GIFT OF * Of A LESSONS IN ASTRONOMY INCLUDING URANOGRAPHY A BRIEF INTRODUCTORY COURSE WITHOUT MATHEMATICS BY CHARLES A. YOUNG, PH.D., LL.D. LATE PROFESSOR OF ASTRONOMY IN PRINCETON UNIVERSITY, AUTHOR OF A "GENERAL ASTRONOMY FOR COLLEGES AND SCIENTIFIC SCHOOLS," OF A "MANUAL OF ASTRONOMY," AND OF "ELEMENTS OF ASTRONOMY" REVISED EDITION WITH ADDITIONS AND CORRECTIONS GINN AND COMPANY BOSTON NEW YORK CHICAGO LONDON ATLANTA DALLAS COLUMBUS SAN FRANCISCO ENTERED AT STATIONERS 1 HALL COPYRIGHT, 1891, 1903, BY CHARLES A. YOUNG COPYRIGHT, 1918, BY G1NN AND COMPANY ALL RIGHTS RESERVED A622.ll i - ct Cfte G1NN AND COMPANY PRO- PRIETORS BOSTON U.S.A. PREFACE TO THE ORIGINAL EDITION THIS volume has been prepare4 to meet the want of certain classes of schools which find the author's " Elements of Astronomy " rather too extended and mathematical to suit their course and pupils. It is based upon the Ele- ments, but with many condensations, simplifications, and changes of arrangement: everything has been carefully worked over and rewritten in order to adapt it to those whose mathematical attainments are not sufficient to enable them to use the larger work to advantage. Of course, such pupils cannot gain the same insight into the mechanism of the heavens as those who take up the subject at a more advanced stage in their education. They must often be contented with the bare statement of a fact without any explanation of the manner in which its truth is established, and thus will necessarily miss much that is most valuable in the discipline to be derived from the study of Astronomy. But enough remains surely there is no other science which, apart from all questions of How or Why, supplies so much to widen the student's range of thought and to make him comprehend his place in the infinite universe. The most important change in the arrangement of the book has been in bringing the Uranography, or " constella- tion-tracing," into the body of the text and placing it near iii iv PREFACE the beginning, a change in harmony with the accepted principle that those whose minds are not mature succeed best in the study of a new subject by beginning with what is concrete and appeals to the senses, rather than with the abstract principles. It has been thought well also to add brief notes on the legendary mythology of the constellations for the benefit of such pupils as are not likely to become familiar with it in the study of classical literature. In the preparation of the book great pains have been taken not to sacrifice accuracy and truth to compactness, and no less to bring everything thoroughly down to date. The Appendix contains in its first chapter descrip- tions of the most used astronomical instruments, and where time permits, might profitably be brought into the course. The second chapter of the Appendix is designed only for the use of teachers and the more advanced pupils. Sees. 431-434, however, explaining how the sun's dis- tance may be found in the simplest way, might well be read by all. 1891. PREFACE TO THE REVISED EDITION SINCE the original publication of this work twelve years ago, a number of editions have been issued in which it was attempted to keep up to date, as far as possible, by such minor changes and corrections as typographical considera- tions would permit. It has now, however, seemed best to reprint the book from entirely new plates, and this has given an opportunity for a thorough revision of the work and the free introduction of all desirable improvements and additions. The former rather unsatisfactory star-maps have been replaced by new ones, and a considerable number of beautiful half-tone illustrations have been added. The publishers have spared no pains or expense in the mechanical execution of the volume, and it is hoped that, so far as its scope permits, the book will now be found to offer a satisfactory summary of the present state of Astronomy. C. A. YOUNG. PRINCETON, N.J., January, 1903. PREFACE TO ISSUE OF 1918 WHILE the greater part of the text remains as it was written by its author, such changes have been made in this issue as are necessary to bring it down to date. ANNE SEWELL YOUNG. MOUNT HOLYOKB COLLEGE, October, 1917. CONTENTS CHAPTER I. INTRODUCTION Fundamental Notions and Definitions The Celestial Sphere and its Circles Altitude and Azimuth Right Ascension and Dec- imation Celestial Latitude and Longitude . . 1-19 CHAPTER II. URANOGRAPHY Globes and Star-Maps Star Magnitudes Names and Designations of Stars The Constellations in Detail . . . . 20-63 CHAPTER III. FUNDAMENTAL PROBLEMS Latitude and the Aspect of the Celestial Sphere Time, Lon- gitude, and the Place of a Heavenly Body . . . 64-77 CHAPTER IV. THE EARTH Its Form and Dimen- sions; its Rotation, Mass, and Density Its Orbital Motion and the Seasons Precession The Year and the Calendar . . .... . . . 78-104 CHAPTER V. THE MOON Her Orbital Motion and the Month Distance, Dimensions, Mass, Density, and Force of Gravity Rotation and Librations Phases Light and Heat Physical Condition Telescopic Aspect and Surface . . . ...... 105-128 CHAPTER VI. THE SUN Its Distance, Dimensions, Mass, and Density Its Rotation, Surface, and Spots The Spectroscope and the Solar Spectrum The Chemical Constitution of the Sun The Chromosphere and Prominences The Corona The Sun's Light Measurement and Intensity of the Sun's Heat Theory of its Maintenance and Speculations regarding the Age and Duration of the Sun .... 129-170 vii viii CONTENTS PAGES CHAPTER VII. ECLIPSES AND THE TIDES Form and Dimensions of Shadows Eclipses of the Moon Solar Eclipses, Total, Annular, and Partial Number of Eclipses in a Year Recurrence of Eclipses and the Saros Occupations The Tides . ._ .171-187 CHAPTER VIII. THE PLANETARY SYSTEM The Plan- ets in General Their Number, Classification, and Arrangement Bode's Law Orbits of the Planets Kepler's Laws and Gravitation The Apparent Motions of the Planets and the Systems of Ptolemy and Copernicus Determination of the Planets' Diam- eters, Masses, etc. Herschel's Illustration of the System Description of Individual Planets The < Terrestrial ' Planets, Mercury, Venus, and Mars . 188-224 CHAPTER IX. PLANETS (Continued ) The Asteroids Intramercurian Planets and the Zodiacal Light The Major Planets, Jupiter, Saturn, Uranus, and Neptune Ultra-Neptunian Planet . . 4 . .225-251 CHAPTER X. COMETS AND METEORS Comets, their Number, Designation, and Orbits Their Constituent Parts and Appearance Their Spectra, Physical Con- stitution, and Probable Origin Remarkable Comets Photography of Cornets Aerolites, their Fall and Characteristics Shooting-Stars and Meteoric Showers Connection between Meteors and Comets . . 252-293 CHAPTER XI. THE STARS Their Nature, Number, and Designation Star-Catalogues and Charts Their Proper Motions and the Motion of the Sun in Space Stellar Parallax Star Magnitudes and Photometry Variable Stars Stellar Spectra , : . . .294-325 CHAPTER XII. THE STARS (Continued) Double and Multiple Stars Clusters and Nebulae The Milky Way and Distribution of Stars in Space The Stellar Universe Cosmogony and the Nebular Hypothesis , 326-357 CONTENTS ix APPENDIX PAGES ASTRONOMICAL INSTRUMENTS. The Telescope, Simple Refracting, Achromatic, and Reflecting The Equatorial The Filar Micrometer The Transit- Instrument The Clock and the Chronograph The Meridian Circle The Sextant . . . . .. .359-378 MISCELLANEOUS (FOR THE MOST PART SUPPLEMEN- TARY TO ARTICLES IN THE TEXT). Hour- Angle and Time Twilight Determination of Latitude Place of a Ship at Sea Finding the Form of the Earth's Orbit The Ellipse Illustrations of Kepler's " Har- monic " Law The Equation of Light and the Sun's Distance determined by it Aberration of Light De 1'Isle's Method of getting the Sun's Parallax from a Transit of Venus The Parabola and the Conic Sec- tions Determination of Stellar Parallax . . .378-397 QUESTIONS FOR REVIEW . . . . . .398-400 TABLES OF ASTRONOMICAL DATA I. Astronomical Constants -_.: . V . 401 II. The Principal Elements of the Solar System . 402 III. The Satellites of the Solar System . . . 403 IV. The Principal Variable Stars . . . . 404 V. The Best Determined Stellar Parallaxes . . 405 The Greek Alphabet and Miscellaneous Symbols . 406 INDEX . . . . . . . . __.;.,,. . . 407-420 STAR-MAPS LESSONS IN ASTRONOMY CHAPTER I INTRODUCTION Fundamental Notions and Definitions The Celestial Sphere and its Circles Altitude and Azimuth Right Ascension and Declination Celestial Latitude and Longitude 1. Astronomy 1 is the science which deals with the heavenly bodies. As it is the oldest of the sciences, so also it is one of the most perfect, and in certain aspects the noblest, as being the most " unselfish " of them all. And yet, although not bearing so directly upon the material interests of life as the more modern sciences of Physics and Chemistry, it is of high utility. By means of Astronomy the latitudes and longitudes of places upon the earth's suface are determined, and by such determinations alone is navigation made secure. More- over, all the operations of surveying upon a large scale, such as the determination of national boundaries, depend more or less upon astronomical observations. The same is true of operations which, like the railway service, require an accurate knowledge and observance of time ; for the fundamental timekeeper is the diurnal revolution of the heavens. 1 The term is derived from two Greek words : astron (a heavenly body) and noinos (a law). 1 2 LESSONS IN ASTRONOMY In ancient times the science was supposed to have a still higher utility. It was believed that human affairs of every kind, the wel- fare of nations, and the life history of individuals alike, were con- trolled, or at least prefigured, by the motions of the stars and planets ; so that from the study of the heavens it ought to be possible to pre- dict futurity. Hence originated the pseudo-science of Astrology, which, baseless and absurd as it has been proved to be, still retains a remarkable hold on the popular mind. 2. The heavenly bodies include, first, the solar system, that is, the sun and the planets which revolve around it, with their attendant satellites; second, the comets and the meteors, which also move around the sun, but are bodies of a very different nature from the planets and travel in different orbits ; and, third, the stars and nebulse. The earth on which we live is one of the planets, and the moon is the earth's satellite. The stars which we see are bodies of the same kind as the sun, shining like him with fiery heat, while the planets and the satellites are dark and cool like the earth and visible only by the sun- light they reflect. As for the comets and nebulae, they appear to be mere clouds, composed of gas or swarms of little particles, perhaps not very hot, but luminous. It is likely, practically certain indeed, that besides the visible stars there are also multitudes of others too cool to shine, some of winch manifest their existence by affecting the motion of certain of the visible stars. It is hardly necessary to add that while with the naked eye we see only a few thousand stars, the telescope reveals millions. 3. As we look off from the earth at night, the stars appear to be all around us, like glittering points fastened to the inside of a huge hollow globe. Really they are at INTRODUCTION 3 very different distances, all enormous as compared with any distances with which geography makes us familiar. Even the moon is eighty times as far away as New York from Liverpool, and the sun is nearly four hundred times as distant as the moon, and the nearest of the stars is nearly three hundred thousand times as distant as the sun ; as to the remoter stars, some of them are certainly thou- sands of times as far away as the nearer ones, so far that light itself is thousands of years in coming to us from them. These are facts which are certain, not mere guesses or beliefs. Then, too, as to their motions. Although most of the heavenly bodies seem to us to be at rest, except as the earth's rotation makes them appear to rise and set, yet really they are all moving, and with a swiftness of which we can form no conception. A cannon-ball is a snail com- pared with the slowest of them. The earth itself in its revolution around the sun is flying eighteen and a half miles in a second, which is more than fifty times as fast as the swiftest rifle bullet. We fail to perceive the motion simply because it is so smooth and so unresisted. The space outside our air contains nothing that obviously obstructs either sight or motion. 4. But this knowledge as to the real distance and motions of the heavenly bodies was gained only after long centuries of study. If we go out to look at the stars some moonless night, we find them apparently sprinkled over the dome of the sky in groups, or constellations, which are still substantially the same as in the days of the earliest astronomers. At first these constellations were figures of animals and other objects, and many celestial globes and 4 LESSONS IN ASTRONOMY maps still bear grotesque pictures 1 representing them. At present, however, a constellation is only a certain region of the sky, limited by imaginary lines which divide it from the neighboring constellations, just as countries are divided in geography. As to the exact boundaries of these constellations, and even their number, there is no precise agreement among astronomers. Forty-eight of them have come down to us from the time of Ptolemy (the greatest astronomer of antiquity, who flourished at Alexandria about A.D. 130), and even in his day many of them were already ancient. About twenty others, proposed by later astronomers, are now generally recognized, and at least as many more have been suggested and abandoned. 5. Uranography, or Description of the Visible Heavens. The study of the constellations, or the apparent arrange- ment of the stars in the sky, is called Uranography. 2 It is not an essential part of Astronomy, but it is an easy and pleasant study; and in becoming familiar with the con- stellations and their principal stars the pupil will learn more readily and thoroughly than in any other way the most important facts in relation to the apparent motions of the heavenly bodies, and the principal points and circles of the celestial sphere. For this reason the teacher is urged to take the earliest opportunity to have his pupils trace such of the constellations as happen to be visible in the evening sky when they begin the study of Astronomy, and to continue it from time to time as the progress of the seasons gives opportunity. 1 Most of these figures follow the designs of Albert Diirer. a From the Greek, ouranos (heavens) and grapM (description). INTRODUCTION 5 6. The Celestial Sphere. 1 The sky appears like a hollow vault, to which the stars seem to be attached like specks of gilding upon the inner surface of a dome. We cannot judge of the distance of this surface from the eye, further than to perceive that it must be very far away. It is there- fore natural and extremely convenient to regard the dis- tance of the sky as everywhere the same and unlimited. The celestial sphere, as it is called, is conceived of as so enormous that the whole world of stars and planets lies in its center like a few grains of sand in the middle of the dome of the Capitol. Its diameter is assumed to be immeasurably greater than any actual distance known, and greater than any quantity assignable. In technical language it is taken as infinite. Since the celestial sphere is thus infinite, any two parallel lines drawn from distant points on the surface of the earth, or even from points as distant as the earth and the sun, will seem to meet at one point on the surface of the sphere. If the two lines were anywhere a million miles apart, for instance, they will, of course, still be a million miles apart when they reach the surface of the sphere; but at an infinite distance even a million miles is a mere nothing, so that, to our observation, the two lines are close together and make apparently but a single point 2 where they pierce the sphere. 7. The Apparent Place of a Heavenly Body. This is simply the point where a line drawn from the observer 1 The study of the celestial sphere and its circles is greatly facilitated by the use of a globe, or armillary sphere. Without some such apparatus it is not easy for a young person to get clear ideas upon the subject. 2 This is the same as the "vanishing point " of perspective. LESSONS IN ASTRONOMY through the body in question, continued outward, pierces the celestial sphere. It depends solely upon the direction of the body, and is in no way affected by its distance from us. Thus, in Fig. 1, A, B, C, etc., are the apparent places of a, b, c, etc., the observer being at 0. Objects that are nearly in line with each other, however great the real dis- tances between them, as h, i, k, will appear close together in the sky. The moon, for instance, often looks to us u very near" a star, which is really of course at an enormous distance beyond her. 8. Angular Measurement. It is clear that we cannot properly describe the appar- ent distance of- two points upon the celestial sphere from each other by feet or inches. To say that two stars are about five feet apart, for in- stance, and it is not very uncommon to hear such an expression, means nothing unless we know how far from the eye the five-foot measure is to be held. The proper units for expressing apparent distance in the sky are those of angle, viz. : degrees (), minutes ('), and seconds (") ; the circumference of a circle being divided into 360 degrees, each degree into 60 minutes, and each minute into 60 seconds. Thus, the Great Bear's tail, or " Dipper-handle," is about 16 long, and the long side of the " Dipper-bowl " is about 10; the moon and the sun are each about half a degree, or 30', in diameter. FIG. 1 INTRODUCTION 7 It is very important that the student in Astronomy should become accustomed as soon as possible to estimate celestial measures in this way. A little practice soon makes it easy, though at first one is apt to be embarrassed by the fact that the sky looks to the eye not like a true hemisphere but like a flattened vault, so that the estimates of distances for all objects near the horizon are apt to be too large. The moon, when rising or setting, looks to most persons much larger than when overhead ; x and the Dipper-bowl, when underneath the pole, seems to cover a much larger area than when above it. 9. Circles and Principal Points of the Celestial Sphere. Just as the surface of the earth in Geography is covered with a network of imaginary lines, meridians and par- allels of latitude, so the sky is supposed to be marked off in a somewhat similar way. Two such sets of points and reference circles are in common use to describe the apparent places of the stars, and a third was used by the ancients and is still employed for some purposes. The first system depends upon the direction of the force of gravity shown by a plumb-line at the point where the observer stands ; the second upon the direction of the axis of the earth, which points very near to the so-called Pole-star; and the third depends upon the position of the orbit in which the earth travels around the sun. 10. The Gravitational or Up-and-Down System. (a) The Zenith and Nadir. The point in the sky directly above the observer is called the zenith; the opposite point, under the earth and of course invisible, the nadir? 1 This is a pure illusion due to physiological causes affecting judg- ment of distance and size. The moon at the horizon is really about 4000 miles more distant from the observer than when nearly overhead, and its apparent diameter, as measured by an astronomical instrument, is actually less by about one-thirtieth. 2 These are Arabic terms. About A.D. 1100 the Arabs were the world's g LESSONS IN ASTRONOMY (b) The Horizon (pronounced ho-ri'-zon, not hor'-i-zon). This is a "great circle '^around the sky, half-way between the zenith and the nadir, and therefore everywhere 90 from the zenith. The word is derived from a Greek word which means a "boundary"; i.e., the line where the earth or sea limits the sky. The actual line of division, which on the land is always more or less irregular, is called the visible horizon, to distinguish it from the true, or astro- nomical, horizon denned above. We may also define the horizon as the great circle where a plane which passes through the observer's eye perpen- dicular to the plumb-line cuts the celestial sphere. 11. Vertical Circles and the Meridian ; Altitude and Azi- muth. Circles drawn from the zenith to the nadir cut the horizon at right angles, and are known as vertical circles. Each star has at any moment its own vertical circle. That particular vertical circle which passes north and south is known as the Celestial Meridian ; while the ver- tical circle at right angles to this is called the prime vertical. Small circles drawn parallel to the horizon are known as parallels of altitude, or almucantars. Fig. 2 illustrates these definitions. By their help we can easily define the apparent position of a heavenly body. Its Altitude is its apparent elevation above the horizon ; that is, the number of degrees between it and the horizon, measured on a vertical circle. Thus, in Fig. 2, the chief astronomers, and have left their mark upon the science in numerous names of stars and astronomical terms. 1 "Great Circles" are those which divide the sphere into two equal parts. INTRODUCTION 9 vertical circle ZMH passes through the point M. The arc MH, measured in degrees, is the altitude of M, and the arc ZM is called its zenith distance. The Azimuth of a heavenly body is the same as its " bearing " in Surveying, but measured from the true meridian and not from the magnetic. 1 It is the arc of the horizon, measured in degrees, intercepted between the FIG. 2. The Horizon and Vertical Circles O, the place of the observer. OZ, the observer's vertical. Z, the zenith ; P, the pole. SWNE, the horizon. SZPN, the meridian. EZW, the prime vertical. M, some star. ZMH, arc of the star's vertical circle. TMR, the star's almucantar. Angle TZM, or arc SH, star's azimuth. Arc HM, star's altitude. Arc ZM, star's zenith-distance. south point and the foot of the vertical circle which passes through the object. There are various ways of reckoning azimuth. Many writers express it in the same way as the "bearing" in Surveying, i.e., so many degrees east or west of north or south. In the figure, the azimuth of M thus expressed is about $, 50 E. The more usual way at present is, 1 The reader is reminded that the magnetic needle hardly anywhere points exactly north. Its direction varies widely at different parts of the earth, and, moreover, is continually changing to some extent. 10 LESSONS IX ASTRONOMY however, to reckon clear around from the south, through the west, to the point of beginning. Expressed in this way, the azimuth of M would be about 310, i.e., the arc SWNEH. Altitude and azimuth, however, are inconvenient for many purposes, because they continually change for a celestial object as it apparently moves across the sky. 12. The Apparent Diurnal Rotation of the Heavens. If we go out on some clear evening in the early autumn, say about 8 P.M. on the 22d of September, and face the north, we shall find the appearance of that part of the heavens directly before us substantially as shown in Fig. 3. In the north is the constellation of the Great Bear (Ursa Major), characterized by the conspicuous group of seven stars known as the " Great Dipper." It now lies with its handle sloping upward to the west. The two eastern- most stars of the four which form its bowl are called the " Pointers," because they point to the Pole-star, which is a solitary star not quite half-way from the horizon to the zenith (in the latitude of New York), and about as bright as the brighter of the two Pointers. High up on the opposite side of the Pole-star from the Great Dipper, and at nearly the same distance, is an irregular zigzag of five stars, each about as bright as the Pole-star itself. This is the constellation of Cassiopeia. If now we watch these stars for only a few hours, we shall find that while all the forms remain unaltered, their places in the sky are slowly changing. The Great Dipper slides downward towards the north, so that by eleven o'clock (on September 22) the Pointers are directly un\der the Pole-star. Cassiopeia still keeps opposite, however, rising i INTRODUCTION 11 towards the zenith ; and if we continue the watch through the whole night, we shall find that all the stars appear to be moving in circles around a point near the Pole-star, revolving in the opposite direction to the hands of a watch FIG. 3. The Northern Circumpolar Constellations ) (as we look towards the north) with a steady motion which takes them completely around once a day, or, to be more exact, once in 23 h 56 m 4 8 .l of ordinary time. They behave 12 LESSONS IN ASTRONOMY just as if they were attached to the inner surface of a huge revolving sphere. Instead of watching the stars by the eye we may advan- tageously employ photography. A camera is pointed up towards the Pole-star and kept firmly fixed while the stars by their diurnal motion impress their "trails" upon the plate. Fig. 4 was made in this way with an ex- posure of about nine hours. To indicate the position of the stars as it will be at mid- night of September 22, the figure must be held so that XII in FIG. 4. -Polar Star Trails the mar S in is at the bottom ; at 4 A.M. the stars will have come to the position indicated by bringing XVI to the bottom, and so on. But at eight o'clock on the next night we shall find things very nearly in their original position. If instead of looking toward the north we now look southward, we shall find that in that part of the sky also the stars appear to move in the same kind of way. All that are not too near the Pole-star rise somewhere in the eastern horizon, ascend obliquely to the meridian, and descend to their setting at points on the western horizon. The next day they rise and set again at precisely the same points, and the motion is always in an arc of a circle, called INTRODUCTION 13 the star's diurnal circle, the size of which depends upon its distance from the pole. Moreover, all of these arcs are strictly concentric. The ancients accounted for these fundamental and obvious facts by supposing that the stars are really fastened to the celestial sphere, and that this sphere really turns daily in the manner indicated. According to this view there must really be upon the sphere two opposite points which remain at rest, and these are the poles. 13. Definition of the Poles. The Poles, therefore, may be defined as those two points in the sky where a star would have no diurnal motion. The exact position of either pole may be determined with proper instruments by finding the center .of the small diurnal circle described by some star near it, as, for instance, by the Pole-star. This definition of the pole is that which would be given by one familiar with the sky but ignorant of the earth's rotation, and it is still perfectly correct ; but knowing, as we now do, that this apparent revolution of the celestial sphere is due to the real spinning of the earth on its axis, we may also define the poles as the two points where the earth's axis of rotation, produced indefinitely, would pierce the celestial sphere. Since the two poles are diametrically opposite in the sky, only one of them is usually visible from any given place. Observers north of the earth's equator see only the north pole, and vice versa for observers in the southern hemisphere. The student must be careful not to confound the Pole with the Pole-star. The pole is an imaginary point ; the Pole-star is only that one of the conspicuous stars which 14 LESSONS IN ASTRONOMY happens l now to be nearest to that point and at present is about li distant from it. If we draw an imaginary line from the Pole-star to the star Mizar (the one at the bend of the Dipper-handle), it will pass almost exactly through the pole itself ; the distance of the pole from the Pole-star (often called Polaris) being very nearly one quarter of the distance between the two "Pointers." 14. The Celestial Equator, or Equinoctial ; Declination. The Equator is a great circle of the celestial sphere drawn half-way between the poles, everywhere 90 from each of them, and is the great circle in which the plane of the earth's equator cuts the celes- tial sphere. It is often called the Equinoctial. Fig. 5 shows how the plane of the earth's equator produced far enough would mark out such a circle in the heavens. Small circles drawn parallel to the equinoctial, like the parallels of latitude on the earth, are known as Parallels of Declination, the Declination of a star being its distance in degrees north or south of the celestial equator ; + if north, - if south. It corresponds precisely with the latitude of a place on the earth's surface ; but it cannot be called " celestial latitude," because that term has been preoccupied by an entirely different quantity (Sec. 20). i See Sec. 126. FIG. 5. The Plane of the Earth's Equator produced to cut the Celes- tial Sphere INTRODUCTION 15 A star's parallel of declination is identical with its diurnal circle. 15. Hour-Circles. The great circles of the celestial sphere which pass through the poles like the meridians on the earth, and are therefore perpendicular to the celestial equator, are called Hour-Circles. Some writers call them " celestial meridians," but the term is objectionable since it is sometimes used to indicate an entirely different set of circles. That particular hour-circle which at any moment passes through the zenith of course coincides with the celestial meridian already defined in Sec. 11. 16. The Celestial Meridian and the Cardinal Points. - The best definition of the celestial meridian is, however, the great circle which passes through the zenith and the poles. The points where this meridian cuts the horizon (the circle of level) are the north and south points, and the east and west points of the horizon lie half-way between them, the four being known as the " Cardinal Points." The student is especially cautioned against confounding the north poinl with the north pole. The north point is on4he horizon; the north pole is high up in the sky. In Fig. 6, P is the north celestial pole, Z is the zenith, and SQZPN is the celestial meridian. P and P' are the poles, PmP' is the hour-circle of m, and amRl V is its par- allel of declination, or diurnal circle. N and S are the north and south points respectively. In the figure, mY is the declination of m, and mP is called its polar distance. The angle made at the celestial pole between the merid- ian and the hour-circle passing through a given star is called the star's Hour-Angle for that moment. It is 16 LESSONS IN ASTRONOMY usually reckoned westward from the meridian, and, for many purposes, in time instead of in arc, i.e., in hours, minutes, and seconds of time instead of degrees, etc. One hour = 15, and one minute of time (l m ) =15 minutes of arc (15'), etc. The hour-angle of a star is always equal to the FIG. 6. Equator, Hour-Circles, etc. 0, place of the observer ; Z, his zenith. S WNE, the horizon. POP', line parallel to the axis of the earth. P and P', the two poles of the heavens. EQWT, the celestial equator, or equi- noctial. X, the vernal equinox, or "first of Aries." PXP', the equinoctial colure, or zero hour-circle. TO, some star. Ym, the star's declination; Pm, its north-polar distance. Angle mPH = a.TG QY, the star's (east- ern) hour-angle ; = 24** minus star's western hour-angle. Angle XPm = arc XY, star's right ascension. Sidereal time at the moment = 24 11 minus XPQ. interval of sidereal time (see Sec. 91) elapsed since the star last crossed the meridian. 17, The Vernal Equinox, or First of Aries. In order to use this system of circles as a means of designating the places of stars in the sky, it is necessary to fix upon some one hour-circle, to be reckoned from in the same way that INTRODUCTION 17 the meridian of Greenwich is used in reckoning longitude on the earth's surface. The " Greenwich of the sky " which has thus been fixed upon is the point where the sun crosses the celestial equator in the spring. The sun and moon and the planets do not behave as if they, like the stars, were firmly fixed upon the celestial sphere, but rather as if they were glow-worms crawling slowly about upon its surface while it carries them in its diurnal rota- tion. As every one knows, the sun in winter is far to the south of the equator, and in the summer far to the north, apparently completing a yearly circuit of the heavens on a path known as the ecliptic. It crosses the equator, therefore, twice a year, passing from the south side of it to the north about March 21 (this is now true, since leap year was skipped in 1900), and always at the same point) neglecting for the present the effect of what is known as "precession." This point, the celestial Greenwich, is called the Vernal Equinox, and is made the starting-point for many astronomical reckon- ings. Unfortunately it is not marked by any conspicuous star; but a line drawn from the Pole-star through Beta Cassiopeise (the westernmost or " preceding " star in the zigzag) (see Map I) and continued 90 from the pole, strikes very near it. In Fig. 6, X represents this point. It is also called the First of Aries, and designated by the symbol f> . 18. Right Ascension. The right ascension of a star is the arc of the celestial equator intercepted between the vernal equinox and the point where the stars hour-circle cuts the equator, and is reckoned always eastward from the equinox and completely around the circle. It may be expressed 18 LESSONS IN ASTRONOMY either in degrees or in hours. 1 A star one degree west of the equinox has a right ascension of 359, or of 23 h 56 m . Evidently the diurnal motion does not affect the right ascension of a star, but this, like the declination, remains practically unchanged for years. In Fig. 6, if X be the vernal equinox, the right ascension of m is the arc XY measured from X eastward. 19. Thus we can define the position of a star either by its altitude and azimuth, which tell how high it is in the sky, and how it " bears," as a sailor would say ; or we may use its right ascension and declination, which do not change from day to day (not perceptibly at least), and so are better adapted to mapping purposes, corresponding as they do precisely to latitude and longitude upon the surface of the earth. Perhaps the easiest way to think of these celestial circles is the following: Imagine a tall pole standing straight up from the observer, having attached to it at the top (the zenith) two half circles coming down to the level of the observer's eye, one of them running north and south (the meridian), and the other east and west (the prime vertical). The bottoms of these two semicircles are con- nected by a complete circle (the horizon) at the level of the eye. This framework, immense but fortunately only imaginary and so not burdensome, the observer takes with him wherever he goes, keeping always at its center, while over it apparently turns the celestial sphere ; really, of course, he and the earth and his framework turn together under the celestial sphere. 1 Twenty-four hours of right ascension or hour-angle = 360 ; one hour = 15. INTRODUCTION 19 The other circles (the celestial equator and the hour- circles) are drawn upon the celestial sphere itself and are not affected at all by the observer's journeys, but are as fixed as the poles and meridians upon the earth; the stars also, to all ordinary observation, are fixed upon the sphere just as cities are upon the earth. They really move, of course, and swiftly, as has been said before, but they are so far away that it takes centuries, as a rule, to produce the slightest apparent change of place. 20. Celestial Latitude and Longitude. A different way of designating the positions of the heavenly bodies in the sky has come down to us from very ancient times. Instead of the equator it makes use of another circle of reference in the sky, known as the Ecliptic. This is simply the apparent path described by the sun in its annual motion among the stars; for the sun appears to creep around the celestial sphere in a circle once every year, and the Ecliptic may be defined as the intersection of the plane of the earth's orbit with the celestial sphere, just as the celestial equator is the intersection of the earth's equator ; the vernal equinox is one of the points where the two circles cross. Before the days of clocks, the Ecliptic was in many respects a more convenient circle of reference than the equator and was almost universally used as such by the old astronomers. Celestial longitude and latitude are measured with reference to the Ecliptic, in the same way that right ascension and declination are measured with respect to the equator, except that celestial longitude cannot be expressed in hours, minutes, and seconds of time like right ascen- sion. Too much care can hardly be taken to avoid confusion between terrestrial latitude and longitude and the celestial quantities that bear the same name. CHAPTER II URANOGRAPHY Globes and Star-Maps Star Magnitudes Designation of the Stars The Constellations NOTE. It is hardly necessary to say that this chapter is to be treated by the teacher differently from the rest of the book. It is to be dealt with, not as recitation matter, but as field-work : to be taken up at different times during the course as the constellations make their appearance in the evening sky. For convenience of reference we add the following alphabetical list of the constellations described or mentioned in the chapter : ARTICLE AndrdmSda 35 Anser, see Vulpe"cula ... 69 Antinoiis, see Aqulla . . . 71 Antlia 62 Aquarius 78 Aqulla (not Aquila) . . . 71 Argo Navis 51 Aries 38 Auriga 41 Bootes 59 Camelopdrdalis 31 Cancer 52 Canes Venatici .... 58 Canis Major 49 Canis Minor 48 Capricornus 73 Cassi6p