ALBERT R. MANN LIBRARY New York Stath Colleges OF Agriculture and Home Economics Cornell University Cornell University Library QB 44.N6 Astronomy for everybody; a popular expos! 3 1924 003 160 540 The original of tliis book is in tine Cornell University Library. There are no known copyright restrictions in the United States on the use of the text. http://www.archive.org/details/cu31924003160540 Total Eclipse of the Sun of May 29, 1900. Photographed by the party o£ the Smithsonian Institution. Science for Everybody ASTRONOMY FOR EVERYBODY A Popular Exposition of the Wonders of the Heavens BY SIMON NEWCOMB, LL.D. Professor, U. S. N., retired FULLY ILLUSTRATED NEW YORK McCLURE, PHILLIPS & CO. 1907 Copyright, 19011, hy McCLURE, PHILLIPS & CO. t^c^7 id Published, November, 1902, N Fourth Impression Preface The present work grew out of articles contributed to McClure's Magazine a few years since on the Unsolved Problems of Astronomy, Total Eclipses of the Sun, and other subjects. The interest shown in these articles suggested an exposition of the main facts of astronomy in the same style. The result of the attempt is now submitted to the courteous consideration of the reader. The writer who attempts to set forth the facts of as- tronomy without any use of technical language finds him- self in the dilemma of being obliged either to convey only a very imperfect idea of the subject, or to enter upon explanations of force and motion which his reader may find tedious. In grappling with this difficulty the author has followed a middle course, trying to present the subject in such a way as to be intelligible and inter- esting to every reader, and entering into technical ex- planations only when necessary to the clear understand- ing of such matters as the measure of time, the changes of the seasons, the varying positions of the constella- tions, and the aspects of the planets. It is hoped that the reader who does not wish to master these subjects will find enough to interest him in the descriptions and illustrations of celestial scenery to which the bulk of the work is devoted. The author is indebted to Mr. Secretary Langley, of the Smithsonian Institution, for the use of the picture which forms the frontispiece. Simon Newcomb. Washington, October, 1902. Contents PART r. TEM CELESTIAL MOTIONS PAGE L A View of the Universe ; 3 What the universe is 6 A model of the xmiverse 7 n. Aspects op the Heavens 9 Apparent daily revolution of the stars 12 Changes ia the motions as we journey south 15 m. EEliATioN OP Time AND Longitude 19 Local time 21 Standard time 21 Where the day changes 23 rv. How THE Position op a Heavenly Body is Defined 25 Circles of the celestial sphere 27 V. The Annual Motion op the Eabth and its Eesttlts ... 31 The sun's apparent path in the sky 32 The ecliptic 33 The equinoxes and solstices 37 The seasons 39 Eelations between real and apparent motions summed up 39 The year and the precession of the equinoxes 41 PART IL ASTRONOMICAL INSTRUMENTS I. The Refracting Telescope 47 The lenses of a telescope 48 The image of a distant object 51 Power and defects of a telescope 53 Mounting of the telescope 55 The making of telescopes 59 Fraunhof er and Alvan Clark 61 viil CONTENTS PAGE n. The Eepmjcting Telescope 67 HE. The Photographio Telescope 71 IV. The Spectroscope 73 Nature and waTO length of light 74 The spectrum 75 How the stars are analysed 76 V. Other Astronomical Instrttments 79 The meridian circle and clock 80 How the position of stars are determined 81 PART III. TBE SUN, EARTH, AND MOON I. An Introdtjctort Glance at the SoixAB System 87 n. The Sun 91 General description 91 Botation of the sun , 93 Density and gravity 94 Spots on the sun 95 The Faculae 98 The prominences and chromosphere 99 How the sun is made up. 101 The source of the sun's heat. 103 m. The Earth 107 Measuring the earth 107 The earth's interior 108 Its gravity and density. . Ill Variations of latitude 114 The atmosphere 116 IV. The Moon 119 Distance of the moon 120 Eevolution and phases 130 The surface of the moon 123 Is there air or water on the moon S 127 Rotation of the moon 138 How the moon produces the tides 129 V, Eclipses op the Moon 133 The nodes of the moon's orbit. 134 Eclipse seasons 134 How an eclipse of the moon looks 136 CONTENTS ix PAGE TI. Eclipses op the Stm 139 Central, total, and annulaT eclipses. 140 Beanty of a total eclipse 141 Ancient eclipses 143 Prediction of eclipses 144 The sun's appendages 145 The corona 147 PART IV. THE PLANETS AND THEIR SATELLITES I. Orbits and Aspects op the Planets 151 Distances of the planets • 154 Bode's law 154 Kepler's laws 155 n. The Planet Meecuet 157 Surface and rotation of Mercury 159 Observations of Schroter, Herschel, Schiaparelli, and Lowell 160 Phases of Mercury 161 Transits of Mercury 162 m. The Planet Venus 167 The morning and evening star 167 Eotation of Venus 169 Atmosphere of Venus , 173 Has Venus a satellite ? 174 Transits of Venus 175 IV. The Plaitet Mars 177 Distance, dates of opposition, etc 178 Surface and rotation of Mars 179 The canals of Mars 180 Probable nature of the channels 184 The atmosphere of Mars 185 Supposed winter snowfall near the poles 187 The satellites of Mars 188 V. The Grottp op Minor Planets 191 Discovery of Ceres 191 Hunting asteroids 192 Orbits of the asteroids 194 Groupings of the orbits 195 The most curious of the asteroids 198 X CONTENTS tAGB TL JupiTEB AND rrs Satblutes 201 Aspect of Jupiter 201 Surface 202 Constitution 205 Rotation 206 Eesemblance of Jupiter to the sun 206 The sateUitea of Jupiter 208 Vn. Satubn and its System 213 Aspects of Saturn 313 Satellites of Saturn. 214 Varying aspects of Saturn's rings 215 What the rings are 219 System of Saturn's satellites 220 Physical constitution of Saturn 324 Vin. UBANtrS AND ITS SATELLITES 335 Discovery of Uranus 235 Old observations 236 Constitution of the planet 337 Its satellites 327 rX. Neptune and its Satelljtb 231 History of the discovery of Neptune 333 Satellite of Neptune 235 X. How the Heavens are Measured 237 Parallax 337 Measurement by the motion of light 239 Measurement by the sun's gravitation 340 Besults of measurements of the sun's distance 243 XI. Gravitation and the WEiOHiNa or the Planets 343 Accuracy of astronomical predictions based on the theory of gravitation 244 How the planets are weighed 346 PART V. COMETS AND METEOBIG BODIES I. Comets 355 Description of a comet 355 Orbits of comets 257 Halley's comet '. 360 Comets which have disappeared 362 CONTENTS xi PAGE Enoke's comet 264 Capture of comets by Jnpiter 265 Whence come comets ? 266 Brilliaut comets of OTir time 267 Nature of comets 274 n. Meteomc Bodies 277 Meteors 277 Canse of meteors 278 Meteoric showers 279 Comiection of comets and meteors 281 The zodiacal light 283 The impTUsion of light 286 PAET VI. THE FIXED 8TARS I. General Review 291 Stars and nebulas 293 Spectra of the stars 293 Density and heat of the stars 296 n. Aspect of the Skt 299 The MUky Way 299 Brightness of the stars 300 Number of stars 301 Colours 303 Collection into constellatious 303 m. Description of the OoNSTBij:.ATioNS 305 To find the sidereal time 306 The northern constellations 307 The autumnal constellations 309 The winter constellations 313 The spring constellations 316 The summer constellations 317 rv. The Distances op the Stabs. 321 V. The Motions op the Stars 325 VL Variable andCompounb Stars. ,,, = ..o....... ........ 329 List of Illustrations PAGE Total Eclipse of the Sun of May 29, 1900. Bhotograplied by the party of the Smithsonian Institution Frontispiece The Celestial Sphere as it appears to ns 13 The Northern Sky and the Pole Star .-. 16 Circles of the Celestial Sphere 27 The Sun Crossing the Equator about -March Twentieth 32 The Orbit of the Earth and the Zodiac 33 How the Obliquity of the Ecliptic Produces the Changes of Seasons 35 Apparent Motion of the Sun along the Ecliptic in Spring and Summer 36 Apparent Motion of the Sun from March till September 37 Precession of the Equinoxes 43 Section of the Object-glass of a Telescope 50 Axes on which a Telescope turns 57 Great Telescope of the Terkes Observatory 65 Section of a Newtonian Eeflecting Telescope 69 Wave Length of Light 74 Arrangement of the Colours of the Spectrum 75 A Meridian Instrument 80 Appearance of a Sun-spot 96 Frequency of Sun-spots in Different Latitudes on the Sun 97 Eevolution of the Moon Eound the Earth 131 Mountainous Surface of the Moon 134 Showing how the Moon would Move if it did not Eotate on its Axis 139 How the Moon Produces Two Tides in a Day 131 The Moon in the Shadow of the Earth 133 xiv LIST OF ILLUSTRATIONS PAGE PaBsage of the Moon through the Earth's Shadow 136 The Shadow of the Moon Thrown on the Earth during a Total Eclipse of the Sun 139 The Moon Passing Centrally over the Sun during an Annular Eclipse 140 Orbits of the Four Inner Planets 153 Conjunctions of Mercury with the Sun 158 Elongations of Mercury 159 Phases of Venus in Different Points of its Orbit 168 Effect of the Atmosphere of Venus during the Transit of 1883. . 173 Map of Mars and its Canals as drawn at the Lowell Observatory 181 Drawings of Lacus Solis on Mars, by Messrs. Campbell and Hussey 183 Separation of the Minor Planets into Groups 195 Distribution of the Orbits of the Minor Planets 196 Telescopic Views of Jupiter, one with the Shadow of a Satellite Crossing the Planet 304 Perpendicular View of the Kings of Saturn 316 Showing how the Direction of the Plane of Saturn's Rings re- mains Unchanged 317 Disappearance of the Bings of Saturn, according to Barnard, when seen edgewise 318 Orbits of Titan and Hyperion, showing their relation 333 Measure of the Distance of an Inaccessible Object by Triangnla- tion 337 Parabolic Orbit of a Comet 357 Donati's Comet, as drawn by G. P. Bond 368 Head of Donati's Comet, drawn by G. P. Bond 870 Great Comet of 1859, drawn by G. P. Bond 373 The Zodiacal Light in February and March 384 Ursa Major, or The Dipper 307 Ursa Minor 308 Cassiopeia 308 Lyra, the Harp ,,.,,.,,.,.,,, , . . . 311 LIST OF ILLUSTRATIONS xv PAGE The Hyades 313 The Pleiades, as seen with the naked eye 313 Telescopic View of the Pleiades, with Names of the Brighter Stais 314 Orion 316 The Northern Crown 317 Aqnila 318 Delphinns, the Dolphin 318 The Great Clnster of Hercules, photographed at the Lick Observ- atory 319 Soorpius, the Scorpion 320 Measnrement of the Parallax of a Star. 323 Axctnros and the Snironnding Stars la Constellation Bootes 328 PART I THE CELESTIAL MOTIONS I A VrEW or the Univeese Let us enter upon our subject by taking a general view of this universe in which we live, fancying ourselves looking at it from a point without its limits. Far away, indeed, is the point we must choose. To give a concep- tion of the distance, let us measure it by the motion of light. This agent, darting through 186,000 miles in every second, would make the circuit of the earth several times between two ticks of a watch. The standpoint which we choose will probably be well situated if we take it at a distance through which light would travel in 100,000 years. So far as we know, we should at this point find ourselves in utter darkness, a black and star- less sky surrounding us on all sides. But, in one direc- tion, we should see a large patch of feeble hght spread- ing over a considerable part of the heavens like a faint cloud or the first glimmer of a dawn. Possibly there might be other such patches in different directions, but of these we know nothing. The one which we have men- tioned, and which we call the universe, is that which we are to inspect. We therefore fly toward it — ^how fast we need not say. To reach it in a month we should have to go a million times as fast as light. As we approach, it continually spreads out over more of the black sky. 4 THE CELESTIAL MOTIONS which it at length half covers, the region behind us being still entirely black. Before reaching this stage we begin to see points of light glimmering here and there in the mass. Continu- ing our course, these points become more numerous, and seem to move past us and disappear behind us in the distance, while new ones continually come into view in front, as the passengers on a railway train see landscape and houses flit by them. These are stars, which, when we get well in among them, stud the whole heavens as we see them do at night. We might pass through the whole cloud at the enormous speed we have fancied, with- out seeing anything but stars and, perhaps, a few great nebulous masses of foggy light scattered here and there among them. But instead of doing this, let us select one particular star and slacken our speed to make a closer inspection of it. This one is rather a small star; but as we ap- proach it, it seems to our eyes to grow brighter. In time it shines like Venus. Then it casts a shadow; then we can read by its light ; then it begins to dazzle our eyes. It looks like a little sun. It is the Sun ! Let us get into a position which, compared with the dis- tances we have been travelling, is right alongside of the sun, though, expressed in our ordinary measure, it may be a thousand million miles away. Now, looking down and around us, we see eight star-like points scattered around the sun at different distances. If we watch them long enough we shall see them all in motion around the sun, completing their circuit in times ranging from three A VIEW OF THE UNIVERSE 5 months to more than 160 years. They move at very different distances ; the most distant is seventy times as far as the nearest. These star-Kke bodies are the planets. By careful examination we see that they differ from the stars in being opaque bodies, shining only by light borrowed from the sun. Let us pay one of them a visit. We select the third in order from the sun. Approaching it in a direction which we may call from above, that is to say from a direction at right angles to the line drawn from it to the sun, we see it grow larger and brighter as we get nearer. Wlien we get very near, we sec it looking like a half -moon — one hemisphere being in darkness and the other illuminated by the sun's rays. As we approach yet nearer, the illuminated part, always growing larger to our sight, assumes a mottled appearance. Still ex- panding, this appearance gradually resolves itself into oceans and continents, obscured o^'er perhaps half their surface by clouds. The surface upon which we are look- ing continually spreads out before us, filling more and more of the sky, until we see it to be a world. We land upon it, and here we are upon the earth. Thus, a point which was absolutely invisible while we were flying through the celestial spaces, which became a star when we got near the sun, and an opaque globe when yet nearer, now becomes the world on which we live. This imaginary flight makes known to us a capital fact of astronomy : The great mass of stars which stud the heavens at night are suns. To express the idea in 6 THE CELESTIAL MOTIONS another way, the sun is merely one of the stars. Com- pared with its fellows it is rather a small one, for we know of stars that emit thousands or even tens of thou- sands of times the light and heat of the sun. Measur- ing things simply by their intrinsic importance, there is nothing special to distinguish our sun from the hundreds of millions of its companions. Its importance to us and its comparative greatness in our eyes arise simply from the accident of our relation to it. The great universe of stars which we have described looks to us from the earth just as it looked to us during our imaginary flight through it. The stars which stud our sky are the same stars which we saw on our flight. The great difference between our view of the heavens and the view from a point in the starry distances is the prominent position occupied by the sun and planets. The former is so bright that during the daytime it com- pletely obliterates the stars. If we could cut off the sun's rays from any very wide region, we should see the stars around the sun in the daj'time as well as by night. These bodies surround us in all directions as if the earth were placed in the centre of the universe, as was sup- posed by the ancients. What the Universe Is We may connect what we have just learned about