^ ELEMENTS ASTRONOMY: DESIGNED AS A TEXT-BOOK uabemws, Btminwcus, anb families. BY Rev. JOHN DAVIS, A.M. FORMERLY PROFESSOR OF MATHEMATICS AND ASTRONOMY IN ALLEGHENY CITY COLLEGE, AND LATE PRINCIPAL OF THE ACADEMY OF SCIENCE, ALLEGHENY CITY, PA. PHILADELPHIA: PRINTED BY SHERMAN & CO.^ S. W. COB. OF SEVENTH AND CHERRY STREETS. 1868. Entered, according to Act of Congress, in the year 1867, by JOHN DAVIS, in the Clerk's OlBce of the District Court of the United States for the Western District of Pennsylvania. STEREOTYPED BY MACKELLAR, SMITHS & JORDAN, PHILADELPHIA. CAXTON PRESS OF SHERMAN & CO., PHILADELPHIA- PREFACE. This work is designed to fill a vacuum in academies, seminaries, and families. With the advancement of science there should be a corresponding advancement in the facilities for acquiring a knowledge of it. To economize time and expense in this department is of as much importance to the student as frugality and in- dustry are to the success of the manufacturer or the mechanic. Impressed with the importance of these facts, and having a desire to aid in the general diffusion of useful knowledge by giving them some practical form, this work has been prepared. Its language is level to the comprehension of the youthful mind, and by an easy and familiar method it illustrates and explains all of the principal topics that are contained in the science of astronomy. It treats first of the sun and those heavenly bodies with which we are by observation most familiar, and advances consecutively in the investigation of other worlds and systems which the telescope has revealed to our view. Thus to enhance the interest and value of this work to the student and casual reader, nearly all of the topics that it contains are fully illustrated by engravings prepared expressly for that purpose. And 4 PREFACE. to promote this object still further, and impart know- ledge in the most impressive manner by a sensible de- monstration of the arrangement, relations, and various motions of the component parts of the planetary system, the Planetelles, Heliotellus, and Lunatellus have been invented by the author and put into public use. Each of these instruments is invaluable in acquiring a know- ledge of astronomy. With these facilities for illus- tration, which address themselves to the mind through the eye, and the plainness and simplicity which charac- terize this entire work, the author would respectfully present it to a generous public, trusting that it may be the means of disseminating a general knowledge of that ennobling science on which it treats CONTENTS. PAGK Planetelles 11 Hbliotellus 13 lunatellus 15 Section I.—History of Astronomy 17 II.—The Planetary or Solar System 19 III.—The Sun 23 IV.—Sun's Atmosphere 24 v.—Solar Spots 25 VI.—Motions of the Sun 26 VII.—Mercury 29 VIIL—Venus 33 IX.—Transits, Conjunctions, and Motions of Mercury and Venus 34 X.—The Earth,—its Annual Motion 36 XL—Daily Motion of the Earth 38 XIL—The Moon 40 XIIL—Phases of the Moon 41 XIV.—Solar and Lunar Eclipses 42 XV.—Lunar Eclipses 45 XVI.—Lunar Influences 45 XVII.—Lunar Atmosphere 48 XVIIL—Lunar Surface 50 XIX.—Mars 54 XX.—The Asteroids 55 XXL—Jupiter 58 XXII.—Jupiter's Moons 60 XXIIL—Saturn and his Kings 62 XXIV.—Saturn and his Moons 66 XXV.—Uranus, Herschel, or Georgium Sidus 68 1* 5 6 CONTENTS. PAGE Section XXVI.—Satellites of Uranus 69 XXVIL—Neptune 70 XXVIIL—Satellites of Neptune 71 XXIX.—Light and Heat on Neptune 72 XXX.—The Earth—one of a Class 74 XXXL—Planets inhabited 75 XXXII.—General Configuration of the Heavenly Bodies 77 XXXIIL—Specific Form of the Earth 79 XXXIV.—The Planets—Oblate Spheroids 81 XXXV.—Position of the Solar System in Relation to the Starry Heavens 82 XXXVI.—Motions of the Planets around the Sun 85 XXXVIL—Centrifugal and Centripetal Forces 87 • XXXVII I.—Motion of the Planets on their Axes 90 XXXIX.—Weight of Objects on the Sun and Planets 91 XL.—Earth's Atmosphere 93 XLL—Refraction 96 XLIL—Twilight 99 XLIII.—Aurora Borealis 101 XLIV.—Shooting Stars 105 XLV.—Meteorites, Aerolites, and Fireballs 108 XLVL—Zodiacal Light 112 XLVIL—Light 115 XLVIIL—Colors of Light 119 XLIX.—Refracting Telescope 123 L.—Reflecting Telescope 125 LI.—Comets—Their Early History 127 LIL—Encke's Comet 129 LIII.—Biela's Comet 132 LIV.—Halley's Comet 135 LV.—The Comet of 1744 137 LVL—The Comet of 1843 140 LVII.—General Form and Appearance of Comets 142 LVIIL—Gravity of Comats 144 CONTENTS. 7 PAQB Section LIX.—Karity of Comets 145 LX.—Nature of Comets 148 LXI.—Comets shine by Eeflected Light 149 LXII.—Anomalous Aspect of Comets 152 LXIII.—Tails of Comets—How formed 154 LXIV.—Superstitious Notions concerning Comets 157 LXV.—Distance to Fixed Stars 159 LXVI.—Classification of Fixed Stars 161 LXVII.—Stars have no Sensible Discs 164 LXVIIL—Keal Magnitude of Stars 165 LXIX.—Method of computing Distances to Fixed Stars by Trigonometry 167 LXX.—Telescopic Method of computing Distances of Heavenly Bodies 168 LXXI.—The Stars are Self-Luminous Bodies 171 LXXTL—Color of Fixed Stars 174 LXXIIL—New Temporary Stars 175 LXXIV.—New Permanen t and Periodical Stars 177 LXXV.—Eelations of Multiple Stars 179 LXXVL—Stars Optically United 179 LXXVIL—Stars Physically United 181 LXXVIIL—Clusters of Stars 183 LXXIX.-Nebulffi 186 LXXX.—The Milky Way 189 LXXXL—Magellan Clouds 190 LXXXII.—Nebulse Proper 191 LXXXIIL—Annular Nebulse 193 LXXXIV.—Stellar Nebulse, and Nebulous Stars 195 LXXXV.—Planetary Nebulae 197 LXXXVI.—Globular Nebula or Clusters 199 LXXXVIL—Spiral Nebula 201 LXXXVIII.—Structure and Harmony of the Universe 203 Definitions of Astronomical Terms and Phrases 333 CONTENTS. PART II. PAGI Circles of the Sphere 207 Elements of Planets 209 Elements of Comets 209 Characters of Planets 209 Aspects of Planets 209 Section LXXXIX—The Visible or Sensible Horizon 211 XC—The Rational Horizon 213 XCL—Zenith and Nadir 214 XCII.—Arrangement of the Planets and the Planes of their Orbits 214 XCIII.—Planes of the Orbits of Asteroids and Comets 217 XCIV.—The Three Great Laws discovered by Kepler.... 218 XCV—Orbits of the Planets 222 XCVI.—Characteristic Points of the Orbits 224 XCVII.—Equinoxes and Solstices 225 XCVIII.—Precession of the Equinoxes 228 XCIX.—Nutation 229 C—Lunar Orbit and Eclipses 281 CI.—Method of Finding the Distance to the Moon, also to the Sun, and from the Sun to the Planets 234 CII.—Method of Finding the Magnitude of the Moon, Sun, and Planets 236 CIIL—Seasons on the Earth 238 CIV.—Seasons of the Planets 242 CV.—Divisions of Time 247 CVI.—Solar Day 24f) CVIL—Equation of Time 252 CVIII.—Tropical, Civil, and Sidereal Year 254 CIX.—The Calendar 257 ex.—Calendar, Synodical, and Sidereal Months 261 CXI.—Tides 263 CXIL—Spring Tides 267 CXIII.—Neap Tides 269 CXIV.—Height of the Tides 270 CXV.—Parallax 273 CXVI.—Proper Motion of the Stars 276 CONTENTS. y PART III. UBANOGRAPHY. PAQB Names and Characters of the Signs op the Zodiac 282 Prominent CoNSTEiii^ATioNS in both Hemispheres: Section CXVII.—Ceplieus, visible in November 283 CXVIII.—Cassiopeia, visible in November 284 CXIX.—Andromeda, visible in November 286 CXX.—Pisces, visible in November 287 CXXI.—Perseus and Head of Medusa, visible in November 289 CXXIL—Aries, visible in December 290 CXXIII—Cetus, visible in December 292 CXXIV.—Auriga, visible in January 293 CXXV.—Taurus, visible in January 295 CXXVI.—Orion, visible in January 296 OXXVIL—Eridanus, visible in January 298 CXXVIIL—Gemini, visible in February 299 CXXIX.—Canis Minor et Monoceros, visible in February 801 CXXX.—Canis Major, visible in February 802 CXXXL—Cancer, visible in Marcli 304 CXXXIL—Argo Navis, visible in March 305 CXXXIII.—Ursa Major, visible in April 307 CXXXIV.—Leo, visible in April 308 CXXXV.—Virgo, visible in May 310 CXXXVL—Centaurus et Crux, visible in May 311 CXXXVIL—Libra, visible in June 313 CXXXVIIL—Bootes et Canis Venatici, visible in June 314 CXXXIX—Ursa Minor, visible in June 316 CXL.—Scorpio, visible in July 317 CXLL—Serpentarius et Serpens, visible in July 319 CXLIL—Hercules, visible in July 820 CXLIIL—Sagittarius, visible in August - 322 CXLIV.—Capricornus, visible in September 323 CXLV.—Cygnus, visible in September , 325 CXLVI.—Aquarius, visible in October 826 CXLVII.—Pegasus, visible in October 328 consteiiliations of minor importance in both hemispheres... 330 Definitions of Astronomical Terms and Phrases 333 ; PLANETELLES. The Planetelles, which is represented by Fig. 1 on the opposite page, was invented by the author of this work, and has been in the course of construction for more than twenty years. Its first conception originated in the conviction that if the solar system, constituted as it is of many parts, and all of these parts having various motions, could be represented by an artificial system like itself, exhibit- ing the same phenomena in the same order, aid of incalculable value would be furnished in the acquisition of knowledge. With the view of rendering this idea practical, the construction of a machine for that purpose was commenced, and pursued, till the one which is here represented was completed. This instrument represents the motion of the sun on his axis, the relative annual motions of the eight primary planets, also their relative diurnal motions, and the various motions of the moon and of all the satellites around their primaries in different periods. It shows the relations existing be- tween the sun and all the primary planets, and their satellites and each other, and is designed to illustrate, also, the phenomena result- ing therefrom. It presents to the eye eighty different motions of these bodies, and illustrates the succession of day and night on each of the primary planets, the inclination of their axes to the planes of their orbits, the change of their seasons, the amount of change in the sun's declination in relation to each, and his rising and setting on each, the different lengths of their days and nights, the difference in the length of their seasons, their conjunctions, and the phases of Mercury and Venus, the retrogression of the moon's nodes, and the length of lunar days and nights, the solar and lunar eclipses, and the changes, phases, fulling, and eclipses, of all the satellites.