Elements of astronomy ... with explanatory notes, and questions for examination

Front matter

I

SI

If li i If I

Preface

" The science of Astronomy, conversant as it is with the sublimest natural phenomena, has ever engaged the attention of mankind, even when, from century to century, scarcely any new revelation of the skies rewarded the labors of the astronomer.

But at the present time, when discovery crowds upon discovery, and the whole field of astronomical research has been wonderfully enlarged, it has suddenly become invested with the charms and freshness of a new science ; and all classes of society listen with wonder and delight to the recital of the lofty truths and amazing facts which it unfolds.

In attempting, therefore, with many honored names, to cause Astronomy to descend from the dignified seclusion of the observatory, that she may walk as a familiar guest amid the lesser temples of knowledge, no apology is required for the motive that prompts the task how much soever it may be needed on account of the execution.

In the present treatise the author has not sought to adapt its subject to the youthful mind, by curtailing the science of its fair proportions, and omitting every thing that requires patient and earnest study ; but he has aimed to preserve its great principles and facts in their integrity, and so to arrange, explain, and illustrate them, that they may stand out boldly defined, and be clear and intelligible to the honest and faithful student, — this is all that can be done for a pupil, if a science is to be taught in its completeness and not in parts.

IV PREFACE.

The hill of science will always be a hill. Inipedi ments and obstructions may be removed and the ascent rendered easier, but the hill cannot be leveled, it must be surmounted.

Several peculiarities are contfined in this text-book, which it is thought will be of material service to the pupil in obtaining a knowledge o/ ihe science. The most important of these we shall now briefly notice.

I. It is usual, in most text-books on this science, to explain many astronomical phenomena by the apparent, and not by the real motions of the celestial bodies. In this treatise the opposite course is taken, wherever practicable ; the explanations being based upon the real motions of the heavenly bodies. By pursuing this method, the subsequent acquisitions of the scholar are built upon the truth itself, and not upon what appears to be true.

II. The mode of ascertaining the distances and magnitudes of the heavenly bodies is so simplified that any student, who understands the rule of proportion, can readily comprehend it.

III. Scientific terms and expressions are explained by foot notes on the pages in which they occur ; and in these notes are likewise embodied such illustrations and information as tend to elucidate the text.

In the preparation of this manual, the author has had recourse to numerous standard works upon Astronomy, and has brought up the subject to the present time. For information respecting recent astronomical discoveries, he is especially indebted to the treatises of Sir John Herschel and Mr. J. Russel Hind.

Hartford, Feb. 19th, 1855.

Contents

Preface.

Page. 3

INTRODUCTORY CHAPTER.

Chapter II. The Horizon

Sensible horizon, 33

Rotional horizon, 34

Plane of the horizon not fixed

in space, 35

Zenith and Nadir, 36

Changing aspect of the heavens arising from the rotation of

theearth, 36

Why the stars appear to describe circles, 38

Why these circles differ in size,

Changing aspect of the heavens arising from change in latitude

Circle of perpetual apparition,. .

Circle of perpetual occultation,.

Latitude of any place equal to the elevation of the pole of

39 41

V]

CONTENTS.

Chapter III

ON THE MODE OF DETERMINING THE PLACE OF A HEAVENLY BODY. Puge.

Pa*

Celestial sphere, poles,

and meridians, 46

Equators, 48

Virtieal Circles 49

The position of a star how determined, 49

Azimuth, amplitude, altitude,

and zenith distance, 50

Declination and right ascension, 51

Ecliptic, ". 53

Latitude and longitude, 53

The signs, 54

Zodiac 54

Chapter IV. Of Refraction

Refraction, 55

Variation of refraction in respect to altitude, 56

The effect of refraction on the position of heavenly bodies, . 58

Its effect on their declination and right ascension, 58

Refraction influenced by the temperature and pressure of the atmosphere, 60

Of parallax, 60

AND PARALLAX.

Parallax — how measured, 62

Variation in parallax — effect of

altitude, 63

Horizontal parallax, 63

Effect of distance, 63

Effect of parallax upon the true

position of a heavenly body, . 64 Its effect on right ascension

and declination, 64

Parallax — its value, 65

CHAPTER Y.

OF THE MEASUREMENT OF TIME.

Transit instrument, 66

Time occupied by the earth in performing one rotation —

how determined, 68

Standard unit of time, 70

Of the sidereal and solar day, 70

Inequality in the length of the

solar days, .75

Modes of reckoning time, 77

Apparent time, 78

Mean solar time, 78

Astronomical time, 78

Equation of time, 79

Chapter VI. Of The Annual Motion Of The Earth

Sun's apparent motion in declination, 81

Sun's apparent motion in right ascension, 82

Sun's apparent path, 83

Sun's apparent motion in declination explained, 83

Sun's apparent motion in right ascension explained, 86

Direction of motion in space explained, 86

Tropical year defined, , .

Its length — how found, The calendar,

Chapter VII, Of The Year

Sothic period,

The Mexican year,.

Greo-orian rule, . . .

90 90 93

CONTENTS.

Vll

Chapter VIII. Of The Precession Of The Equinoxes, Change Of The Pole-star

AND NUTATION.

Of the precession of the equinoxes,

Sidereal year,

Change of the pole-star,

Effect of precession on the right ascension and longitude of the stars,

Its effect on the declination and latitude,

Terrestrial latitude constant, . .

Page.

96 91

Relative positions of the signs and constellation of the zodiac

variable, 99

Cause of the precession, .. . 100

Influence of the sun, 100

Influence of the moon and

planets, 101

Obliquity of the ecliptic affected by nutation, 103

Chapter IX. Of The Earth S Orbit

Sun's apparent diameter, 105

Anomalistic year, 107

Apparent angular motion, 108

Variation in the earth's orbitual

velocity, 109

Form of the earth's orbit ascertained by angular velocities,. 110

Product of the square of the distance into the angular velocity — constant, 110

Kepler's laws, 112

Extent of the earth's orbit, 112

How ascertained, 112

Chapter X. Of The Seasons

The seasons, 115

Spring, 118

Summer, 118

Autumn, 120

Winter, 120

Polar winter — effects of refraction, 121

Twilight and its influence, .... 1 22

The cause of the unequal distribution of heat over the surface of the globe, 123 '

The summer of the southern hemisphere not hotter than that of the northern, 124

ELLIPTICITY OF THE EARTH'S ORBIT, ITS EFFECT ON THE SEASONS 125

Chapter I. The Sun

Real diameter of the sun, 12S

Its size, 130

Quantity of matter in the sun,. 131 Weight of bodies at the sun's

surface, 131

Solar spots, 133

Their size and number, 133

Motion of the spots, 134

Rotation of the sun, 136

Inclination of the sun's equator

to the plane of the ecliptic,. . 139 Physical nature of the sun, ... 140 Temperature at the sun's surface, 142

VI 11

CONTENTS.

CHAPTER II. Page

The distance of the moon, 143

Diameter in miles, 144

Moon's phases, 146

From new moon to the first

quarter, 146

From the first quarter to full

moon, 147

From full moon to the third

quarter, 147

From the third quarter to new

moon, 147

What the phases prove, 150

Sidereal month, 150

Lunar month, 150

Physical aspects of the

moon, 152

Lunar mountains, 153

The heights measured, 154

Names and heights of the lunar

mountains, 156

Lunar craters, 157

Lunar volcanoes 159

Page.

Lunar atmosphere, 160

The bulk, mass, and density of

the moon, 161

Moon's orbit, 161

Its figure determined, 162

Plane of the moon's orbit, 162

The line of the nodes, 163

The line of the apsides, 164

Increased apparent size of the

moon when in the zenith, . . 165 The moon always turns the

same face toward the earth, . 166

Libration in longitude, 167

Libration in latitude, 168

Diurnal libration, 169

Length of the lunar day, 169

The appearance of the earth as

seen from the moon, 170

Acceleration of the moon's motion in her orbit 171

The moon's path in space,. . . . 172

Chapter III. Eclipses Of The Sun And Moon

Lunar eclipses, 173

Of the earth's shadow, 175

Extent of shadow traversed hy

the moon, 177

Of the penumbra, 177

Duration of a lunar eclipse, ... 178

Red light of the disk, 179

Earliest observations of lunar

eclipses, 180

Eclipses of the sun, 180

Form of a solar eclipse, 180

Shadow of the moon, 181

Altitude of the moon, — its effect

on eclipses, 182

Total eclipses of the sun, 184

Duration of a solar eclipse,,. . . 186 Solar and lunar eclipses — points

of difference, 186

Frequency of eclipses,. ....... 187

Quantity of an eclipse,. ....... 187

The period of the eclipses — the

Saros, 189

CHAPTER IV central forces and gravitation.

Of gravity, 192

Its variation, 193

Universal gravitation discovered, 194

Universal gravitation defined,. . 197

Chapter V The Planets

Their distances, 198

Kepler's law of distances, 199

Bode's law of distances, 200

Of their magnitudes, 200

Division of the planets, 202

Inferior planets, 202

CONTENTS.

IX

Page.

Mercury, 203

His solar distance, 203

Orbit — inclination of its plane,. 204

Size — apparent — real, 204

Periodic time, 205

Rotation on its axis, 205

Phases, 206

Transit of Mercury, 206

Splendor of Mercury, 207

Mass and density, 208

Ancient observations of Mercury, 209

Venus, 209

Distance and periodic time, . . . 209

Apparent diameter, 209

Real diameter, 209

Rotation, 210

Orbit, — Inclination of its plane

to that of the ecliptic, 211

Phases, 211

Splendor of Venus, 212

Mass and density, 214

Atmosphere of Venus, 214

Transit of Venus, 214

The Earth, 217

Superior planets, 217

Mars, 218

Distance — Orbit — Inclination of

the plane of the orbit, 218

Real and apparent diameter, . . .218

Phases, 219

Physical aspect — Atmosphere,. 220 Rotation, — Inclination of the

axis,— Ellipticity, 221

Density and mass, 221

Intensity of solar light, 222

THE ASTEROIDS, 222

Ceres, 224

Pallas, 225

Vesta, 226

Astrea 227

Flora, 229

Metes, 230

Hygeia, 230

Parthenope, 231

Victoria or Clio, 232

Egeria, 232

Irene, 233

Eunomia, 233

Page.

Psyche, 234

Thetis, 234

Melpomene, , 234

Fortuna, 235

Massalia, 235

Lutetia, 235

Calliope 235

Thalia, .' 235

Themis, 236

Phoccea, 236

Proserpine, 236

Euterpe, 236

Bellona, 236

Amphitrite, 237

Urania, 237

Euphrosyne, 237

Pomona, 237

polymnia, » 237

The system pursued in naming

new planets, 238

Olber's theory, 238

Jupiter, 239

Periodic time — distance 239

Diameter — apparent — real . . . 239

Ellipticity— bulk, 239

Physical aspect of Jupiter —

Belts, 239

Rotation, 240

Velocity of rotation, 241

Mass— Density, 241

Satellites of Jupiter — their discovery, 241

Their magnitudes, diameters, distances, and periods of

revolution, 242

Kepler's laws — applicable to the

satellites — their rotation, .... 242 Transits and eclipses of the

satellites, 243

Velocity of light, 244

Saturn, 245

Distance — Periodical revolution

and inclination of orbit, 246

Form — diameter, 246

Bulk, diameter, intensity of

light, 246

Physical aspect — atmosphere,. . 246 Rotation and inclination of its

axis, 247

Ring of Saturn — its discovery, 247

Form — constitution,. 248

CONTENTS.

Page.

Rotation — Position — Inclination

to the ecliptic, 248

Phases of the ring, 249

Vanishing of the ring — Three

^auses, 250

Divisions of the ring, 251

Dimensions of the rings, 252

Satellites of Saturn, 253

Mimas, 253

Enceladus, 253

Tethys, 253

Dione, 254

Rhea, 254

Titan, 254

Hyperion, 254

Japetus, 255

Diameter of the satellites, .... 255 Ancient observations of Saturn, 255

Uranus or Herschel, 256

Aspect, — Diameter, — Mass, —

Density, 256

Page.

Rotation, 256

Distance, — Inclination of Orbit,

— Periodic time, 257

Satellites of Uranus, 257

Intensity of light, 258

Neptune,. 258

History of its discovery, 258

Name, — Diameter, — Mass, —

Density, 259

Orbit, — Inclination of orbit, — Distance, — Periodic time, . . 259

Intensity of light, 259

Has Neptune a ring, 259

The satellite of Neptune, 260

Real and apparent motions

of the planets, 260

Causes of the apparent motions, 261

Apparent motions explained,.. 261 The planets at times stationary, 262

Chapter VI. Comets

Their constitution, 262

Number of comets, 263

Splendor and size, 265

Velocity, 265

Temperature, 266

Comets shine by reflected light, 266 Orbits, — Perihelion distances, . 267 Inclination of the orbits, — Direction of motion, 268

Elements,— Identity, 268

Halley's comet, 269

Encke's comet, 270

Biela's comet, 270

Faye's comet, 271

De Vico's comet, 271

Comets of 1843, 272

Physical nature of comets, 273

Collision with the earth, 273

CHAPTER Vn.

Tides defined, 274

Cause of the tides, 276

Why high tides occur on opposite sides of the globe, .... 276 Why low tides occur on opposite sides*)f the globe, .... 277

Solar influence, 278

Spring and neap tides, 279

Time of the tide, 281

Priming or lagging of the tide, 282 Effect of declination on the

height of the tide,

Actual heights of the tide,

Derivative tides, .....' 284

No tides except on the ocean,

and on seas connected with

it,

282 283

CONTENTS.

Chapter VIII. Terrestrial Longitude

Pa<je.

Longitude ascertained by four

methods, 28G

By chronometers, 286

By eclipses,

By the electric telegraph, By the lunar method,

XI

Page.

287 288 288

Chapter I. Of The Fixed Stars In General And The Constellations

The fixed stars,

Magnitudes,

Number of stars, .

Distance of the fixed stars, ....

Parallax and distance of Alpha

Centauri,

Parallax and distance of 61

Cygni,

Nature and intrinsic splendor

The constellations, .

Their use,

290 290 291 292

295 295

The stars in the constellations,

— How designated, 296

Principal constellations, 296

Constellations north of the

zodiac, 296

Constellations of the zodiac, . . . 297 Constellations south of the

zodiac, 297

How to study the heavens, .... 298

The celestial globe, 298

Star maps, ^300