Astronomy in a Nutshell

Part I—the Celestial Sphere.

PAGES

Definition of Astronomy—Fundamental Law of the 3-64 Stars—Relations of the Earth to the Universe—Ordinary Appearance of the Sky—The Horizon, the Zenith, and the Meridian—Locating the Stars—Altitude and Azimuth—Circular or Angular Measure—Altitude Circles and Vertical Circles—The Apparent Motion of the Heavens—The North Star and Phenomena Connected with it—Revolution of the Stars round the Pole—Locating the Stars on the Celestial Sphere—Astronomical Equivalents of Latitude and Longitude—Parallels of Declination and Hour Circles—“The Greenwich of the Sky”—Effects Produced by Changing the Observer's Place on the Earth—The Parallel Sphere, the Right Sphere, and the Oblique Sphere—The Astronomical Clock—The Ecliptic—Apparent Annual Revolution of the Sun round the Earth—Inclination of the Ecliptic to the Equator: its Cause and its Effects—The Equinoxes—Importance of the Vernal Equinox—The Equinoctial Colure—The Solstices—Poles of the Ecliptic—Celestial Latitude and Longitude—The Zodiac—The Precession of the Equinoxes: its Cause and Effects—Revolution of the Celestial Poles—Past and Future Pole Stars

Part Ii—the Earth.

PAGES

Nature, Shape, and Size of the Earth—The Polar 67-123 Compression and Equatorial Protuberance, and their Cause—The Attraction of Gravitation—The Mass of the Earth: how Found—How the Earth Holds the Moon, and the Sun the Planets—The Tides—How the Moon and Sun Produce Tides—Spring Tides and Neap Tides—The Atmosphere—The Law and Effects of Refraction—Dip of the Horizon—The Aberration of Light—Time: how Measured—Sidereal, Apparent Solar, and Mean Solar Time—The Clock and the Sun—Day and Night—Where the Days Begin—The Seasons—Effects of the Varying Declination of the Sun—Polar and Equatorial Day and Night—The Tropics and the Polar Circles—Inequality of Length of the Seasons—When the Seasons in the Two Hemispheres will be Reversed—The Calendar, the Year, and the Month—Reformations of the Calendar—Different Measures of the Month

Part Iii—the Solar System.

PAGES

The Sun—Distance, Size, and Condition of the 127-215 Sun—Temperature of the Sun—Solar Heat on the Earth, and its Mechanical Equivalent—Peculiar Rotation of the Sun—Sun-spots, their Appearance and Probable Cause—Faculæ—The Photosphere—Solar Prominences—Explosive Prominences—The Solar Corona—Parallax, and the Measure of Distances—Spectroscopic Analysis—How the Elements in the Sun Reveal their Presence—List of the Principal Solar Elements—The Moon—Origin of the Moon—Appearance of its Surface—Gravity on the Moon—The Phases of the Moon—Causes of the Absence of a Lunar Atmosphere—Eclipses—How the Moon Causes Eclipses of the Sun—The Laws Governing Eclipses—The Shadow during a Solar Eclipse—Eclipses of the Moon—Number of Eclipses in a Year—The Saros—The Planets—Kepler's Laws of Planetary Motion—Mercury—Venus—Mars, and its So-called Canals—Theories about Mars—Jupiter, its Belts and its Satellites—The "Great Red Spot”—Saturn, its Rings and its Satellites—Composition of the Rings—Uranus and Neptune—Comets, and the Laws of their Motion—Composition of Comets—The Pressure of Light and its Connection with Comets' Tails—Breaking up of Comets—Meteors and their Relations to Comets—The November Meteors and Other Celebrated Showers—Meteorites or Bolides which Fall upon the Earth—The Question of their Origin

Part Iv—the Fixed Stars.

PAGES

Division of the Stars into Magnitudes—Division of 219-257 the Stars according to their Spectra—Stars Larger and Smaller than the Sun—The Distances of the Stars—Variable Stars—Double and Binary Stars—Spectroscopic Binaries and how they are Discovered—Proper Motions of the Stars—Number of the Stars—New, or Temporary Stars—The Milky Way—The Nebulæ—The Two Kinds of Nebulæ—Spiral Nebulæ—The Nebular Hypothesis—Applications of Photography to Stars and Nebulæ—The Constellations—How to Learn the Constellations—Their Antiquity—Description of the Principal Constellations Visible from the Northern Hemisphere at Various Times of the Year

INDEX 259

------------------------------------------------------------------------

ILLUSTRATIONS

PAGE

THE LUNAR “CRATER” COPERNICUS Frontispiece

PHOTOGRAPH OF SOUTH POLAR 8 REGION OF THE MOON

THE MOON NEAR THE “CRATER” 20 TYCHO

DRAWING OF JUPITER 28

DRAWING OF JUPITER 28

JUPITER 38

SATURN 46

SATURN 46

THE MILKY WAY ABOUT CHI CYGNI 58

THE GREAT SOUTHERN 64 STAR-CLUSTER IN CENTAURI

PHOTOGRAPH OF A GROUP OF 76 SUN-SPOTS

POLAR STREAMERS OF THE SUN, 89 ECLIPSE OF 1889

SOLAR CORONA AT THE ECLIPSE OF 89 1871

MOREHOUSE'S COMET, OCTOBER 15, 96 1908

MOREHOUSE'S COMET, NOVEMBER 96 15, 1908

HEAD OF THE GREAT COMET OF 105 1861

HALLEY'S COMET, MAY 5, 1910 105

THE SIX-TAILED COMET OF 1744 112

SPIRAL NEBULA IN URSA MAJOR (M 124 101)

THE WHIRLPOOL NEBULA IN CANES 124 VENATICI

TRESS NEBULA (N. G. C. 6992) 132 IN CYGNUS

THE GREAT ANDROMEDA NEBULA 140

SPIRAL NEBULA IN CEPHEUS (H. 154 IV. 76)

NEBULOUS GROUNDWORK IN TAURUS 154

NEBULA IN SAGITTARIUS (M. 8) 162

THE GREAT NEBULA IN ORION 180

PHOTOGRAPHS OF MARS 200

SCHIAPARRELLI’S CHART OF 220 MARTIAN “CANALS”

ILLUSTRATIONS IN THE TEXT

THE RATIONAL AND THE SENSIBLE 12 HORIZON

ALTITUDE AND AZIMUTH 14

RIGHT ASCENSION AND 35 DECLINATION

THE ECLIPTIC AND CELESTIAL 51 LATITUDE AND LONGITUDE

HOW THE EARTH CONTROLS THE 75 MOON

THE TIDAL FORCE OF THE MOON 79

REFRACTION 85

DIP OF THE HORIZON 87

SIDEREAL AND SOLAR TIME 93

THE CHANGE OF DAY 101

THE SEASONS 107

PARALLAX OF THE MOON 139

PARALLAX OF THE SUN FROM 141 TRANSIT OF VENUS

SPECTRUM ANALYSIS 147

THE PHASES OF THE MOON 160

ORBITS OF MARS AND THE EARTH 183

ELLIPSE, PARABOLA, AND 203 HYPERBOLA

THE NORTH CIRCUMPOLAR STARS 244

KEY TO NORTH CIRCUMPOLAR STARS 245

------------------------------------------------------------------------

PART I.. The Celestial Sphere.

THE CELESTIAL SPHERE.

------------------------------------------------------------------------

PART I.

THE CELESTIAL SPHERE.

=1. Definition of Astronomy.= Astronomy has to do with the earth, sun, moon, planets, comets, meteors, stars, and nebulæ; in other words, with the universe, or “the aggregate of existing things.” It is the most ancient of all sciences. The derivation of the name from two Greek words, aster, “star,” and nomos, “law,” indicates its nature. It deals with the law of the stars—the word “star” being understood, in its widest signification, as including every heavenly body of whatever kind. The earth itself is such a body. Since we happen to live on the earth, it becomes our standpoint in space, from which we look out at the others. But, if we lived on some other planet, we would see the earth as a distant body in the sky, just as we now see Jupiter or Mars.

Astronomy teaches us that everything in the universe, from the sun and the moon to the most remote star or the most extraordinary nebula, is related to the earth. All are made of similar elementary substances and all obey similar physical laws. The same substance which is a solid upon the earth may be a gas or a vapour in the sun, but that does not alter its essential nature. Iron appears in the sun in the form of a hot vapour, but fundamentally it is the same substance which exists on the earth as a hard, tough, and heavy metal. Its different states depend upon the temperature to which it is subjected. The earth is a cool body, while the sun is an intensely hot one; consequently iron is solid on the earth and vaporous in the sun, just as in winter water is solid ice on the surface of a pond and steamy vapour over the boiler in the kitchen. Even on the earth we can make iron liquid in a blast furnace, and with the still greater temperatures obtainable in a laboratory we can turn it into vapour, thus reducing it to something like the state in which it regularly exists in the sun.

This fact, that the entire universe is made up of similar substances, differing only in state according to the local circumstances affecting them, is the greatest thing that astronomy has to tell us. It may be regarded as the fundamental law of the stars.

=2. The Situation of the Earth in the Heavens.= One of the greatest triumphs of human intelligence is the discovery of the real place which the earth occupies in the universe. This discovery has been made in spite of the most deceptive appearances. If we accepted the sole evidence of our eyes, as men once did, we could only conclude that the earth was the centre of the universe. In the daytime we see the sun apparently moving through the sky from east to west, as if it were travelling in a circle round the earth, overhead by day and underfoot at night. In the night-time, we see the stars apparently travelling round the earth in the same way as the sun. The fact is, that all of them are virtually motionless with regard to the earth, and their apparent movements through the sky are produced by the earth's rotation on its axis. The earth turns round on itself once every twenty-four hours, like a spinning ball. Imagine a fly on a rotating school globe; the whole room would appear to the fly to be revolving round it as the heavens appear to revolve round the earth. It would have to be a very intelligent insect to correct the deceptive evidence of its eyes.

The actual facts, revealed by many centuries of observation and reasoning, are that the earth is a rotating globe, turning once on its axis every twenty-four hours and revolving once round the sun every three hundred and sixty-five days. The sun is also a globe, 1,300,000 times larger than the earth, but so hot that it glows with intense brilliance, while the substances of which it consists are kept in a gaseous or vaporous state. Besides the earth there are seven other principal globes, or planets, which revolve round the sun, at various distances and in various periods, and, in addition to these, there are hundreds of smaller bodies, called asteroids or small planets. There are also many singular bodies called comets, and swarms of still smaller ones called meteors, which likewise revolve round the sun.

Continue reading Astronomy in a Nutshell free in the Wunder reader →