eastward, growing smaller all the time as she goes farther away from us, and showing a slower apparent movement, which gives one an agreeable sense of a reluctant parting, until after a little more than seven months she will have reached the sun and will again be in superior conjunction. She has then been a morning star for nine and a half months, and has been visible for about the same length of time that she was when she shone as an evening star.

This is a brief outline of a typical journey of Venus through one synodic revolution. She began one of these journeys on July 5, 1912, being then in superior conjunction. During the autumn of this year and the winter of 1912–13 she may be seen shining with great brilliancy in the west at sunset, and a few hours thereafter. Early in November, 1912, she and Jupiter will both be in Scorpio, where they will approach within two degrees of each other; and there is no doubt that their presence will add much charm to that region of the sky during the entire autumn.

About the middle of February, 1913, Venus will appear half-way up to the zenith at sunset. She will then be at her greatest distance east of the sun, and will be very bright; but, though a little nearer the sun, she will be still brighter shortly after the middle of March. A month later she will be invisible, and inferior conjunction will occur on April 24th. During most of May and all of June and July she will be a morning star, and her brilliant beauty will well repay an early morning outlook. She will get back to superior conjunction on February 11, 1914, and in that year she will be in an ideal situation for us to cultivate a more intimate acquaintance with her. From the latter part of March to November, 1914, she will be the brightest star in the western evening sky, and will do much to enhance the beauty of the pleasant summer evenings of that year. The sturdy, red-faced Mars will meet her on August 5th, a little more than a month before greatest eastern elongation, and might almost kiss her pale cheek as they pass within one-sixth of a degree of each other, a distance equal to less than one-third of the diameter of the moon.

The next long period when Venus will shine as an evening star will comprise the spring and early summer of 1916. She will be at her greatest distance from the sun during the last week of April, and will not pass from view until about the first of July. Then again she will be an evening star, and so seen in the west during the autumn of 1917 and the winter of 1917–18, reaching greatest eastern elongation during the first few days of December, 1917. Her next return to the evening sky will be for the first eight months of 1919, and the next will be for the winter of 1920–21 and the spring of 1921.

The synodic period of Venus is nearly five hundred and eighty-four days, or a little more than one year and seven months. That is, the planet returns to the same position with relation to the sun and the earth at intervals of about that length. The intervals do vary, however, as much as a week or more, owing to the various motions and situations of the planet and the earth. But every eight years Venus and the earth come around to almost exactly the same relative position with each other and the sun and the stars, and thus the appearances of Venus at the various seasons practically repeat themselves every eight years. The full splendor that she is to offer us in the summer of 1914 will be repeated in 1922, just as that of 1914 will but repeat that which she showed in 1906. And in each of the intervening years she will have again the same appearances that she had eight years before.

With the following table as a guide, the appearances of Venus can be followed through a number of years with sufficient accuracy for any but a close student of her movements. The exact dates of elongations and conjunctions will vary a few days, but for at least two or three multiples of eight years not enough to make any material difference in her various aspects.

1913—1921—1929—1937

Greatest eastern elongation, February 12th. Inferior conjunction, April 24th. Greatest western elongation, July 3d.

1914—1922—1930—1938