Astronomy for schools and general readers

Preface

Astronomy is not studied in the lower and intermediate schools of the United States as much as its importance and interest demand. Its phenomena are so striking, so well calculated to awaken thought, and so much objects of common notice, that an intelligent appreciation of their causes and relations is greatly to be desired.

This book is believed to be written so that any person of ordinary education and intelligence can understand it. No knowledge of mathematics beyond arithmetic is necessary, except that in a few cases trigonometrical solutions of important problems have been given in foot-notes for the benefit of those who understand such methods. Special effort has been made to render ckar the abstruse points in the science, — with what success can be judged from the explanations of the Transit of Venus, the Precession of the Equinoxes, the Tides, etc. Particular care has been taken to distinguish between theories and established facts, even when the former seem to be highly probable; while mere speculations are altogether excluded. The illustrations have been carefully chosen. They are

PREFACE.

believed to be better and more numerous than are usually found in books of this character, and it is hoped that they will render considerable help in making the subject clear and interesting.

The most original feature of the work is the direction everywhere given for observations with the naked eye and with small telescopes. As illustrations of this may be mentioned the methods of observing meteors, variable stars, and the phenomena of Jupiter's satellites. This plan of setting students at practical work has been so successful in chemistry, botany, and other sciences, that it seems to be quite time to use it in astronomy. It may be that many of the readers of this little book will be surprised at the large amount of interesting and valuable observation that can be made with the aid of a very small glass, and even with the unassisted eye.

Preface To The Fourth Edition

The present edition has been carefully revised. The recent discoveries of importance have been included, as well as the best of new theories. The authors thankfully acknowledge the receipt of valuable suggestions from teachers as to better methods of presenting certain portions of the subject, which they have freely used. The book is believed to be not only adapted to class use, but reliable and thoroughly modern.

Contents

INTKODUCTIOK

PAGE

History of Astronomy 9

General View of the Heavens 18

Usefulness of Astronomy 26

PAET I.

THE SOLAR SYSTEM.

Chapter I. — General View of the Solar System 28 III.— The Inferior Planets 65

Mercury 66

Venus 70

IV.— The Earth 79

The Tides 118

y._The Moon 124

VI.— Eclipses 144

VII.— The Superior Planets 153

Mars ,153

The Minor Planets 159

VIII.— Comets and Meteors 189

Comets 189

Meteors 204

Relation between Comets and Meteors . .214 1* 5

CONTENTS.

PAET II.

THE SIDEREAL SYSTEM.

jPAGE

Chapter I. — The Constellations . 217

Description of the Constellations . . . 230

II.— Double Stars 249

Clusters and Nebula . . % . . . . 260

Structure of the Universe 272

PAET III.

Properties of Light, and Astronomical Instruments 279

APPENDICES.

I. — List of Large Telescopes II. — Astronomical Symbols III. — Lengths of Days, Months, and Years IV. — Statistics of Planets, Sun, and Moon

V. — Periodic Comets

VI. — List of Noted Double Stars .

VII. — List of Noted Clusters and Nebulae

SUGGESTIONS TO TEAOHEES.

Aids. — A celestial globe, twelve inches, or thereabouts, in diameter, is most useful in illustrating and explaining many astronomical phenomena, and in finding the constellations and principal stars. Be sure that the globe has a horizontal ring about the middle. A Planisphere is a tolerable substitute for a globe, and much cheaper. A Star Lantern is also very convenient. A good star-map is important. A tele scope of any size, or even a good spy-glass or pair of opera-glasses, will add much interest to the study.

Methods of Instruction. — Each teacher has his own method of conducting recitations, but the authors' experience leads them to prefer the topical method, and whenever possible they would have the student learn the topics in their order, so as to get a complete and connected knowledge of the subject. The headings of the paragraphs and the arrangement of the topics will facilitate this. Eeviews here, as elsewhere, will be found to be very valuable. Distances, dimensions, etc., as given in round numbers, should be learned and made perfectly familiar by frequent repetition. Particular attention ought to be paid to the questions and

8 SUGGESTIONS TO TEACHERS.

suggested problems in the foot-notes. This will test the pupil's knowledge and make it more thorough. A teacher should never be content until his class understands each point thoroughly ; and it must not be forgotten that no one can explain clearly to a class what he does not clearly understand himself. It is hoped that every teacher who essays to teach this subject will acquaint himself thoroughly with it by making use of standard works upon astronomy ; and he may rest assured that no knowledge that he can acquire will be more interesting, or more valuable everywhere and anywhere, than this.

Practical Work. — Above all things, the teacher must not neglect the practical work. Let him take his class out under the clear sky and point out the constellations, principal stars, and planets. Let him make himself, and lead his students to make, the observations described in the following pages. He will be surprised at the interest awakened, and at the valuable results. A common household almanac will be of great aid here. There is much more in an almanac than most people see.

Introduction

History of Astronomy.

1. Early History. — Astronomy, the science of the heavenly bodies, is probably the oldest of all the sciences. So old is it that there is no trustworthy account of its origin ; indeed, almost every famous nation of antiquity claimed the honor of originating it. Nor is it hard to see why this science should have been cultivated so early. The first men had no books to occupy their time, hence they observed nature. The most striking occurrence was the succession of day and night, the one lighted up by the brilliant sun, the other dark, or feebly illuminated by the wonderful stars and the curiously changing moon. These changes were a very natural division of time, the only ones they had. As men knew not the true God, they naturally turned to the heavens for objects of worship, and this led to careful study and observation of the heavenly bodies by priests and other ministers of religion. Besides, the occupations and modes of living of our earliest ancestors were most favorable to the study of astronomy. As hunters, shepherds, and farmers, their lives were spent in the open air, by night as well as by day. In travelling over the thinly-peopled earth, and the sea as well, the stars were their guides. It is not surprising, then, that these men, with no instruments, no books, no schools, knew much about astronomy. They seem, in fact, to have known more about the appearance and phenomena of the heavens than we generally do.

2. Astronomy of the Chaldeans. — According to the Greek historians, the Chaldeans were the first astronomers. These people lived along the Euphrates River in Asia, in and about the city of Babylon. They kept careful records of the movements and phenomena of the heavenly bodies. By these records they discovered that the eclipses of the sun and moon are almost exactly repeated every eighteen years, and thus successfully predicted eclipses. But of the real causes of eclipses, or of the nature, distance, or real motions of the heavenly bodies, these ancient astronomers knew nothing.

3. Astronomy among other Ancient Nations. — The Egyptians, like the Chaldeans, studied astronomy in very ancient times. Some writers contend that their famous pyramids are so built as to show great astronomical knowledge, but very little is certainly known about this matter, or about their advancement in astronomy. It seems to be proved that the Chinese had a knowledge of astronomy very early, — more than four thousand years ago, according to their own claim ; but the evidence of this extreme age of the science among them is doubtful. Their records relate that about that date Ho and Hi were the two royal astronomers, whose duty it was to predict all eclipses, but that, giving themselves up to the pursuit of pleasure, they neglected their duties, and an eclipse of the sun occurred without being predicted. The whole nation was thus exposed to the

IXTRODVCTIOX. \\ anger of their gods, because of the omission of the religious ceremonies always performed upon such occasions. The unfortunate astronomers were immediately put to death. It is certain, however, that the Chinese made reliable astronomical observations, some of which are of use to us, at least two thousand five hundred years ago.

The Hindoos also claim to have been the first to study astronomy. They have proved their claim to an extensive knowledge of the subject, but whether they borrowed this knowledge from the neighboring nations, or gained it by observation, is uncertain.

4. Greek Astronomy. — Astronomy was a favorite science with the ancient Greeks. But, as was the case with a great part of their science, their astronomy was imagined rather than observed. Some of their astronomers advanced surprisingly correct theories of the heavenly bodies, but seem to have made little effort to prove them. Certain of their earliest philosophers taught that the earth is a sphere, a belief not original with Columbus, as some people think, but one taught in Greece two thousand years before Columbus was born. Later some of the Greeks taught that the sun is the centre of the system of planets to which the earth belongs, and that all revolve about the sun ; others taught that day and night are caused by the revolution of the earth upon its axis. These great truths are the foundation of modern astronomy, but the Greek philosophers brought forth so little evidence in support of these guesses, and mingled so many absurdities with them, that they were not generally believed, and were soon forgotten. Notwithstanding their general habit of neglecting experiments for theories, the Greeks achieved some substantial results. They made observations which were of use to succeeding astronomers, they greatly improved the reckoning of time, and determined the length of the year to be three hundred and sixty-five and one-fourth days, which is wonderfully near its exact length. 1

5. The Alexandrians. — For a few hundred years before and after the Christian era, 2 the city of Alexandria in Egypt was famous for its learning. Its astronomers were the most skilful that had yet lived. They attempted to find the relative distances of the sun and moon from the earth. The method employed was a correct and very ingenious one, but from the imperfections of their observations their results were far from the truth. They determined the width of the torrid zone with great exactness, and found the circumference of the earth with surprising accuracy, using the method

1 The ancients found the length of the year by means of a gnomon (no'mon). This was a pillar set up to cast a shadow, which was measured at noon every day. When the noonday sun was lowest down in the sky the shadow of the gnomon was longest, as a little reflection will show. This time of year is called the winter solstice, and marks the time when the sun is farthest south of the equator and is shining directly down upon the tropic of Capricorn ; according to our reckoning, this is about the 21st of December. After that date the noonday shadow grows shorter, because the sun gets farther north every day. Now, if the day upon which the gnomon's shadow is shortest is found, and the days are carefully counted until the shortest shadow comes again, the length of the year is found. It is interesting to know that the obelisks of Egypt, one of which has lately been brought to New York and set up in the Park there, are thought to have been used as gnomons.

If the gnomon were south of the equator, would it make any change in this explanation ? Could the time of noon be found by measuring the length of the shadow ?

2 What is meant by this ?

which is still used as the very best one known. It will be described farther on in the book. Euclid, 1 who gave us the geometry which in substance is still universally used, lived in Alexandria during this period, and contributed to the advancement of astronomy.

6. Hij)par chics. 2 — This was the greatest of the ancient astronomers, and well deserves his title, " Father of Astronomy." He lived upon the island of Rhodes, in the Mediterranean Sea, about 150 B.C. 3 Hipparchus determined the length of the year to within about four minutes of its true length. He discovered that the distance from the sun to the earth varies throughout the year, and he made several most important discoveries in the movements of the heavenly bodies. He made the first catalogue of the stars, fixing the position of over a thousand of them. This catalogue is one of the most valuable possessions of modern astronomy. Hipparchus invented the science of trigonometry, and first used latitude and longitude to determine the positions of places on the earth.

7. Ptolemy* and his System. — This most famous astronomer of antiquity lived at Alexandria about 130 a.d. 3 He made few observations himself, but collected the results of other men's work and wrote them down, together with some important investigations of his own, and it is to him that we owe almost all our knowledge of ancient astronomy. His great work upon astronomy, the " Almagest," still exists, and for fourteen hundred years it was the highest and the only authority upon

1 Euclid (yoo'klid) nourished about 300 B.C.

8 What is meant by this? How long ago were these times ? * Pronounced Tol'e-my.

the subject. The foundations of the Ptolemaic system are that the earth is a sphere, that it is the centre of the universe, and that it is stationary, while all the heavenly bodies revolve about it every twenty-four hours. That the earth is a sphere Ptolemy proved by the fact that at places west of the observer the sun rose and set later, and at places east, earlier ; l and also because as one goes north the pole-star rises higher in the sky, while it sinks lower as he goes south. That the earth stands still while the sun and stars revolve about it, Ptolemy argued was simply common sense. And he took some pains to show the absurdity of the belief that these phenomena are caused by the turning of the earth upon its axis. Ptolemy's theory explains the apparent motions of the sun, moon, and stars pretty well, but the apparent motions of the planets 2 are so peculiar, as will be explained when these are treated of, that he was forced to conclude that these bodies do not move in circles about the earth, but in very complicated circular paths, composed of series of loops. This is the theory of the universe which was accepted everywhere without question until the sixteenth century. 3

1 A little thought, aided perhaps by a diagram, will make this reasoning clear. At St. Louis the sun rises and sets an hour later than at Philadelphia ; hence St. Louis time is an hour behind Philadelphia time. How would this affect travellers ? How does it affect railroadtrains ?

2 A few of what are commonly called stars are planets, and are comparatively near to us. They resemble the earth in many respects. The others are properly called stars, and are suns, situated at immense distances from us. The word planet is derived from a Greek word, meaning a wanderer, because these bodies wander among the stars,

3 The sixteenth century began at the beginning of the year 1501

8. Copernicus 1 and his System. — It has already been mentioned that some of the old Greek astronomers held and taught the true theory of the heavenly bodies, but, substantiated by no proofs and borne down by the great authority of Ptolemy, their teachings had long since been forgotten. And it was not until about 1500 a.d. that Copernicus, a Prussian mathematician and astronomer, revived and firmly established the essential truths of astronomy. He showed that the earth and planets revolve about the surf as a centre, and that the daily risings and settings of the heavenly bodies are caused by the turning of the earth upon its axis. Although his theories were not strictly original with him, and although he left them very incomplete, yet Copernicus has been honored greatly and justly for bringing forward and clearly stating the true principles of astronomy, at the same time showing good reasons for his belief; as well as for his courage in thus breaking away from the ignorance and superstition of his age. His work upon the subject was not published until just at the close of his life, and the first printed copy of it was put into his hands only a few hours before his death. In his honor our theory of astronomy is still called the Copernican System.

9. Kepler. 2 — This great mathematical astronomer followed Copernicus. His whole life was spent in laborious calculations. His name is most frequently mentioned in connection with three great laws, which explain the paths, motions, and distances of the planets.

and ended at the close of the year 1600. "What century is this ? When did it begin, and when will it close ?

2 Kepler, a German, 1571-1630.

These three laws (see page 36), which would scarcely fill a half of one of these pages, cost him seventeen years of hard work. When the third one was established, he said of the book containing it, " It may well wait a century for a reader, as God has waited six thousand years for an observer."

10. Galileo. 1 — This famous Italian first used the telescope in astronomy. The first telescope was made in Holland in 1608; a vague report of the invention reached Galileo the next year, and from this hint, after one night's reflection, he was able to construct one which magnified objects three times, and he finally made one which magnified thirty-two times. He discovered the moons of Jupiter, the spots upon the sun, and many other wonderful things. His brilliant discoveries convinced the world of the truth of the Copernican theory, but brought on him the condemnation of the Church for teaching heresies, and the closing years of his life were saddened by its persecutions. Natural philosophy is as greatly indebted to this remarkable man as astronomy.

11. Newton. 2 — Within a year of the day on which Galileo died, Sir Isaac Newton was born in England. While Newton did not discover the law of gravitation, as is sometimes stated, yet he first proved that the force which brings the apple to the earth binds the planets and sun into one system. 3 This establishment of the

3 The well-known story that while driven into the country by the Plague in London, Newton noticed an apple falling from a tree, and that this suggested the idea that the motions of the planets might be controlled by the same force, is worth remembering. This discovery

INTR OD UCTIOK 1 7 fact that gravity is the force which controls the motions of the heavenly bodies was of the greatest importance : a large part of the science of astronomy depends upon it. 1 Newton, like Kepler, was a mathematical astronomer, not an observer. He discovered and proved many other important facts in astronomy, besides making many and valuable discoveries in natural philosophy and other sciences. He also occupied important positions under the English government. Sir Isaac Newton was probably the greatest scientist that the world has yet seen. His great work is called the " Principia." La Place, 2 the only man who could have disputed Newton's pre-eminence as a mathematical astronomer, pronounced this work the greatest production of the human intellect.

12. Modern Astronomy. — Since Newton a host of eminent astronomers and mathematicians have given their lives to the advancement of our science. Every generation and every civilized country has furnished its share. As it was the earliest begun, so it is the farthest advanced of the sciences. Its strides seem to be growing longer rather than shorter. Our own generation and our living astronomers are inferior to none of their predecessors in ability or in the value of their discoveries. And there is every reason to expect these discoveries to go on with increased rapidity. In the

was made by Newton while he was absent from London on account of the Plague, but the rest of the story is not supported by sufficient evidence ; it is not at all improbable, however.

1 It may not be amiss to remark that, while the laws and effects of gravity are well known, the cause of this force has never been discovered.

2 La Place (La-plass'), a great French mathematician and astronomer, 1749-1827.

2* astronomical work of the last generation our own country has done its full share. Our astronomers and observatories have no superiors. Our contributions to the world's store of knowledge have been greater in this direction than in any other. The history of the important discoveries in astronomy made since Newton's day would fill a much larger book than this. We can only give these discoveries in their proper places in a general account of the subject.

General View of the Heavens.

13. Introductory. — If a person will carefully watch the heavens, he will see much that will tend to excite his curiosity. What are all the glittering lights? How far are they away ? Why do they seem to move around him in a circle every day? Why do some of them change their position among the others ? Many such questions as these will come up, and the best method of arriving at a correct answer is first to observe carefully all that can be seen. The ancients did this much more faithfully than we do, and the various generations of men have accumulated a great number of facts and laws of which we can now have the benefit. % It is the object of a book on astronomy to explain these points so that an observer can better comprehend the causes of what he sees. But careful watching must accompany the study if the phenomena are to be fully understood.

14. The Heavens by Day. — But what can the unaided eye see ? In the daytime there is usually only the sun, and this presents the same general appearance every day. We will find that continual changes are taking place on his surface, but these changes are not visible to the eye. His position in the heavens is, however, perceptibly changing. Every one is familiar with the motion which occurs each day, — his rising in the east, reaching the highest point at noon, and setting in the west. A careful observer will notice, besides this, a change of place at different seasons of the year. He is in the south every day at noon, but in the summer he is higher up in the sky than in the winter. It will be noticed, too, that he does not rise and set in the same place through the year. If the point of setting be noted every evening, beginning with the first of the year, it will be found to be moving towards the north as the winter progres&es. This will go on till the middle of summer, when the place of setting will be far to the north of the west. Then it will slowly change back again towards the south through the fall and early winter. So with the time of rising and setting : it will be noticed that the farther to the north the sun rises, the earlier in the day it will rise and the later it will set. 1

15. Horizon and Zenith. — The circle where the earth and sky appear to meet is called the horizon. On the ocean it is a perfect circle, but on land it is broken up with the irregularities of the surface. When the sun rises it passes above this circle, and when it sets it sinks

1 Let the student carefully note, by reference to a tree or some distant object, the point in the horizon where the sun rises or sets, at intervals of a week or two, and this change of place will be readily manifest. He must be careful to occupy the same point of observation at the different times. Let him also with a watch observe the exact time, and thus notice the gradual change.

20 1NTR OD UCTIOK below it. The point in the sky directly over the head of the observer is called the zenith. 1

16. The Heavens by Night — In the night there is much more to attract attention in the sky. The moon seems to follow nearly in the path of the sun. If carefully observed, she will be seen to change her place among the stars, being each night a little farther to the east than the preceding. The changes in her appearance from crescent-shaped to full, and from full to crescent-shaped, are also striking. There will be certain nights each month when she cannot be seen ; after this a glimpse of her can be obtained in the west just after sunset; she will then be crescent-shaped, and her horns will point directly away from the sun. She will then grow in size for about two weeks, all the time appearing farther and farther away from the sun at sunset, till when quite full she will rise in the east just as the sun is setting in the west. Then she will go through the changes in a reverse order for two weeks more.

It will also be noticed that certain of the brighter stars appear, like the moon, to change their places among the others. The ancients called these planets, or " wandering stars." Those that can readily be seeri by the naked eye are* Venus, Mars, Jupiter, and Saturn.

But the great majority of the stars preserve exactly their relative positions. They appear night after night looking precisely the same. A given star will always

1 Towards what point will a plumb-line, extended upwards, point? Does the zenith change with a change of position on the earth ? Is the sun ever seen in the zenith in the northern hemisphere ? On which side of the zenith is the sun at noon ? In what time of year does the sun pass nearest the zenith ?

rise in the same point in the horizon, though not at the same time ; it will always follow in the same path throughout the year, and set in the same place in the west.

But it will be noticed that the paths which different stars describe are very different. If we look in a northerly direction towards a point nearly half-way from the horizon to the zenith, we shall see a star of medium brightness which does not change place at all ; it is the pole-star, or Polaris. Around this star all the northern heavens seem to revolve in circles. If these northern or circumipolar stars be watched, such as are between the pole-star and the horizon will move towards the east ; such as are on the east of the pole will ascend ; such as are above will move westward ; and such as are to the west of the pole will descend. Those stars situated a little farther from the pole than the northern horizon is will just dip below it and remain set but a short time. Those that rise in the east will be visible just twelve hours and set in the west; they will not, however, pass through the zenith, but south of it, always remaining the same distance from the pole-star. Still farther to the south the stars will be but a short time above the horizon, passing over from southeast to southwest. 1

1 In order to obtain a correct idea of this diurnal motion, the student should watch stars in different parts of the heavens at intervals of a few hours, so as to notice the paths they are describing. It is also advisable to set a globe so that the axis about which it revolves will point nearly to the pole-star. The horizontal ring encircling the globe will then represent the horizon. By turning the globe on its axis, it will be seen that the part around the pole will not pass below the horizon, and the various circles of latitude will represent the paths of stars in different parts of the sky. Some portions around

17. Diurnal Motion. — This general motion of the sun, moon, planets, and stars, which carries them apparently around the earth every twenty-four hours, is called the diurnal motion. The heavens appear to us to be the concave surface of a sphere, called the celestial sjihere. The celestial sphere and all the heavenly bodies revolve about the earth every day, while the sun, moon, and planets have a separate motion of their own, which causes them to change their places among the stars.

18. Cause of Diurnal Motion. — A quiet motion often gives the impression of rest. A sailing vessel will glide along through still water so quietly that a person on board can easily conceive that he is at rest and surrounding objects are in motion in the opposite direction. Now the earth is turning on its axis from west to east with a perfectly noiseless and smooth motion. The effect produced on us is that all the heavenly bodies are passing over from east to west The apparent diurnal motion of the heavens is therefore due to a real motion of the earth. Instead of the sun, moon, and stars rising above the horizon, the eastern horizon is really falling away from them. Instead of their setting, the western horizon is rising to obscure them. The reason that they appear to climb the sky is because the portion of the earth on which we are is turning more directly under them ; and the reason that they sink is because we are revolving away from them. All the effects of diurnal motion above described are readily explained by the rotation of the earth on its axis, this axis pointing nearly towards the pole-star.