A new astronomy for beginners

Front matter

Cllap.^^...., ^^right No.,

[ma

UNITED STATES OF AMERICA.

Preface

IV TEGLECT hitherto of the adaptabiHty of astronomy to a laboratory ^ ^ course has mainly led to the preparation of this JVew Astrononiy for Begiiuiers. Written purely with a pedagogic purpose, insistence upon rightness of principles, no matter how simple, has everywhere been preferred to display of mere precision in result. To instance a single example : although the pupiPs equipment be but a yardstick, a pinhole, and the 'rule of three,' \\ill he not reap greater benefit from measuring the sun for himself (page 259), than from learning mere detail of methods employed by astronomers in accurately measuring that luminary ?

Astronomy is preeminently a science of observation, and there is no sufficient reason why it should not be so studied. Thereby will be fostered a habit of intellectual alertness which lets nothing slip. Sixteen years' experience in teaching the subject has taught me many lessons that I have endeavored to embody here. Earth, air, and water (merely material things) are always with us. We touch them, handle them, ascertain their properties, and experiment upon their relations. Plainly, in their study, laboratory courses are possible. So, too, is a laboratory course in astronomy, without actually journeying to the heavenly bodies ; for light comes from them in decipherable messages, and geometric truth provides the interpretation. But the student should learn to connect fundamental principles of astronomy with tangible objects of the common sort, somewhat as in phvsics and chemistry ; and I have aimed to indicate practically how teachers and pupils of moderate mechanical deftness can themselves make the apparatus requisite for illustrating many of these principles. All of it has been repeatedly constructed ; and its use should pave the way to better equipment for more advanced study.

Especial attention has been accorded the recommendations of ' The Committee of Ten' on secondary school studies (1892); the specifications concerning astronomical instiuction published by the Board of Regents of the state of New York (i89'5) ; and the Action of the

4 Preface

Editorial Board of The Astrophysical Journal with regard to Standards in Astrophysics and Spectroscopy (1896).

In order to secure the fullest educational value, I have aimed to piesent astronomy, not as mere sequence of isolated and imperfectly connected facts, but as an inter-related series of philosophic principles. The geometrical concept of the celestial sphere is strongly emphasized ; also its relation to astronomical instruments. But even more important than geometry is the philosophical correlation of geometric systems. Ocean voyages being no longer uncommon, I have given rudimental principles of navigation in which astronomy is concerned. Few young students may ever see the inside of an observatory ; but that is reason for their knowing about the instruments there, and prizing opportunities to visit such institutions.

Everywhere has been kept in mind the importance of the student's thinking rather than memorizing. Mere memorizing should be rendered facile ; in treating of the planets, I have therefore presented our knowledge of those bodies, not subdivided according to the planets themselves as usually, but according to especial elements and features. The law of universal gravitation has received fuller exposition than commonly in elementary books, its significance demanding this. Biographic notes, intrusions in the text, have been relegated to the Index.

In conclusion, I desire to thank Professor Newcomb of Washington, Professor Pickering, Director of Harvard College Observatory, and my colleague, Professor Kimball, for helpful suggestions on the proof sheets. A few illustrations have been reengraved from the Lehrbuch der Kosmischen Physik of Miiller and Peters. P^or many of the excellent photographs, reader, publisher, and author are indebted to the courtesy of astronomers, in particular to M. Tisserand, late Director of the Paris Observatory, to the Astronomer Royal, to Professor Pickering, to Professor Hale ; also to Dr. Isaac Roberts and Professor Barnard, both of whose series of astronomical photographs have received the highly honorable award of the gold medal of the Royal Astronomical Society.

DAVID P. TODD.

Amherst College Observatory.

Contents

I. Introductory ......

II. The Language of Astronomy .

III. The Philosophy of the Celestial Sphere

Chapter I

ASTRONOMY is the science pertaining to all the bodies of the heavens. Parent of the sciences, it is the most perfect and beautiful of all. Sir William Rowan Hamilton, the eminent mathematician, has called astronomy man's golden chain between the earth and the

I r.e Yerkes Observatory, Professor George E. Hale, Director

visible heaven, by which we * learn the language and interpret the oracles of the universe.' This noble science is to man a possession both old and ancestral, passing with resistless progress from simple shepherds of the Orient watching their flocks by night, to the rulers of ancient

8 Iiit7^odiictory

empires and the giants of modern thought ; until to-day the civiHzed world is dotted with observatories equipped with a great variety of instruments for weighing and measuring and studying the celestial bodies, each of these observatories vying with the other in pure enthusiasm for new knowledge of the infinite spaces around us.

Astronomy a Useful Science. — Many devoted lives have been grandly spent in pursuit of this branch of learning ; and it would hardly be possible for any one who has given even a general glance at their unselfish history to make the vulgar inquiry, * What's the use t ' Only a very small and unaspiring mind ever asks this question about any science which adds to the sum total of our actual knowledge, least of all with reference to this, — one of the most practical of all sciences. Astronomy binds earth and heaven in so close a bond that it even maps the one by means of the other, and guides fleet and caravan over wastes of sea and sand otherwise trackless and impassable. By faithful study, even for a short time, it is possible to discover many of these uses. They may not at once appear to put money into men's pockets or clothes upon their backs ; but we have passed the pnxnitive stage of a rudely toiling community, where material progress alone is the thought and aim.

Especial Uses. — To specify in part the relations in which astronomy is useful: (i) In chronology, — fixing many disputed dates of ancient battles, the reigns of kings, and other important historic events, and establishing the exact length of the units of time requisite for the calendar. For example, the surest basis of the chronology of ancient Assyria rests upon an eclipse of the sun observed in Nineveh in the middle of the reign of Jeroboam the Second, which modern astronortrfeaj calculations prove to have taken place on the 15th of Juii-er-B.c^^63. (2) In navi-

Especial Uses gation, — conducting ships from port to port, almost without risk, thereby saving human Hfe and lessening the cost of many of the necessaries of existence. The great national observatory^^ at Greenwich (page 433) is one of those founded for the especial and practical purpose of improving the astronomical means of navigation. {^\\). geodesy ana in ' surveying, — enabling us to ascertain the size of the earth, make accurate maps of its continents and oceans, and run boundaries of countries and estates. (4) In determining exact time, '

— a vast convenience in all the affairs of life, particularly in the operation of railways. In many large cities, the dropping of a ball on a high tower indicates exact noon.

Every good watch has been carefully rated by an accurate clock (perhaps in some observatory), which again has been corrected by observations of the fixed stars

— a knowledge of the precise positions of which depends upon the faithful patience of a multitude of astronomers who have given their lives to this work in the past. Indeed, it is hardly an exaggeration to say that there is no civilized person in existence whose comfort is not enhanced, whose life is not rendered more worth the

I

The Time-ball at New York lO living, or who is not affected, at least indirectly, by the work of astronomers, and by those who, though not astronomers, are yet practically applying the principles of this science to the affairs of everyday life.

The Sun by Day. — Singularly few persons regard the daytime sky. Yet this beautiful and ever-varying spectacle may be seen and enjoyed by all ; perhaps that is one reason why it is so little thought of.

Even the sordid city court, the worst tenement district, may have its strip of blue above, far away from noise and uncleanliness.

No buildings are high enough to shut out this heavenly gift entirely. The study of the sky in daylight, especially its clouds, is properly part of a separate science, — meteorology as distinguished from astronomy. The marvelous sun, too, by wTiich, as will be seen, we live and m.ov€ and^ have our being, is held hardly less a matter of course. Here it is that meteorology joins on the boundary of the science we take up to-day ; for the sun is one of the chief objects of study in modern astronomy, — its distance, its

Clouds of the Daytime Sky (photographed by Henry)

Pi,ATO II,— Th^ Sun as R^vkai^ed by Tki^^copk and Spectroscopy.

{TrouveloL)

The Sta7^s by Night

II vast size, its apparent motion, the sources of its intense light and heat, its constantly changing spots, its constitution, the hydrogen prominences, which seem to spring from its edge as tongue-like flames, and its energies, tirelessly radiated into space and regnant in all the forms of life upon the earth, no less than in all those phenomena of the atmosphere which we call weather. Many of the spots on the sun are larger than our globe, like the one here pictured. Without fine in- ^" ^"^^^^^ ^""^p°^ ^^^''^^^ struments carefully adjusted, the prominences cannot be seen except during total eclipses of the sun. ^^^ The Stars by Night — But this sense of everyday t'^ usualness in great part gives way, once the sun has set, and the stars have come forth, as if from their daytime hiding. Of course they fill the sky just as truly when the world is flooded with sunlight, shining all in their appointed places, where the brighter ones may be seen with the telescope during the day ; but their feebler light is conspicuous only when this greater brilliance is withdrawn from our horizon, or when the moon comes in between us and the sun, causing a total eclipse. Immanuel Kant, a great German philosopher, has said that two things filled him with ceaseless awe, — the starry heavens above and the moral law within. Even the most prosaic cannot but notice and revere the night-time sky, and few are so

Introduciory hopelessly unimaginative as not to be impressed by the dark blue dome spangled with its myriad stars. The positions of the stars with reference to one another seem to remain constant, although they are continually changing . their places relatively to objects on the earth. Hence the term fixed stars. But this is only seemingly the proper expression. In reality, all are speeding through space at

The Night-time Sky in a Great City

very. high velocities, but so infinitely removed are the stars from us that they appear to be at rest. Although quite the reverse, as we now know, from 'fixed,' the term is still used, because in the astronomically brief period from generation to generation, the changes are so slight that the naked eye is powerless to detect them.

Number of the Brighter Stars. — In ancient times the brilliant host of the nightly sky was thought to be countless ; but surprising as it may seem, the stars actually visible to the unaided eye at a single place in the United States do not exceed 2000 or 3000, and only upon ex-

The Milky Way near the Star 15 Monocerotis, yR = 6 h. 35 m., Decl. N. 10° (photographed by Barnard, 1894. Exposure 3k hours)

ceptionally favorable nights may so many be counted without a telescope. As an average, on what may be termed clear nights, the number thus ordinarily seen at any given time is rather less than 2000 ; but this number varies greatly with changing conditions of our atmos-" phere. If one were to keep count, through the year, of all the stars visible to the naked eye in all that part of the heavens ever seen from a single place in the United States, the total number would be about 4000.

Number of the Telescopic Stars. — By the use of a small telescope, or even an opera glass, the number of visible stars is increased enormously. Even in Galileo's time, his ' optick tube ' revealed an unsuspected and unnumbered host, beyond the dreams of any primitive astronomer. With our modern telescopes (in which the object glass of almost every famous new one has been an advance in size upon all its predecessors) the 'blue field of heaven' is estimated to contain at least 100,000,000 stars. Beyond what is shown even by these telescopes are the remarkable revelations of celestial photography, which reproduces unerringly upon the sensitive plate uncounted millions of other stars too faint for the eye to detect, even when aided by the most powerful optical means at our command. In a single field embracing but a slight fraction of the whole sky, recently charted with the Bruce telescope of Harvard Observatory (the largest photographic instrument in existence), there were counted no less than 400,000 stars. And who can say where this stupendous array ceases 1

The Constellations. — The names and positions of the brighter stars are very easy to remember. By even a casual glance at the sky on any clear night, it will be seen that the stars make all sorts of figures with one another, — squares, triangles, half circles, — and fanciful combinations mav be traced in all directions. The ancients called these

The Yerkcs Telescope

The Yerkes Telescope of the University of Chicago

This great telescope was mounted in 1896-97 at Williams Bay, Wisconsin. It is the principal instrument of the Yerkes Observatory, and cost about ^125,000. The glasses for its 40-inch lenses, the largest in the world, were made by M, iMantois of Paris, ground and figured by Alvan Clark & Sons of Cambridgeport ; and the tube and all the intricate- machinery for handling the telescope with ease and precision were built by Warner & Svvasey of Cleveland.

i6 various figures after their gods and heroes, dividing them into 48 groups, largely named after the characters associated with the voyage of the fabled ship Argo, Although these constellations bear little real resemblance to the men, animals, and other objects named, they too are easily learned. Properly that is not astronomy, but merely geography of the heavens ; yet it is an interesting and popular branch of knowledge, often leading to farther studies into the most absorbing and uplifting of sciences.

The Moon. — Of all celestial bodies, meteors alone excepted, the moon is the nearest to us, and apparently of about the same size as the sun ; but this is the result of a somewhat curious coincidence, by which the sun, although 400 times larger than the moon, is also very nearly 400

-:.c ...wv^.. vL^.^OLographed by the Brothers Henry) ,. c ,y

^ times farther away.

Even with a small telescope we may generally see the deep craters and the rugged mountain peaks of the moon, partly

M illuminated by sunlight, while the rest of our satellite is turned away from the sun, lying in shadow and seen very faintly by the sunlight falling upon it after reflection from the earth. Our companion world is dead and cold, its air and water almost certainly gone, so that no amount of brightest sunshine can of itself bring back any warmth of life. Earth and other planets are dark, too, on the surface, save for what the sun bestows of brightness and warm.th ; but our own planet and some of the others are blessed with an encircling atmosphere, best gift after sunlight itself, to save and store for our use the sun's heat shed lavishly upon us.

The Planets. — When frequent looking at the nightly sky has somewhat familiarized the evening constellations, — different at the same hour at the various seasons of the year, — one may notice three or four very bright stars which do not twinkle. A few evenings' watching will show that they are slowly changing their positions relatively to other and fainter stars about them. These are the planets (^wanderers '), and will at first be thought and called stars ; but although speaking in the most general terms, it is proper to refer to them as stars, they are worlds, among w^hich the earth is one, traveling round the sun in nearly circular paths.

Like our own planet, they receive their light from the central orb, and reflect it afar. The planets

and all their moons (called Satel- jupltsr in a Small Telescope

lites), as well as our moon, give

light only as reflected sunshine, — second-hand. Some of the planets are brighter than most stars, only because they are very much nearer to us and to the sun. todd's astron. — 2 i8

Differences between Stars and Planets. — Besides the noticeable change of position of the planets, and their shining by reflected light, another difference between planets and fixed stars is that, when seen through a telescope, planets appear larger in size than with the naked eye. This the stars never do. Most planets have an appreciable breadth, called the disk ; and this seems to

The Planet Saturn in 1894 (drawn by Barnard with the Lick Telescope)

grow larger as the power of telescopes is increased. Stars, on the contrary, seem to be mere points of light, intensely luminous, and infinitely far away. They increase only in brilliancy with the size of our largest glasses ; and even the strongest lenses cannot produce the slightest effect upon the apparent size of these stupendously distant blazing suns. Also some of the planets as seen in the telescope show phases ; in particular, Venus, the brightest planet, a familiar glory of the western sky, passing through all the changing phases of our moon, — full, quarter, and crescent. A planet called Saturn is surrounded by a thin ring, as shown in above engraving. It suggested a process of evolution called the nebular hypothesis, by which stars, planets, and satellites seem to have developed into present forms through the operation of natural laws.

The Distances of the Stars 19

The Fixed Stars are Suns. — All these fixed stars are suns like our own — singularly similar, the modern revelations of the spectroscope tell us, as to material elements composing them. Probably, at their inconceivable distances from us, these suns afford light and heat to uncounted worlds not unlike those in the system of planets to which our earth belongs. But if such planets exist, they are too near their own central luminaries, and too faint for their reflected light ever to reach our far-off eyes. One must think of the vaster brilliance of the sun as due almost wholly to our relative nearness to him. Were the earth to be removed as far from the sun as it is distant from the stars, our lord of day would shrink to the feeble insignificance of an average star.

The Distances of the Stars. — The nearest star is so far from us that its distance in figures, however expressed, remains unapprehended by the human mind. Who can conceive of 25 millions of millions of miles .'^ Yet so remote is our closest stellar neighbor. As the stars v^ry enormously in their distances from us, so they are equally diverse in their relations to each other. We see them all by the light they emit — light which does not come to us instantaneously, yet with speed almost inconceivably great. While one is taking two ordinary steps, at an average walking pace, light will travel a distance equal to eight times round the world (nearly 200,000 miles). Now, to realize in some sense the enormous distance of the nearest fixed star from our earth, open a Webster's International Dictionary, which contains over 2000 pages of three columns each, or the equivalent. Begin to read as rapidly as you can, and imagine a ray of light to have just left the nearest fixed star at the instant you began. By the time you have finished a single page, the star's light will have sped onward toward the earth no less than 100,000,000 miles.

Imagine that you could keep right on reading, tirelessly and without ceasing, day and night, just as light itself travels — how many pages would you have read when the ray of light from Alpha Centauri, the nearest fixed star, had reached the earth ? You would have read it completely through, — not once, or twice, but nearly a hundred times. So enormously distant is this nearest of the stars that, if it were blotted out of existence this present moment, it would continue to shine in its accustomed place for more than three years to come. And other stars whose distances have been measured are a hundredfold more remote.

The Shooting Stars and Comets. — Very frequent celestial sights, especially in April, August, and November, are

the swarms of swiftly-falling meteors. They flash across the skv and seem to vanish into the blackness whence they came, burning sparks in the starry firmament. On rare occasions a fragment of a meteor falls down upon the surface of the earth, and many thousands of such specimens are preserved as collections of meteorites in various scientific centers, — Vienna, London, Paris, and Washington.

Sometimes they are of iron, and sometimes of stone.

Much less common than the spectacle of shooting stars is that of a majestic comet, whose long and graceful tail sweeps many degrees along the sky, sometimes for weeks

The Great Comet of 1858

Gravitation - 2 1

or even months together. All these wandering visitors, too, must be studied in their place.

General Outline. — We know that the stars are suns ; that our sun is one of them, seemingly larger only because very much nearer ; that he conducts with him through space our earth and her companion planets with their moons, or satellites; that the stars are all moving through the celestial spaces with great velocity, though at such enormous distances from us that they appear to be almost at rest; that meteors and comets flash into our firmament, the former to perish after one bright, sparkling clash with our atmosphere, while the latter have their known and regular orbits, or paths, some of them coming back within our sight at predicted intervals.

Gravitation. — The mighty power called gravitation holds all these whirling, flying, incandescent or white-hot, or cold and dead bodies from swerving outside their paths in space; and, little by little, the patience and ingenuity and genius of man have interpreted many of the laws governing them, and have brought to our knowledge manifold facts about them, — their weights and distances, sizes and motions, and even the elemental substances of which they are composed. Their physical appearances, as revealed by telescope and camera, will be abundantly emphasized. But perhaps the most striking fact in all astronomy is that unerring precision with which the heavenly bodies move through the celestial spaces in accordance with this great law of gravitation, whose action enables us to foretell with great accuracy, hundreds of years in advance, the places of planets in the starry heavens, and the exact hour, minute, and second, when eclipses will happen. And progress through the chapters of this book will unfold in part the knowledge gained by astronomers through centuries of careful investigation.

Chapter II

NO one can understand even the simplest truths of astronomy without first learning the language of precision which astronomers use. Only a few terms in this language will be necessary at the outset, and they wall be illustrated and ideas of them conveyed by means of com-

How to find True North (Approximately)

mon objects and simple processes. First, the four cardinal points, east, north, west, and south, — terms in constant use from the remotest antiquity.

Plumb-line, Zenith, and Nadir

How to find the Cardinal Points. ^ Any sharply-pointed object, firmly set, may be used as a gnomon for finding the cardinal points. But the following method has greater advantages. Place a carefully leveled board or table so that the sun may fall freely upon it, from about nine o'clock in the morning until three in the afternoon. Fasten securely. Near the sunward end of the table, and about eight inches above it, fix firmly a card with a smooth pin hole through it. This will give a small, oval image of the sun on the table, and its position must be marked at nine o'clock, at a quarter past, and at half after nine ; again at half after two in the afternoon, a quarter to three, and three o'clock. The principle involved is that of the gnomon of Anaximander in very compact form. Take especial care that the marked surface, whether board or paper, shall not have moved meanwhile. Draw three straight lines joining the sun marks, as indicated in the picture opposite ; connect the nine o'clock mark with the three o'clock one; draw a second line connecting the 9:15 mark with that made at 2 : 45 ; and a third, joining the 9 : 30 and 2 : 30 marks. These three lines will be nearly parallel, and they mark the direction east and west approximately, the east end being indicated by the three afternoon marks. Three pairs of points are better than one, because clouds may interfere with the afternoon observations ; also, we can take the average direction of three lines, which will give true east and west more accurately than a single line. By the simple construction in geometry indicated in the illustration, draw a perpendicular to this average line ; this perpendicular, then, wdll lie in the direction north and south, north lying on the right hand as one faces west. Extend these two straight lines indefinitely, and they will mark the four cardinal points called east, north, west, and south.

Plumb-line, Zenith, and Nadir. — Suspend any heavy object by a delicate cord attached to a firm support, and allow it to come to rest. Draw it to one side or the other from its support, and let it swing freely. Such an object capable of swinging is called a pendulum. The force causing it to swdng back and forth is called the attraction of gravity. We shall see subsequently that this is the same force that makes all bodies fall to the earth ; also that it holds the moon, our satellite, in its monthly path, or orbit, about us. After swinging back and forth many times, the pendulum will come to rest ; and it will do so more quickly if the weight or bob of the pendulum is freely suspended in a basin of water. A pendulum that has stopped swinging becomes a plumb-line.

A Plutnb-:ine

Imagine the cord of the plumb-line extended both upward to the sky and downward through the earth indefinitely. The point overhead where the plumb-line intersects the sky is called the zenith ; the opposite point is called the nadir.

The Apparent or Visible Horizon. — Looking up to the sky, it seems to be arched over us like the inside of a great hollow sphere. The dome of the sky is nearly hemispherical, and seems to most eyes less distant overhead. In ordinary inland regions the sky seems to meet the earth in an irregular and broken line. This is called the appa-

Plane of the Sensible Horizon cuts through the Mountains

rent or visible horizon ; and nearly every point of it, even in locations not especially mountainous, will usually be considerably above the level of the eye. In cities the surrounding buildings, the trees in the park, and the spires of churches will lift themselves into our vision, too near by to allow any observation of the sky at the exact level of the eye. In the country, in Massachusetts, for example, it is not always easy, without ascending some great height, to reduce the obstacles forming the apparent horizon to a minimum ; and usually the sensible horizon lies far below them all. Objects relatively near, then, whether

The Sensible Horizon houses, grain elevators, churches, forests, or mountains, make irregular curves and broken lines which limit the outward view in every direction. Their outline marks the observer's apparent, or local, or visible horizon.

The Sensible Horizon. — From the surface of the ocean, or from a widely extended plain or prairie, the dome of the sky appears to join the earth in a nearly perfect circle about 25 miles in diameter. In Boston, for instance, we may take the steamer for Nahant, and for a portion of even that short trip our perfect ocean horizon on one side will hardly

■1

The Visible Horizon on the Ocean

be interfered with. In New York a boat trip to Far Rockaway or Long Branch will give us a similar opportunity. In Chicago we have a choice of ways to get a complete view of the sensible horizon. A car ride in almost any direction — to Evanston, perhaps — will show widely extended prairies, seeming to stretch to the sky on all sides ; far out upon Lake Michigan, the effect upon the observer is like that of the ocean; or perchance the Auditorium tower may be ascended, and if the distant view is clear, a far-away horizon of the sensible order is within sight. Practically in this circle, the four cardinal points are located. Imagine a plane passed through these four points. It will pass through the eye of the observer, and will essentially be the plane of his sensible horizon, neglecting only a small angle called the dip of the horizon, a