Astronomy; a popular handbook

Front matter

Class Q B44- Book. ■ J3

CDFHUGHT DEPOSIT.

MACMILLAN & CO., Limited

Preface

The present volume has been prepared with a double purpose, and upon a plan somewhat unusual. First, an effort has been made to meet the wishes of the ordinary reader who may desire to inform himself as to the present state of astronomic science, or to secure a simple explanation of the many phenomena constantly exhibiting themselves in the universe about him ; and the further purpose has been to produce a satisfactory textbook for use in high schools and colleges.

Thus, for the general reader, it has been thought necessary to eliminate all formal mathematics ; for the student, on the other hand, the occasional use of elementary algebra and geometry are essential. To satisfy these two apparently contradictory conditions, the book has been written in two parts ; the first free from mathematics, the second a series of extended elementary mathematical notes and explanations to which appropriate references are made in the first part of the book. Thus the general reader may confine his attention to the non-mathematical part ; the student should master the whole volume.

Attention is directed especially to Chapter I, in which is presented a brief summary of the entire science. It is hoped that this will serve to strengthen in most readers a desire for further and more detailed information. To the student this chapter should furnish as much knowledge as he must have in his possession before beginning a direct

vii

PREFACE

study of the sky with a telescope. In the author's extended experience as a college teacher of elementary astronomy, he has found it most desirable to give life to the subject by requiring frequent evening visits of students at the observatory. These should begin almost immediately upon commencing the study of the science ; and the first chapter is therefore intended to give the students something to work upon, even in their earliest observatory visits. At Columbia and Barnard colleges, these visits are required on frequent dates, regularly assigned throughout the year, without regard to the state of the weather. When clear, the telescope is used ; when this is impossible, oral and informal discussion takes place upon the work done in the classroom. Attention is also given to the daylight study of solar shadows, all students being required to construct a practical sundial, as explained in Chapter V.

The author has, of course, drawn freely upon many othei books, especially in the preparation of numerous diagrams, and in arranging the various parts of the subject in order. But most of the diagrams are new, and all have been simplified as much as possible. In a few cases, illustrations were copied from very old astronomic textbooks: references are then always given, in the hope that some readers, at least, will be led to examine these fine venerable classics of the science.

Almost all the inserted plates are photographic reproductions of actual photographs. For these the author is under deep obligations to Professor E. E. Barnard, of the Yerkes Observatory, and to the astronomers of the Lick Observatory. g J.

Columbia University, May, 1913.

Vlll

Table Of Contents

CHAPTEE PAGE

I. The Universe 1

Introductory. General view of the science. Its practical use: navigation, coast and boundary surveying, timekeeping for mankind. Value as a culture study.

Chapter I

If a company of men and women should chance to be gathered together on some clear, quiet evening under the dome of the sky, and if there should happen to come into that company one known to possess an acquaintance with the facts and the theories of astronomic science, — if these things should occur, inevitably there would descend upon that astronomer a shower of questions. These he would answer in simple language, after a kindly fashion for many centuries the habit of his guild ; and, as he passed on, he would once more marvel, as he had done many times before, at the changelessness of man's desires. For these questions of the multitude, welcome ever to the star-man, to-day still resemble those that were laid of old as problems before his predecessors at the side of the pyramids.

Why, for instance, does the moon appear at times as a full round disk, at others as a tiny crescent ? Why do certain bright stars called planets seem to wander about among the multitude of their fellows? Why is summer hot and winter cold ? How do navigators find their way across the trackless ocean by observing the heavenly bodies ?

Let us begin by attempting to set forth as best we may the answer to some of these many eternal questions from the skies. For the astronomer is not always present ; and even B l if he were, it is often better to gather our information in silence, by means of the process called reading. The very name Astronomy tells us what our science is. Derived from two Greek words, Astronomy means "the law of the stars." Where does the law of the stars hold sway? Throughout all space. What is space ? Space is the place where astronomy has its being. When does astronomy enforce its laws? Throughout all time. What is time? Time is the period during which astronomy has its being. Astronomy needs no logical definitions of space and time. They belong to it ; they are part of it.

Somewhere, then, in the endless void of space our universe is suspended ; the visible universe. Is that visible universe but one of many? Are there invisible universes without number scattered through the vastness of space like continents in an endless ocean? The human mind loses itself in speculations such as this : nor do such speculations here concern us ; for astronomers consider only the ascertainable phenomena of the universe that unfolds itself to our senses.

There is in existence a vast quantity of matter and a vast quantity of active energy, or force. It is not necessary at this point to define these terms ; but we should remember that according to accepted theory the total of matter and the total of energy in the universe do not change. Matter is never destroyed ; and the accepted law of the conservation of energy tells us that the quantity of energy in the universe is likewise constant and unvarying in amount. None ever disappears out of existence. But both matter and energy may and do undergo changes in form and appearance. Thus, water may appear as steam or as ice ; and the energy of a moving body may be transformed into heat, light, or electricity.

When we examine this visible universe of ours at night with the unaided eye, we see several different kinds of objects : nebulae, or small luminous clouds ; star clusters, like the famous group called the Pleiades ; individual stars ; the moon ; and, occasionally, comets or meteors. In the day we see the sun ; sometimes the moon ; and very rarely indeed a particularly bright star or comet. We shall give here a brief outline of existing knowledge concerning these various celestial objects, leaving a detailed description of their peculiarities to later chapters. They are all composed of matter ; all, if in motion, move in accordance with the laws of mechanical science which govern the operation of energy ; and all, if they change, undergo only changes such as accord with the laws of physics and chemistry.

First, then, the nebulae. We shall begin with these because they probably represent the form in which matter shows itself to us in its most primitive stage of development. Only one or two can be seen with the unaided eye; and these only on very clear nights when the moon is invisible. In the telescope they appear as patches of luminous cloud, often more or less irregular in form. They were once thought to be simply conglomerations of small stars, so close to each other that the optical powers of existing telescopes were unable to separate them into constituent units. This view gained in probability for a long time, because, as the power of telescopes increased with the increase of skill among opticians, astronomers were constantly resolving new nebulae, as they used to call it ; separating them into simple close clusters of faint stars.

But the invention of an instrument called the spectroscope, in the middle of the nineteenth century, put us in possession of a means, previously non-existent, for distinguishing with certainty between the light of incandescent gases and that derived from incandescent or luminous matter in the liquid or solid stage. With the spectroscope astronomers have been able to ascertain that there are many nebulae in a truly gaseous condition; that probably most of these objects are gaseous bodies ; that they could not be resolved into stars, even if terrestrial man possessed to-day telescopes more powerful than he is likely ever to have at his command.

According to many modern theorists, we may take the nebulae to be matter not yet fashioned into stars. This means, of course, that certain forces are at work in the nebulae ; forces of irresistible power, slow in action, as all cosmic changes must be slow when measured by the life of human generations ; but sure in action, too, with that infinite sureness which belongs in celestial spaces. These forces doubtless produce motions of vast import within the body of the nebula ; heat is doubtless engendered ; condensations occur at certain points ; nuclei are formed ; probably, finally, one or more stars take the place of these nuclei ; and so, perhaps, is the original nebular material transformed into stars such as men see clustered upon the sky of night.

Certainly the force of gravitation must be active. Since the time of Newton, in the seventeenth century, it has been known that there is a force of gravitation ; that under the influence of that force every particle of matter in the universe attracts or pulls every other particle of matter ; that the combined effect is always motion of some sort, each particle pursuing in space some determinate path or orbit under the influence of gravitational attraction exerted by all the particles.

The most recent observations of nebulae have brought out aBBM^SMiwBBi

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PLATE 2. Spiral Nebula.

the fact that they are extremely numerous ; probably many hundreds of thousands exist, although only about ten thousand have been catalogued. This fact is of importance ; for if we are to regard the stars as a product of development or evolution from the nebulae, we should expect these gaseous bodies to exist in numbers comparable with the number of the stars themselves.

But of even greater interest is another recent observation of the nebulae. The predominant type seems to be spiral in form; a species of central hub, carrying two attached curved spires, like a whirling wheel with two very flexible spokes but no rim. There can be little doubt that these nebulae are subject to internal motions, probably rapid in themselves, but appearing infinitely slow to us because of the almost inconceivably vast distance by which we are sundered from them.

According to the foregoing theory, which admits the existence of irregular as well as spiral nebulae, we should expect to find the stars in groups, a certain number assembled comparatively close together near certain former nebulous regions within the sidereal universe. And this is precisely what we do find. Usually the number of stars thus belonging together is small ; very frequently but a single star can be detected with the telescope. But many of the constituent stars of a group may be too faint to show themselves on account of their distance ; often they are all probably too faint, except the large one that may have resulted from the former hub or center of the parent nebula, if it was one of true spiral form. Furthermore, gravitational attractions and orbital motions may have commenced among the stars of every group even before they had become separate bodies. While they were still numerous, frequent collisions must have brought about the coalescence of two or more into a single larger unit. In short, we have here an outline of a fairly consistent explanation to carry us forward from the nebular stage to that of stellar development, a theory that leads us to expect star-groups ranging all the way from a single visible luminous object to a detached assemblage closely packed in a globular cluster.

And the stars are simply suns ; our sun is a star. There can be no doubt that the stars are not the tiny twinkling points of light they seem to be. Their apparent lack of size or volume is simply a result of the distance by which they are separated from us ; their twinkling comes from undulations or other irregularities in the ocean of terrestrial atmosphere, or "air," through which we are compelled to view them. We know the stars to be self-luminous, masses of glowing incandescent solids, liquids, or gases. We know the stars to be composed of chemical elements practically identical with those found on the earth. We know the stars to be subject to the law of gravitation ; that every particle of matter in each one of them is endowed with that mysterious quality postulated by Newton, the power of pulling all other matter in the universe. From all these known facts, and reasoning by analogy, we are led to believe the stars to be suns, more or less like our own sun, though by no means necessarily in the same stage of cosmic development. All are doubtless cooling gradually and steadily by the constant radiation of heat into space ; some have probably reached temperature conditions similar to those existing in our sun ; and there may very probably be some that are attended by planets like our earth.

The stars are classified according to their so-called magnitudes, by which astronomers mean simply their lucidity or brightness, not their actual dimensions ; the first-magnitude stars are the brightest, the fifth-magnitude stars the faintest usually visible to the unaided eye under ordinary conditions. There are in all about sixteen hundred stars of the first five magnitudes ; and only about one half of these can be seen at any one time, when the sky is perfectly cloudless, because the other half are always concealed from view below the horizon. The stars are also divided into a series of so-called constellations ; very irregular, even grotesque imaginary figures of men, animals, and other objects, placed in the sky by the astronomers of old, and retained there in a somewhat simplified form by the moderns, principally on account of an unwillingness to destroy the ancient landmarks of this venerable and venerated science.

The stars so far described are called fixed stars, which means that they do not change their relative positions in space ; that any two of them now close together have been thus in proximity from the beginning, and will remain so to the end. But modern researches have brought out the fact that these apparently fixed stars are not really fixed absolutely. They have motions in space ; these motions seem to us extremely slow and minute simply because the stellar distances are so vast. For at a sufficiently great distance, even large and rapid motions will necessarily appear reduced and retarded. And it is, in fact, quite inconsistent with what we know of the laws governing gravitational attraction to suppose any particle of matter in the universe to be really fixed in position absolutely. Everything must move; must be following some duly appointed path, ever contrasting the intricate complexity of nature with the wondrous simplicity of nature's order and nature's law. Even our sun, regarded as a star, cannot be fixed in space, but must be moving majestically through the void, drawing with it our attendant earth, and ourselves upon it.

And if the stars are incandescent suns, we must expect to find, and we do find, that some among them undergo internal changes that make their visible brightness vary. In certain cases slowly, in others more rapidly, their luminosity waxes and wanes with a more or less periodical regularity. Now and again, rarely and at long intervals, some special catastrophe takes place ; some convulsion of nature, whereby a new star is made to blaze forth into view where previously had been only darkness. Possibly we witness in such cases the result of a sudden collision in space between two ancient suns previously cooled through the ages, and long since bereft of luminosity and of life. The stars that change their brilliancy are called variable stars ; those that blaze forth suddenly are "new stars," or novce.

As we have already stated, the sky contains stellar systems other than those involving but a single visible object. Of these probably the most interesting are the double stars, composed of two individuals, often of different colors. These double stars appear but single to the unaided eye; only when the powers of a telescope of some size are brought into play, is it possible to resolve them into their component parts. In the field of view of such an instrument the stars all appear as brilliant points of light, occasionally glittering and sparkling, but the glitter and sparkle are imperfections caused by terrestrial atmospheric effects, and by the impossibility of constructing telescope lenses whose surfaces are ground to the right theoretic shape with absolute exactness. In other words, the stars appear in the telescope much as they do to the eye : only when the star is a double, the telescope often shows it as such, while the eye is unable to see between the two components. And it is a very impressive sight, when we turn a telescope upon one of these double stars, to see the two tiny points of light projected on the deep, fathomless background of the night sky, and to realize that the speck of darkness between them is a bit of abysmal space.

Sometimes the close proximity of the components of a double star is fortuitous merely. The two objects may simply appear close together through happening to lie in almost exactly the same direction from us. But one of them may in reality be behind the other, and at a distance from us immeasurably greater than the first. In this respect astronomic observation differs from the viewing of ordinary objects on the earth. If, for instance, we should happen to notice two men, both standing at points almost exactly north of us, but one ten times as far away as the other, we would at once detect a difference of distance from the fact that the distant man would appear much smaller than the near one. But in the case of the stars, which we see as points of light merely, we could gather no such information. Even if one of the stars should be brighter than the other, this extra brilliancy might be due to a higher intrinsic light-giving power, and in no sense a result of greater proximity.

When two stars thus appear close together, though in reality separated by a great distance, they probably have nothing in common, and are of lesser interest. But in certain cases the two stars will appear close together through really being near each other in space. Then they must belong to a single system; have probably originated in a single nebula ; true twin suns, bound one to the other and the other to the one ; held by the invisible, intangible, but indestructible power of gravitational attraction.

In addition to these fixed stars, whose motions were unknown to the ancients ; whose motions are so slow that generations of men must come and go before they can reveal themselves to the unaided eye, — in addition to these fixed stars, the night sky contains five other bright stars called of old the planets, from the Greek word irkavrJTrjs, the wanderer. They have been named Mercury, Venus, Mars, Jupiter, and Saturn. The most conspicuous thing about them, when viewed without a telescope, is their peculiar and rapid motion among the fixed stars. They can be seen to make an entire circuit of the heavens, traveling apparently among the fixed stars, in brief periods of time ranging from about a year to about thirty years. Of course we now know the cause. These planets are not properly stars at all; they are like the earth, attendants of our sun, revolving around the sun in perfectly definite paths or orbits, and in perfectly definite periods of time. Compared with the fixed stars, they are all extremely near the sun. And being all thus comparatively near the sun, they are of course also all comparatively near each other ; and our earth being one of the number, they are all comparatively near the earth, too. But we have just seen that the extreme apparent slowness of stellar motion is really only a result of the extraordinarily great distance by which we are separated from the stars ; as this immensity of distance does not exist in the case of the planets, of course their apparent motions must and do appear to us comparatively rapid.

Their apparent motions are also complex in a high degree. Two of them, Mercury and Venus, move around the sun in orbits smaller than that of the earth, and therefore entirely within the earth's orbit; the other three, Mars, Jupiter, and Saturn, are exterior to the earth. Mercury has the smallest orbit of all. It is always actually quite close to the sun, and therefore always appears near the sun when seen projected on the sky. Of course, it cannot be seen when the sun is visible on account of the overwhelming luminosity of the sun itself. Therefore we can observe Mercury occasionally only, just after sunset, near the point of the horizon where the sun has disappeared ; or just before sunrise, near the point of the horizon where the sun is about to make its appearance. It is thus always seen in the evening or morning twilight, and was called of old the evening star or the morning star. The same is true of the planet Venus, which attains, however, a much greater apparent distance from the sun.

The exterior planets, Mars, Jupiter, and Saturn, may be seen at certain times throughout a wide range of space on the sky, and at any hour of the night, all of which phenomena will be explained in detail in a later chapter. Still other planets exist ; but they are mostly too faint for the unaided eye; they have been discovered telescopically in modern times. All, together with our sun itself, are probably the result of gradual changes in a parent nebula.

The planets are unlike the stars in still another important particular. We have seen that the stars are self-luminous, incandescent ; the planets are quite different, and give out no light of their own. They shine only by reflected light which they receive from the sun. The light goes from the sun to the planet ; illumines it ; and then we see the planet by solar light, just as we see objects in a room by reflected solar light, which we call daylight. This produces a rather curious telescopic planetary phenomenon called phase, a phenomenon which is most conspicuous also in the case of our moon. The planets are globular in shape, and therefore only one hemisphere can be illumined by the sun at any one time. But the planet does not usually happen to turn its illuminated hemisphere directly towards the earth. Therefore we usually see only a part of the bright hemisphere, and this often looks more or less like what is called a half-moon. In other words, we always see a hemisphere of the globular planet, but it is not the same hemisphere which is turned toward the sun, and which is therefore bright. If the hemisphere we see and the bright hemisphere are mutually exclusive, we see a dark or " new-moon" phase. If the bright hemisphere and the one we see overlap, we see a crescent, half-moon, or other phase, as the case may be. Among the planets, Mercury, Venus, and Mars show the most conspicuous phase phenomena.

Sir John Herschel has given a good illustration of dimer - sions in our solar and planetary system. Represent th3 sun by a globe two feet in diameter. Then Mercury will be a grain of mustard seed on a circle 164 feet in diameter with the sun near its center ; Venus, a pea, 284 feet distant ; the earth, also a pea, 430 feet away; Mars, a pin's head, 654 feet ; Jupiter and Saturn, oranges, distant respectively half a mile and four-fifths of a mile. The nearest fixed star, on the same scale, would be distant about 8000 miles, not feet. This illustration brings out clearly the comparatively minute dimensions of the solar system in relation to the vastness of stellar distances.

In actual appearance the planets differ greatly in the telescope; and they differ especially from the fixed stars. For even our most powerful optical apparatus will not suffice to magnify the latter so as to make them appear otherwise than as minute points of light. Many of them doubtless possess globular dimensions greatly exceeding anything we find in the solar system; but the vast distances cause these dimensions to shrink into mere nothings even in our largest telescopes.

But in the case of the planets these great distances d not exist, and therefore the telescope shows their spheric? ( size in the plainest possible way. But the planets difTf greatly one from the other. Jupiter shows a bright, near! round disk, crossed by a few dark straight lines or.ba It is accompanied, in small telescopes, with four sate! or moons, which can be seen to revolve around the plane At times they pass behind the planet and disappear; and again, one or other of them is so placed that the planet interposes between it and the sun. Then, too, it disappears ; for the satellites also shine by reflected solar light ; and, of course, they receive none when Jupiter is placed between them and the sun. Finally, at certain other times a satellite may pass between Jupiter and the sun ; and then it can be seen to cast a small round shadow dot on the bright surface of the planet. Such phenomena are called eclipses.

Saturn is the most beautiful of the planets, viewed with a telescope of moderate size. It has a number of moons or satellites, mostly too small to be seen in a glass of low power ; but its most conspicuous feature is the famous ring of Saturn. This is a flat disk surrounding the planet, and, in the words of Huygens, who was the first to explain it correctly, nowhere "sticking to" the planet. The ring, like the other bodies of our system, shines by reflected solar light ; and it is always distorted in appearance, as seen from the earth, into a flattened oval or ellipse, like a cart-wheel seen nearly edgewise. At certain times we actually do see it exactly edgewise, and then it appears, of course, like a thin, straight, bright line against the dark sky background. And when the ring appears opened up to a considerable extent, we can see this dark background of the sky by looking through the openings between the ring and the ball of the planet.

Tvlars and Venus show us plain bright disks of moderate size, exhibiting in small telescopes little or no detail of any kind in the way of markings or bands. Their most conspicuous feature is the phase, which is much more marked than j is in the case of Jupiter and Saturn, whose phase henomena are practically altogether unnoticeable. This ollows, of course, from the fact that the quantity of visible phase is due to proximity ; and Mars and Venus, being the planets nearest to our earth, must, of course, show more phase than the distant planets Jupiter and Saturn.

Mercury, as we know, is seen only in the twilight, showing in the telescope a small disk with marked phases.

Comets are occasional visitors to the solar system. They come presumably from outer space in the course" of their orbital motions under the influence of gravitational and perhaps other forces ; remain for a time in the vicinity of the solar system ; are consequently visible to us ; and finally retire again into the depths of space whence they came. When bright enough to be observed without the telescope, they commonly exhibit to our view a brilliant cometary "head," containing a central condensation or nucleus surrounded by a mass of tenuous luminous haze, and to it often attached a long visible streamer or tail, in olden times dreaded by all as a possible harbinger of wars and pestilence.

All these cometary phenomena are well seen in the photograph reproduced as a frontispiece in the present volume. The tail in this case has more than one streamer ; and its length, as photographed, is about 11°, or nearly one-eighth the distance from the horizon to the zenith,

The tail actually seen by astronomers was at one time twice as long. The little curved lines on the photograph are star-images. We should of course expect these to be round dots in the picture ; but in photographs of this kind they are almost always drawn out into little curves, for a very simple reason. The telescope is aimed accurately at the comet when the exposure of the photographic plate is commenced, and it is kept thus pointed at the comet during the whole duration of the exposure. This of course makes a "moved picture" of the stars, as photographers would call it. For the comet will " wander " among the stars, like a planet, in consequence of its orbital motion in space ; and if the telescope's movement upon its stand is adjusted correctly to allow for the comet's motion, the photographic images of the stars must suffer.