UNIVERSITY OF CALIFORNIA DEPARTMENT OF EDUCATION Gift of Received \ ASTRONOMY FOR SCHOOLS AND GENERAL READERS BY ISAAC SHARPLESS, Sc.D., PRESIDENT OF HAVERFORD COLLEGE, AND GEO. MORRIS PHILIPS, PH.D., PRINCIPAL OP STATE NORMAL SCHOOL, WEST CHESTER, PA FO UBTH EDITION RE VISED. PHILADELPHIA: J. B. LIPPINCOTT COMPANY !>3 --.'..* -ceo*, Copyright, 1832, by J. B. LIPPINCOTT A Co. Copyright, 1892, by J. B. LIPPINCOTT COMPANY. EDUCATION DEPT, PREFACE. ASTKONOMY is not studied in the lower and inter- mediate 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 un- derstand it. To knowledge of mathematics beyond arithmetic is necessary, except that in a few cases trig- onometrical solutions of important problems have been given in foot-notes for the benefit of those who un- derstand such methods. Special effort has been made to render clear the abstruse points in the science, with what success can be judged from the explanations of the Transit of Venus, the Precession of the Equi- noxes, 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 3 60SG66 4 PREFACE. believed to be better and more numerous than are usu- ally 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 direc- tion 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 thank- fully 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, INTEODUCT1ON. MM 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 II. The Sun . . 44 III. The Inferior Planets ' 65 Mercury ,-'., .66 Venus . . . . . . . 70 IV. The Earth 79 The Tides . . 118 V. The Moon 124 VI. Eclipses 144 VII. The Superior Planets 153 Mars 153 The Minor Planets 159 Jupiter 163 Saturn 173 Uranus 181 Neptune . . 183 VIII. Comets and Meteors .189 Comets 189 Meteors 204 Relation between Comets and Meteors . .214 1* 6 6 CONTENTS. PAKT II. THE SIDEREAL SYSTEM. jPAGE CHAPTER I The Constellations . . . ... 217 Description of the Constellations . . . 230 II. Double Stars . . . . . . 249 Variable and New Stars 253 Clusters and Nebulas . . . , __. . . 260 Structure of the Universe . . 272 PAET III. PROPERTIES OF LIGHT, AND ASTRONOMICAL INSTRUMENTS 279 APPENDICES. I. LIST OP LARGE TELESCOPES 305 II. ASTRONOMICAL SYMBOLS . . . 306 III. LENGTHS OF DAYS, MONTHS, AND YEARS . . , 307 IV. STATISTICS OF PLANETS, SUN, AND MOON . ' " . - . 308 V. PERIODIC COMETS . . . . , . % . . 309 VI. LIST OF NOTED DOUBLE STARS . . . . . 310 711. LIST OF NOTED CLUSTERS AND NEBULA , . .311 SUGGESTIONS TO TEACHERS. Aids. A celestial globe, twelve inches, or there- abouts, 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' ex- perience 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. Reviews 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. Par- ticular attention ought to be paid to the questions and g< : &Tr&80N8 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 un- derstands 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 ob- servations described in the following pages. He will be surprised at the interest awakened, and at the valu- able 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 sci- ences. 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 culti- vated 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 an- cestors were most favorable to the study of astronomy. As hunters, shepherds, and farmers, their lives were snent in the open air, by night as well as by day. In travelling over the thinly-peopled earth, and the sea as 9 S; >i *.- *t i ...; ^ INTRODUCTION. 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 astrono- mers. 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 ex- actly repeated every eighteen years, and thus success- fully 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 Egyp- tians, 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 knowl- edge of astronomy very early, more than four thou- sand 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 du- ties, and an eclipse of the sun occurred without being predicted. The whole nation was thus exposed to the INTRODUCTION. \\ anger of their gods, because of the omission of the religious ceremonies always performed upon such oc- casions. The unfortunate astronomers were imme- diately 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 sci- ence 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 astron- omers 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 phi- losophers 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 12 INTRODUCTION. achieved some substantial results. They made obser- vations which were of use to succeeding astronomers, they greatly improved the reckoning of time, and de- termined the length of the year to be three hundred and sixty-five and one-fourth days, which is wonder- fully near its exact length. 1 5. The Alexandrians. For a few hundred years be- fore 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 at- tempted 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 imperfec- tions 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 re- flection 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 short- est shadow comes again, the length of the year is found. It is inter- esting 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 ? * What is meant by this ? INTRODUCTION. 13 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 uni- versally used, lived in Alexandria during this period, and contributed to the advancement of astronomy. 6. Hipparchus. 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 discov- eries 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 posi- tions of places on the earth. 7. Ptolemy 4 and his System. This most famous astron- omer of antiquity lived at Alexandria about 130 A.D. 5 He made few observations himself, but collected the results of other men's work and wrote them down, to- gether 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) flourished about 300 B.C. 8 Pronounced Hip-ar'kus. What is meant by this ? How long ago were these timeg ? 4 Pronounced Tol'e-my. 14 INTRODUCTION. 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; 1 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 com- plicated circular paths, composed of series of loops. This is the theory of the universe which was accepted everywhere without question until the sixteenth cen- tury. 3 1 A little thought, aided perhaps by a diagram, will make this rea- soning 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 railroad- trains ? 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. 8 The sixteenth century began at the beginning of the year 1501 INTRODUCTION. 15 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 sun as a centre, and that the daily risings and settings of the heav- enly 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 ? 1 Copernicus (ko-per'ni-kus), 1473-1543. * Kepler, a German, 1571-1630. 16 INTRODUCTION. 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 estab- lished, 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 tele- scope 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 dis- covered the moons of Jupiter, the spots upon the sun, and many other wonderful things. His brilliant dis- coveries convinced the world of the truth of the Co- pernican 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 re- markable man as astronomy. 11. Newton? 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 1 Galileo (Gal-i-lee'o), 1564-1642. New'ton, 1642-1727. 8 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 INTRODUCTION. 17 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 astron- omer, not an observer. He discovered and proved many other important facts in astronomy, besides making many and valuable discoveries in natural phi- losophy and other sciences. He also occupied impor- tant 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 as- tronomer, pronounced this work the greatest produc- tion of the human intellect. 12. Modern Astronomy. Since Newton a host of emi- nent astronomers and mathematicians have given their lives to the advancement of our science. Every gen- eration and every civilized country has furnished its share. As it was the earliest begun, so it is the far- thest advanced of the sciences. Its strides seem to be growing longer rather than shorter. Our own genera- tion and our living astronomers are inferior to none of their predecessors in ability or in the value of their dis- coveries. 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 dis- covered. 2 La Place (La-plass'), a great French mathematician and astrono- mer, 1749-1827. a 18 INTRODUCTION. 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 New- ton'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 care- fully 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 accom- pany the study if the phenomena are to be fully under stood. 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 INTRODUCTION. 19 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 progresses. This will go on till the mid- dle 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 dis- tant 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 observa- tion at the different times. Let him also with a watch observe the exact time, and thus notice the gradual change. 20 INTRODUCTION, 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 seen 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 ? INTRODUCTION. 21 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 north- erly 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 circumpolar 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 situ- ated 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 south- east to southwest. 1 1 In order to obtain a correct idea of this diurnal motion, the stu- dent 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 22 INTRODUCTION. 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 con- cave surface of a sphere, called the celestial sphere. 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 sur- rounding objects are in motion in the opposite direc- tion. 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 ivest. The appar- ent 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 set- ting, 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. the south pole will not pass above the horizon. There are some verj brilliant southern stars that we never see in this latitude. INTRODUCTION. 23 Ir; some explanations it is easier to consider that the sun moves about the earth, as it seems to do. When we speak in this way, it must be remembered that we refer to the apparent and not the real motion. 1 19. Celestial Measures. The heavenly bodies being apparently on the inner surface of a sphere, the line joining their positions on this sphere is an arc of a circle. Hence we do not measure distances in the heavens by miles or other linear units, but by circular measure. Every circle is divided into three hundred and sixty degrees (), each degree into sixty minutes ('), and each minute into sixty seconds ("). The distance from the zenith to the horizon is a quarter of a circle, or ninety degrees. It is well for the student to have a correct idea of the size of small meas- ures in the heavens. The fol- lowing will aid in obtaining it. There are two stars which con- tinually point to the pole-star. They are two of the seven Fl0 ' 1 ' which form what is often called the Dipper, revolving continually about the pole-star and just touching the northern horizon. These two "pointers" are just 1 In the summer of 1881 there was a bright comet, the tail of which pointed nearly to Polaris. It partook of the diurnal motion of the heavens, and being near the pole-star was seen all night. When in the northwest in the evening, its tail pointed upwards and to the right. When it got around to the northeast in the morning, the tail pointed upwards and to the left. Many people who saw it in both of these positions, not understanding about the diurnal motion, thought there were two comets. Let the student think of this matter till he sees how it was that the comet thus changed the direction of its tail with reference to the horizon. 24 INTRODUCTION. about five degrees apart. The diameter of the sun and that of the full moon are each about half a degree or thirty minutes long. If two stars are nearer to- gether than three or four minutes, they will appear as one to the eye. 1 20. The Heavens at the Equator and at the Poles. As the observer changes his position on the earth, the ap- pearance of the heavens will also change. If he move eastward or westward, his horizon will move the same way, and the time of rising and setting of the stars will vary. If the movement be eastward, the same stars will pursue the same course through the sky, but they will rise earlier and set earlier ; if westward, the reverse will be the case. If, however, the observer move towards the north or south, the whole aspect of the heavens will change. The reason that the pole-star does not seem to move is because the axis about which the earth revolves points almost directly towards it. 2 There is no change of the horizon with reference to it. To an observer at the equator the pole-star would be at the horizon, be- 1 The following additional measurements will assist in estimating distances. The stars may be found on a map or globe, or some one knowing them may point them out in the heavens. The extreme stars of the three in the belt of Orion are about three degrees apart ; Castor and Pollux about four degrees. Near Vega is a faint star, which by a good eye can be seen to be made up of two stars. They are three and a half minutes apart. The height of the pole-star above the horizon is about equal to the latitude of the place. In the Middle States this is nearly forty degrees. 8 The axis of the earth does not point directly towards the pole-star, Vat about one and a half degrees from it. The pole-star, therefore, describes a small circle about the pole of the heavens, though, roughly peaking, it may be said to correspond with it. INTRODUCTION. 25 cause the axis of the earth is pointing in that direction. If a globe be set with its axis horizontal, it will show the motion of the heavenly bodies to a person at the equator. The stars that rise in the east will pass di- rectly overhead and set in the west ; every star will be just twelve hours above the horizon ; those around the poles will describe small circles, those farther away larger ; there will be no stars that never rise, and none that never set. As the person moves towards the north pole, the pole- star will rise above the horizon, its height being equal to the latitude of the person ; l that is, if the observer is at latitude 40, as at Philadelphia, the pole-star will be forty degrees above the horizon. When the ob- server reaches the pole, the pole-star will be in his zenith ; the heavens seem to move as in the case of a globe with its axis vertical; only one-half the stars will be ever visible; those in the horizon will con- 1 The elevation of the pole-star may be shown to be equal to the latitude by the aid of Fig. 2. Let BAG .be a meridian of the earth, P the north pole, and E the point where the meridian cuts the equator. The axis of the earth, OP, will cut the celestial sphere at a point, P', very near the pole-star. Let A be the posi- tion of the observer on the earth. Then the arc EA, or the angle EGA, is the latitude of the place J of the observer, and BOP is the elevation of the pole of the heavens above the horizon. We wish to prove that EOA = BOP. BOA is a right angle, as is also POE, because the equator is ninety degrees from the pole. Hence BOA = POE. Taking from these equals the angle POA, we have BOP==AOE, which is what we wished to prove. 26 INTRODUCTION. tinually skirt around the horizon ; none will ever rise, and none set. The same changes would be noticed if the observer moved southward from the equator, except that there is no star to mark the position of the south pole. Usefulness of Astronomy. 21. Astronomy, besides being a very grand and interesting science, has great practical usefulness. Every day there is telegraphed over the country from the Washington and other observatories the accurate time of noon ; this is determined by astronomical ob- servations, without which it would be almost impos- sible to keep our clocks and watches correct. Every captain of a vessel when he starts out on a long voyage takes with him a chronometer 1 which has been previously tested at an observatory, and a nautical almanac? in which the positions of the sun, moon, and principal stars are given with great accuracy. With these and some simple observations he is a*ble to tell his position on the ocean and thus to direct his move- ments. The basis of our calendar is astronomical. The lengths of the year, month, and day are governed by phenomena of the heavenly bodies, and are determined by observations of them. Our common almanacs are calculated from the nautical almanacs, which are issued from the national observatories. 1 A clock swinging in rings, so that the motion of the vesse" will not affect it. 2 This will be further explained on page 169. INTRODUCTION. 27 All the maps of the surface of the earth are depend- ent for their accuracy on astronomical observations; the methods of finding latitude and longitude are largely astronomical. Astronomy is also a help to geology, to meteorology, and to other sciences. Hence we see that it is one of the very practical sciences; and it will probably be found that some of its researches, which do not now seem to be of any use to man, will in the future be in some way closely related to his welfare. It will be of use to students also, if they study it rightly, to teach habits of observation, to strengthen their powers of thought, and to give correct ideas of the method by which the Creator of the universe works. PART L THE SOLAR SYSTEM. CHAPTER L VIEW OF THE SOLAR SYSTEM. 22. Parts of the Solar System. The group of bodies to which the earth belongs is called the solar system. It consists of the sun, the planets, their satellites or moons, the comets, and meteoroids. 1 The earth is one of the planets, and the moon one of the satellites. These bodies are closely connected with one another, and, comparatively speaking, are close together. The sun is very much the largest and most important member of the system : hence the name solar 2 system. The stars are all situated at immense distances from us, and, aside from their light, exert little or no in- fluence upon us. 23. Arrangement of the Solar System. The sun is the centre of the solar system, and about it all of the other 1 " Shooting stars" are meteoroids which have come into our at- mosphere. a Solar, from Latin sol, the sun. 3* 29 30 ASTRONOMY. members revolve. The time that it takes one of these bodies to revolve about the sun is called its year. If an observer could be at the sun and watch the other members of the solar system, they would revolve about him in apparent circles, just as we see the moon re- volving about the earth. But from one of the planets these motions do not seem so simple, and it was a long time before men found out that the earth and the rest of these bodies revolve about the sun. FIG. 3. THE ORBITS OF MARS, THE EARTH, AND VENCS. One inch = 100,0(10,000 miles. The arrows show the direction in which the planets move, as seen from the north side of their orbits. The path in which a body moves about the sun is called its orbit. Fig. 3 shows the orbits of the earth and the planets next to it on either side, Mars and GENERAL VIEW OF THE SOLAR SYSTEM. 31 Venus. Those planets whose orbits are inside of the earth's orbit, as Venus, are called inferior planets, be- cause they are nearer to the sun. Those outside are called superior planets. 24. Positions and Apparent Motions. When a heav- enly body is on the side of the earth opposite to the sun, it is said to be in opposition ; thus, if Mars is at M, with the earth at E, Mars is in opposition. When a heav- enly body and the sun are on the same side of the earth, the body is in conjunction ; thus, if Mars is at M', 1 with the earth at E, Mars is in conjunction. It is evi- dent from the figure that an inferior planet has two conjunctions. With the earth at E, Venus at V is in inferior conjunction, but at V is in superior conjunction. If in going between the earth and the sun Venus should happen to pass directly across the face of the sun, it would be called a transit. 2 This rarely happens ; the inferior planets usually cross a little below or above the sun. A superior planet may be seen at any height in the heavens; it may be in opposition to the sun, when it would rise about the time the sun sets, and would shine all night. An inferior planet can never be in opposition to the sun, but in revolving about the sun seems to us to pass back and forth, from one side of the sun to the other, as Fig. 3 shows is the case with Venus. An inferior planet, then, is never far from the sun ; it is only seen a little while after sunset or before sunrise. When Venus is at V" or V", with the earth at E, it seems to be farthest from the sun ; it is then said to be at its greatest elongation. The planets all 1 M' is read M prime; V, V prime; V", V second; V", V third, etc. 2 Can a superior planet ever transit ? 32 ASTRONOMY. move about the sun in the same direction, from west to east. To an observer north of them (as anywhere in the United States north of Florida) they would seem to move roin the right over to the left, or in a direc- tion opposite to the motion of the hands of a clock. 1 Although the planets always move around the sun in the same direction, our position upon the earth makes them seem to move differently sometimes. With the earth at E, Yenus seems, while moving from V" to Y /r/ , to move in the proper direction, from right to left, but while moving from V" to V", across between us ?,nd the sun, it seems to move in the opposite direc- tion. 2 This is called its retrograde (backward) motion. A superior planet retrogrades when the earth passes between it and the sun. The earth leaves the planet behind, and it seems to move backward, just as trees seem to move backward when we pass them in the cars. If we imagine ourselves at E, watching Yenus pass us, or Mars as we pass him, it will be clear. 25. Shapes of the Orbits. The orbits of the planets are not circles, but ellipses. An ellipse is an oblong curve, so made that the sum of the distances from any 1 This motion must not be confounded with the apparent diurnal motion of the heavenly bodies. All of the planets and stars seem to move every night from, east to west, which, as has been explained, is caused by the revolution of the earth upon its axis. But the motion here referred to is one which the planets have in the opposite direction among the stars, just as the moon moves to the east among the stars, although it rises and sets with them. The planets move more slowly than the moon, but if one of them be watched from night to night, its motion eastward among the stars may be seen. It is very im- portant to have this matter perfectly clear. 2 For the sake of simplicity the earth is here supposed to be station- ary ; the earth's motion really shortens very much the time of retrogra- dation. GENERAL VIEW OF THE SOLAR SYSTEM. 33 point of it to two fixed points is always the same. Fig. 4 represents an ellipse. The sum of ES and EF is just equal to the sum of E'S and E'F. 1 If the ends of a string be fastened at two points (S and F) upon a table, so as to lie loosely between them, and a pencil held against the string so as to stretch it (as at E) be moved FIG. 4. ELLIPSE. FIG. 5. PARABOLA. along, it will mark an ellipse. S and F are called the foci (fo'si). In the orbits of the planets the sun is al- ways at one focus (fo'kus). If the foci are nearer to the centre C, the ellipse is nearer circular. The eccen- tricity of an ellipse is the distance OS divided by CP ; it is usually expressed in a decimal fraction : the eccen- tricity in Fig. 4 is .8, or . 2 The eccentricity of an ellipse shows whether it is nearly circular or more ob- long. The orbits of the planets have very little eccen- tricity, as the table in Art. 27 shows. It must be re- membered, then, that the elliptical shape of a planetary 1 Measure the lines in the figure, and see if SB and EF taken to- gether are equal to SB' and E'F. Try the sum of the distances to any other point on the curve. Measure CS and CP, and see if OS is .8 (or f ) of CP. 34 ASTRONOMY. orbit, as shown in Fig. 4, is greatly exaggerated. An exact figure of a planet's orbit could not be distin- guished by the eye from a circle. Fig. 3 shows the real shapes of the orbits of Mars, Earth, and Venus. If the sun and the other orbits be covered, no one of these can be distinguished from a circle. That point of a planet's orbit which is nearest to the sun is its perihelion; 1 the point which is farthest from the sun is its aphelion. 2 In Fig. 4, P is the perihelion, and A the aphelion. The difference between the distances of these two points from the sun may be very consider- able, even if the orbit does seem to be almost a circle. In the case of the earth the difference is three millions of miles, and with most of the other planets the differ- ence is greater. Some of the comets are thought to move not in ellipses, but in parabolas? The two sides of this curve (Fig. 5) keep separating farther and far- ther forever. The parabola is not a closed curve like the circle and the ellipse. 26. Characteristics of all the Planets. Next to the sun the planets are the most important parts of the solar system. They are alike in many points. Besides moving about the sun in the same direction in ellip- tical orbits, they all seem to revolve upon their axes in the same direction, giving them all day and night. Their paths all lie nearly in the same plane. They are all of the same shape. They all shine by reflected sunlight. 1 Perihelion, from the Greek peri, near, and helios, the sun. 2 Aphelion, from the Greek apo, from, and helios, the sun. 8 The paraVola is so drawn that every point of the curve is equally distant from a fixed point and a fixed straight line. As in Fig. 5, CD and OS are equal ; S is the/ocws, and DD' the directrix. GENERAL VIEW OF THE SOLAR SYSTEM. 35 27. Statistics of the Sun and Planets. Name. Average dist. from the sun. Diameter in miles. Length of year. Length of day. Mass (times the weight of the earth). Density (times the weight of water). Eccentricity of orbit. Millions of miles. I Times the | earth's dist. Sun 866,000 3000 7630 7918 4200 20 to 300 (?) 86,000 73,000 32,000 35,000 days. 88 225 365J 687 years. 3 to 7 12 29* 84 164* 25 days. 88 days ? ^25 days? 23h. 56m. 24h. 37m. unknown. 9h. 55m. lOh. 14m. unknown, unknown. 330,000 f unknown. 312 93 }? 11 6| 4f 1 unknown. If ij 1} Mercury 36 67 93 142 200 to 325 483 886 1,782 2,790 * IJ 2 to ? , 9 9 J 30 0.2056 0.0068 0.0168 0.0933 0.02 to 0.38 0.0483 0.0560 0.0464 0.0090 Venus h arth Mars Planetoids. ..J Jupiter Saturn Uranus Neptune This table is not to be committed, as the most im- portant of these statistics will be given in round num- bers in connection with the separate planets, but some of its striking facts should be noticed. The sun is by far the largest body in the solar system. His mass is seven hundred times that of all of the planets together. The planets are divided into three groups. Nearest the sun are four small planets, not differing very greatly in size ; of these the earth is the largest. Next to these is a large number of ve^ small planets, or planetoids. Then come four giant planets, which in several re- spects resemble one another. The four small planets are of heavy material; the sun and the four large planets are all about as light as water. Two of the four small planets have days about twenty-four hours long, while all of the large planets whose axial rev- olutions have been determined have days of only ASTRONOMY. ten hours. Figs. 6 and 7 will assist in giving clear ideas of the sizes and distances of the planets. In Fig. 7 two of the planetoids are put between Mars and .Jupiter. These planetoids are very small planets. They all come between Mars and Jupiter, and are close to- gether. Little is known about them. The planetoids and the two farthest planets, Uranus and Neptune, were unknown to the ancients. 28. Satellites. All of the principal planets except the two inner ones, Venus and Mer- cury, have one or more satellites. The earth has one satellite, the moon; and the satellites of the other planets are often called their moons. These satellites all revolve around their planets, just as the planets re- volve about the sun, and are carried with them by the planets in their journey about the sun. 29. Kepler 7 s Laws. As has been said, Kepler discov- ered three important laws, by which the motion of all the planets and their satellites is interpreted. These are : FIG. 6. THE COMPARATIVE SIZE OF THE PLANETS. GENERAL VIEW OF THE SOLAR SYSTEM. 37 I. The planets move in ellipses, with the sun in one focus. Before the discovery of this law, astronomers had always assumed that the planets move in circles, and it must not be forgotten that these ellipses are almost circles. When it is at perihelion, or nearest the sun, a planet moves fast- est; if it did not, the increased at- traction of the sun would cause the planet to fall into it. This is be- tween P 2 and P 3 in Fig. 8 ; but as the planet moves from P 3 to P 4 , the sun's attraction, pulling it back, makes its Fia - 7. THE COMPARATIVE SIZES OF THE SUN, AS SEBN , . -, , FROM THE DIFFERENT PLANETS. motion slower and slower, until between P 4 and P 5 it is slowest of all. If this were not the case, the sun's attraction upon it here would be too weak to hold it in its place, and it would fly off into space. As it turns and passes through P and P 1 , the sun's attraction pulls it forward and contin- ually increases its velocity, so that at perihelion the planet's motion is swift enough to carry it past the sun without falling into it. 4 38 ASTRONOMY. II. TJie radius-vector of each planet sweeps over equal areas in equal times. The radius-vector is the line drawn from the sun to any point of the orbit, as SP, SP 1 , SP 2 , etc., in Fig. 8, P P' Fia. 8. TLLUSTPATIXO KEPLER'S PKCOXD LAW. In this figure suppose that PP 1 , P 2 P 3 , and P 4 P 5 each represent the path of a planet for two weeks. Then the three shaded parts will be equal in area. III. The squares of the times of revolution of two planets are proportional to the cubes of their distances from the sun. To illustrate this law, let us compare the times and distances of Mercury and Mars, from the table on page 35, and by this law we shall have : 88 2 : 687 2 : : 36,000,000 3 : 141,000,000 3 /Mercur.v'sX /Mars's \ /Mercnry's\ / Mars's \ V period. / \ period.; V distance. / V listancej If this be worked out, the product of the means will be found to be nearly equal to the product of the ex- GENERAL VIEW OF THE SOLAR SYSTEM. 39 tremes. 1 The same proportion will be true, for any other pair of planets. Observation has fixed the times of revolution of the planets very exactly, and when the distance of the earth from the sun is found, 2 the third law enables us to find the distance of any other planet from the sun by the proportion : Square of . square of . . cube of . cube of earth's period planet's period earth's distance planet's distance. Knowing the first three terms of this proportion, the last is found by arithmetic. This is the method used by astronomers to find the distances of the planets. It also follows from this law that the planets near the sun move much faster than the distant ones. The table shows that Neptune is eighty times as far from the sun as Mercury, and its orbit is then eighty times as long. But it takes Neptune seven hundred times as long to complete its circuit. Mercury must move nearly nine times as fast as Neptune. Every planet moves faster than the planets outside of it. If it did not, it could not keep from being pulled in to- wards the sun by his greater attraction. 30. Ecliptic. As has been said, the earth revolves about the sun once a year, but, as in all such cases, it seems to us that the earth is stationary, and that the sun moves about it. The apparent yearly path of the 1 The products will not be found to agree exactly, chiefly because the distances and times used above are not quite exact. If the exact distances and times were used, the agreement would be still a little imperfect, because the different planets influence the motions of one another slightly. 8 The method of finding the distance from the earth to the sun will be explained in chapter iii. 40 ASTRONOMY. sun among the stars is called the ecliptic. 1 The earth's axis is not perpendicular to the plane in which the sun moves, but is inclined to it. The angle between the ecliptic and the equator is 23J degrees. Fig. 9 shows this leaning of the earth's axis. SS' is the ecliptic. Fio. 9. THE ECLIPTIC. EQ is the equator. The plane of the ecliptic cuts the earth along TT'. The angle EVT is the angle of 23| degrees. When the sun is at S it is directly over T, which is 23J degrees south of the equator. This is the winter solstice ; 2 it comes on the 21st of December. This is the shortest day of the year, the sun being far- thest south. As the sun moves around from S towards S' it shines directly down upon the line TVT', and is getting farther north, nearer the equator. On the 20th of March the sun is half-way from S to S', and then shines directly down upon the equator at V. This is the vernal equinox f or spring equinox. On the 21st of June 1 So named because eclipses can occur only when the moon is near this line. 2 Sol'stice, from the Latin words sol, the sun, and sfo, to stand, because the sun seems to stand still here a short time before turning to the north. * E'qui-nox, from the Latin words equus, equal, and nox, night, because the nights and days are here equal. Vernal, from the Latin adjective vernalis, spring. The dates of the solstices and equinoxes may vary a day, because 365 or 366 days do not make an exact year. GENERAL VIEW OF THE SOLAR SYSTEM. 41 the sun is at S', the summer solstice, and is now farthest north, being directly over T'. Half- way from S' to S, on September 22, the sun again crosses the equator, giving us the autumnal equinox. The student must again carefully distinguish this motion of the sun among the stars from its apparent daily motion from east to west. If the stars could be seen in daytime, the sun would be seen to be slowly moving among them towards the east, just as the moon does at night ; it is this path that is the ecliptic. The ecliptic is divided into twelve equal arcs of 30 degrees each, called signs. They begin at the vernal equinox, and take their names from the names of twelve constellations, or groups of stars. Their names which are all Latin and. symbols are these : From the Vernal Equinox. A'RI-KS, (the ram) . , . ' . to 30 TAU'RUS, & (the bull) . . . . 30 to 60 GEM'I-NI, n (the twins) . . . . 60 to 90 CAN'CER, 05 (the crab) . . . '. 90 to 120 LE'O, St (the lion) . . . . . 120 to 150 VIR'GO, v% (the virgin) . . . . 150 to 180 LI'BRA, =& (the balance) .... 180 to 210 SCOR'PIO, n\, (the scorpion) . . . 210 to 240 SAGITTARIUS, / (the archer) . . . 240 to 270 CAPRICOR'NUS, VJ (the goat) . . . 270 to 300 AQUA / RIUS, c# (the waterman) \ . 300 to 330 PISCES, X (the fishes) . . _. . 330 to 360 The sun enters the sign Aries at the time of the ver- nal equinox, about March 20, and about a month later enters the second sign, Taurus, and so on through them all during the year. These signs and their sym- bols are in the first part of our common almanacs. 81. The Celestial Equator. The celestial equator is a 42 ASTRONOMY. great circle around the heavens, right above the equa- tor on the earth. It cuts the ecliptic at the equinoxes, making an angle with it, of course, of 23 J degrees. If the equator were visible in the sky, it would appear as an arch, passing across our southern sky, cutting the horizon just east and west of us. The path of the sun on March 20, or September 22, is on the equator. In summer the sun's path is higher up in the sky than the equator ; in winter it is lower. Latitude and longitude are used to fix the position of places on the earth, and in the same way places in the sky are located; but, unfortunately, astronomers use other names than latitude and longitude to indicate corresponding distances. The distance of a star north or south of the equator is called its declination. In- stead of the meridian of Greenwich or Washington to reckon longitude from, the meridian passing through the vernal equinox is used. And the distance that a star is east of the vernal equinox is its right ascension. 1 Both declination and right ascension, like latitude and longitude, are reckoned by degrees. 1 Declination, like latitude, is measured both north and south from the equator to the poles, but right ascension is measured around by the east only. So that a heavenly body may have any right ascen- sion up to 360. What is the greatest possible declination that any point can have ? where is that point ? What is the dec. of the sun on June 21 ? on September 22? What is the dec. of your zenith ? (See Fig. 2). What is the K. A. (right ascension) of the sun on March 20? on June 21 ? on December 21 ? In which sign is the sun when his R. A. is 50 ? when it is 140 ? 250 ? When the sun's K. A. is 110 is its dec. north or south ? when its K. A. is 180 ? when it is 300 ? What is the sun's dec. when its K. A. is 90? when 180? when 270? when 360? Bight ascension is usually reckoned in hours by astronomers, 1 hour being 15 degrees. GENERAL VIEW OF THE SOLAR SYSTEM. 43 32. The Zodiac. 1 The zone of the heavens, extending about eight degrees on each side of the ecliptic, is called the zo'diac. It too is divided into twelve signs, which have the same names and order as the signs of the ecliptic. These signs roughly coincide with twelve constellations, or groups of stars, and it was to these constellations that the ancients gave the names Aries, Taurus, etc. When these names were given, the sun entered the constellation Aries at the time of the ver- nal equinox, and the signs of the ecliptic, through which the sun moves, coincided with the constellations marking the signs of the zodiac. But the vernal equi- nox, the point where the sun crosses the equator in the spring, moves very slowly backward, so that now the sun comes to the vernal equinox about a month before it enters the constellation Aries. The sun, therefore, is in the sign Aries while it is in the constellation Pisces, and in the sign Taurus while in the constellation Aries, etc. The signs of the ecliptic are about one place ahead of the corresponding signs and constellations of the zodiac. Although the planets all move about the sun in the same direction, yet their orbits do not lie in the same plane. But the angles which the planes of the orbits make with each other are all small, and the planets are always found within the zodiac. Their paths are ap- parently circles, cutting the ecliptic at two points 180 degrees apart. These points are called nodes. Since the planets are always so close to the ecliptic, when- ever they can be seen they show us just about where the ecliptic lies in the sky. 1 From the Latin Zoon, an animal. So named from the animals with which the ancients supposed it peopled. See page 43. 44 ASTRONOMY. CHAPTER II THE SUN. Distance from the Earth, 93,000,000 Miles. 1 Diameter, 866,000 Miles. Axial Rotation, 25 Days. Specific Grav- ity, 1.4. 33. The Sun's Parallax. In finding the distance from the sun to the earth, astronomers have generally tried to determine first the sun's parallax. 2 The parallax of a heavenly body is the angle that the earth's radius would make if seen from that body. And so the sun's parallax is the angle that the earth's radius of nearly four thousand miles would make, or, more properly speaking, would subtend, if looked at from the sun. FIG. 10. THE SUN'S PARALLAX (grefltly exaggerated). Fig. 10 will make this clear. E is the centre of the earth, and AE is the earth's radius. Then, if S repre- 1 In kilometres, now so frequently used for scientific measurements, the sun's distance is between 149 and 150 millions. A kilometre is nearly two-thirds of a mile. 2 Par / al-lax, from a Greek word spelled almost exactly the same way, and having the same meaning. THE SUN. 45 sents the sun, the angle ASE is the sun's parallax. 1 An accurate measurement of the sun's parallax is exceed- ingly difficult, but so great is its importance that many efforts have been made to determine it. Some of the most successful methods will be explained later in the book, Arts. 56, 57. It is a very small angle; the best measurement so far makes it 8.81". 2 The angle at S in Fig. 10 is greatly exaggerated ; it is almost three thousand times as large as the real angle. To represent it exactly in a figure is of course impossible. It is the angle which a foot-rule would subtend at a distance of four and a half miles. 34. Distance and Size of the Sun. Since the earth's radius is known very exactly (Art. 65), when we know the angle that it subtends at the sun, it is an easy problem in trigonometry to calculate the distance of the sun, 3 which will be found to be a little less than 93,000,000 miles. This distance has been aptly called the yard-stick of the universe. Our measurements of the distances and dimensions of all the other planets, 1 Properly speaking, this is the horizontal parallax, that is, the angle subtended by the radius running to our feet when the sun is on the horizon. It is easily seen that if the sun were above its position in Fig. 10, the angle ASE would be smaller. And if the sun were directly above A, this angle would be zero. 2 A few of our readers may need to be reminded that this is 8.81 seconds, and is angular measure. It must not be confounded with seconds of time, which are never indicated by these two strokes // t but always by s, or sec. 8 The following proportion will make this clear to those who under- stand trigonometry. Using the triangle in Fig. 10, in which A is a right angle and ASE is the parallax, we have : Sin parallax : sin 90 : : earth's radius : dist. from sun to earth ? or, sin 8.81" : sin 90 : : 3959 : required distance. 46 ASTRONOMY. and even of the distances of the fixed stars, depend upon it. If the distance to the sun is determined more accurately, all these distances and dimensions as given in this book should be proportionately changed. On this account these figures will be found to differ in different astronomies. By measuring the apparent angular diameter 1 of the sun, and knowing its distance from us, another simple trigonometrical solution gives us its diameter, 2 which is about 109 times the earth's diameter. And since the volumes of spheres are as the cubes of their diam- eters, the sun's volume is 109 3 , or about 1,300,000 times that of the earth. But the density of the sun is only about one-fourth of the earth's density, so that while it would take 1,300,000 worlds as large as ours to make one as large as the sun, yet it would only take one-fourth of this number, or about 325,000, to make one as heavy as the sun. The force of gravity upon the sun is much greater than upon the earth, and, as the weight of a body depends upon gravity, anything would weigh nearly twenty-eight times as much upon the sun as upon the earth. A man who weighs one hundred and fifty pounds here would weigh more than two tons upon the sun, and would be crushed to death by his own weight. THE SUN AND HIS SURROUNDINGS. 35. The Sun's Outer Atmosphere. If it were possible 1 The angular diameter of the sun is the angle which its diameter subtends as seen from the earth. 2 If a right-angled triangle be drawn, having the line from the centre of the earth to the centre of the sun as its hypothenuse, and its right angle at the surface of the sun (because the line along which the edge of the sun is seen is a tangent), we have : Sin 90 : sin of half of sun's angle : : 93,000,000 : sun's radius. THE SUN. 47 to visit the sun, one would first enter the corona? a very light atmosphere extending several hundred thou- sands of miles on all sides. It is never seen except during a total eclipse, and then is a bright cloud-like circle of light surrounding the darkened sun. A great part of the corona is made up of streamers of light extending from the sun in various directions. Some- times these streamers stretch away in two opposite directions only; often they project in four directions, giving the corona a four-sided appearance. At the eclipse of 1878 these streamers were noticed by some observers to extend as far as 9,000,000 of miles from the sun. The corona is never twice of the same shape, and even during the same eclipse its shape appears very different to different observers. 2 Photographs of it taken from different points on the earth at about the same time will, however, show the same general features. Fig. 11 represents a sketch of the corona as seen by Prof. Stone 3 during the eclipse of 1878. The spectroscope (Art. 254) shows that the corona is composed mostly of hydrogen, which is the lightest known gas upon the earth, and some unknown gas or vapor even lighter than hydrogen, which has been 1 Cor-o'na, Latin corona, a crown. 2 This is remarkable. Different observers of the same eclipse, even when sitting side by side, make totally different drawings of the same corona. This is probably because one observer's attention is attracted mainly or even only by those features of the corona which strike him as most prominent, perhaps the great length or breadth of certain streamers. Another might notice particularly, and therefore draw only, the brighter parts of the corona. And owing to the short time that the eclipse lasts and to the excitement of the observers, probably none of them will notice all the parts of the corona. 3 0rmond Stone. 1847 , director of the Observatory of the Univer- sity of Virginia. 48 ASTRONOMY. called coronium. These gases give out light of them- selves, and not merely reflect the sunlight. They are exceedingly thin and rare, more so than our own atmo- FIG. 11. THE CORONA AS SEEN IN 1878. sphere, and the streamers are probably more like the streaks of "Northern Lights" than anything else we know of on the earth. They also have a certain re- semblance to the tails of comets, and may owe their origin to electrical action. 36. The Sun's Lower Atmosphere. The lower part of the sun's atmosphere, which rests directly upon the sun itself, is called the chromosphere. 1 It is a sheet of flame several thousands of miles deep surrounding the sun. The spectroscope shows that the chromosphere is made up of the burning vapors of iron, copper, sodium, and some 1 ChrO'mo-sphere, from the Greek chrdma, color, and sphere. It Us this layer of burning vapors that causes the dark lines in the sun's spectrum, as is explained in Art. 263. THE SUN. 49 twenty or more other substances ivhich we find upon ike earth. Besides these, there are several substances burning in the chromosphere which have never been found upon the earth. This discovery of the substances which compose the chromosphere is one of the most remarkable of modern times. It was made by Prof. G. R. Kirchoif (kirk'hof ), of Germany, in 1859. The chromosphere cannot be seen with the naked eye, nor with an ordinary telescope, except during a total eclipse of the sun. But by having a spectroscope attached to a telescope (Art. 254), and directing it to the edge of the sun, the chromosphere can be observed on any clear day. 37. The Solar Prominences. Terrible storms are con- stantly raging in the chromosphere. From every part FIG. 12. CHANGES IN A SUN PROMINENCE DURING TEN MINUTES, OBSERVED BY PROFESSOR YOUNG, OCTOBER. 7, 1869. of the sun's surface great masses of the burning vapors are frequently hurled up to a height which not uncom- monly reaches 100,000 miles. Prof. Young, in 1880, 5. 50 ASTRONOMY. saw one thrown up to the enormous height of 350,000 miles. These are the red prominences seen during total eclipses of the sun, and now, with the aid of the spectroscope, watched every day. These masses are frequently thrown up with a velocity of 100 miles, and sometimes even 200 miles, per second. They are largely composed of burning or glowing hydrogen, but some- times, near the base, of the burning vapors of the metals and heavy elements which make up the sun. They must be caused by great eruptions or explosions in the sun or the chromosphere. Fig. 12 shows the sudden changes in one of these prominences, as seen by Prof. Young 1 in 1869. Others of the prominences remain unchanged in form and position for days. These may be great masses of clouds thrown up by an explosion, which remain floating in the sun's at- mosphere. 38. The Surface and Interior of the Sun. Below the corona and chromosphere we come to the surface of the sun itself, the only part of it ever seen by most people, called by astronomers the photosphere. 2 This is now generally believed to be a shell of clouds surround- ing the unseen mass of the sun beneath. Every one knows that the clouds about the earth are made up of tiny drops of water, that clouds are in fact precisely like fogs, except that they are floating high up in the air. The clouds which make up the sun's surface are not composed of water, but of tiny drops of fiery-hot melted iron, copper, and other substances that consti- tute the chromosphere. 1 Charles A. Young, 1834 , Professor of Astronomy at Princeton, New Jersey. * ^fcO'to-sphere, from Greek phos, light, and sphere. THE SUN. 51 Within the photosphere is the body of the sun, and, strange as it may seem, it is now generally "believed that this is a great hall of gas ; in fact, an enormous bubble. The great pressure makes this gas denser than water, so that it is not light and thin like the air around us, but probably as thick as tar or jelly. This gas is no doubt composed of the vapors of the various substances which make up the chromosphere. These are all kept in the condition of vapor by the intense heat. 39. Sun-spots. With a small telescope the only thing to be seen on the sun's surface is a greater or less Fio. 13. SUN-SPOTS AND FACUI-;E. (From Young's The Sun.) number of dark spots. The shapes of these are very various and irregular. The central part of a spot, called the nucleus, or umbra* is black, while around the 1 Um / bra, Latin umbra, a shadow. 52 ASTRONOMY. edge is a lighter, grayish border, the penumbra. 1 Fig. 14, a drawing of a sun-spot seen through a large tele- scope in 1860, shows very clearly the features of a sun-spot. Here filaments of the penumbra stretch entirely across the umbra ; but this is unusual. These spots are of all sizes, from those just visible in large telescopes to occasional monstrous ones 100,000 miles in diameter. They are very commonly found in groups, and are not distributed over the whole surface of the sun, but are confined to two zones, one on each side of the equator. These zones begin about 10 from the equator, and end about 35 from it. Close to the sun's equator spots are rarely seen, and close to the poles, never. As the sun turns upon its axis, the spots are carried along with it, and so pass across the sun's disk in twelve or fourteen days ; it is by the motion of the spots that we can tell that the sun rotates, and determine the time of its rotation. Besides being thus carried around by the sun, the spots have some motion of their own over the sun's surface. Careful observa- tions have shown also that the spots in different lati- tudes have different rates of rotation. Spots on the equator revolve in tw