Elements of astronomy ...

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Preface

The science of Astronomy, conversant as it is with tt»«» Bublimest 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 jrssent 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.

K

IV PREFACE.

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

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

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 Hersohel and Mr. J Russel Hind.

Hartyord, Feb. 19 th, 1855

PREFACE

TO THE LAST REVISED EDITION.

During the last few years the progress of Astronomy has been very rapid ; for the number of known planetoids has been greatly increased, and a new value assigned to the solar parallax, which involves changes in the elements of the heavenly bodies running throughout the Solar System, and extending even to the fixed stars.

Within this period is also embraced the wondrous discoveries due to the spectrum analysis, which have revealed to us, to some extent, not only the physical nature of the sun and planets, but also that of comets, fixed stars, nebulae, and meteoric bodies.

It has been the aim of the author in this edition, to bring the work fully up to the present (1870) state of the science. The chapter on the sun has been entirely rewritten, and many other changes made which the advance of Astronomy demanded. New subjects have also been added, as for instance the nature of Meteorites and Shooting-stars, a description of the principal Constellations, and a History of Astronomy. Hartford, Conn., July, 1870.

Contents

I'age.

Preface,.. « 2

Preface to the last revised edition, 4

INTRODUCTORY CHAPTER.

Astronomy defined, 13

Interior Planets, 14

Asteroids, 14

Exterior Planets, 14

Symbols of the Planets 14

Mode of conducting astronomical investigations, 15

EXPLANATORY CHAPTER.

Angle, 17

Right angle,. 18

Triangles,. 19

Similar triangles, 19

A plane surface,. 20

A plane figure, 20 j

An ellipse 20

To construct an ellipse, 21

Eccentricity , 21

A sphere, 22

Poles of a circle of a sphere, 22

Part First. The Earth In Its Relation To Other Heavenly Bodies

ClIAPTEli I.

ITS FORM, SIZE, ROTATION, AND DENSITY.

The form of the earth, 24

Its size, 26

Its rotation, 29

Deviation of falling bodies from a

vertical line, 31

Variation in the weight of bodies,. . . 32

Density 33

Chapter II. The Horizon

Sensible horizon, 33

Rational 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 the earth, 36 Why the stars appear to describe circles...... 38

Why these circles differ in size, 38

Changing aspect of the heavens arising from change in latitude, 39

Circle of perpetual apparition, 41

Circle of perpetuaUoccultation,. ... 42 Latitude of any place equal to the elevation of the pole of the heavens, 45

VI

CONTENTS.

ON'

CHAPTER in.

MODE OF DETERMINING THE PLACE OF A HEAVENLY BODY.

Celestial sphere, poles, axes, and

meridians, 46

Equators, 48

Vertical circles, 49

The position of a star how determined, 49

Page.

Azimuth, amplitude, altitude, and

zenith distance, 50

Declination and right ascension,. ... 51

Ecliptic, 53

Latitude and longitude, 63

The signs, 54

Zodiac, 54

Chapter IV. Of Refraction And Parallax

Refraction, 55

Variation of refraction in respect to altitude, 66

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

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

Transit instrument,

Time occupied by the earth in performing one rotation — how determined,

Standard unit of time,

Of the sidereal and solar day,

Inequality in the length of the solar days,

Chapter V. Of The Measurement Of Time

Modes of reckoning time, 77

Apparent time, 78

Mean solar time, 78

Astronomical time, 78

Equation of time, 79

Astronomical clock — sidereal time —

error and rate, 80

Right ascension — how determined,. . 81

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,

Sun's apparent motion in right ascension explained,

Direction of motion in space explained,

S3

Chapter VII. Of The Year

Tropical year defined , 87

Its length — how found, 87

The calendar, 89

Sothic period, 90

The Mexican year, 90

The christian calendar— Julian correction, 91

Gregorian correction 91

Gregorian rule,. 93

Chapter VIII

OF THE PRECESSION OF THE EQUINOXES, — CHANGE OF THE POLE-STAR, AND NUTATION.

Of the precession of the equinoxes , . . 93

Sidereal year, 96

Change of the pole-star, 97

Effect of precession on the right ascension and longitude of the stars, 98

CONTENTS.

Vll

Page.

Its effect on the declination and latitude, 98

Terrestrial latitude constant, 98

Relative positions of the signs and constellation of the zodiac variable, 99

Cause op the precession, 100

Page.

Influence of the sun, 100

Influence of the moon and planets,. 101

Nutation, 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 orbital 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. Op The Seasons

The seasons, 115

Spring, 118

Summer, 118

Autumn, 120

Winter, 120

Polar winter — effects of refraction,. 121

Twilight and its influence, 122

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

CHAPTER I. Real diameter of the sun

.. 126

.. 127

Mass and Density, 128

Force of gravity at the sun, 128

Solar surface— general appearance , . 128

Nature of the spots, 128

Their extent, number, and periodicity, 129

Their position, 130

Apparent motion of the spots, 131

Their proper motion, 132

Faculae, 133

Pores j granules, and willow leaves,. 133

Rotation of the sun on it3 axis, 131

Inclination of the sun's equator to

the plane of the ecliptic, 137

SUN.

Nature of the sun, 138

Spectrum analysis, 138

Spectra of flames containing metallic

and other vapors— bright lines,. . 138 Experiments of Bunsen and Kirch-

Dark lines — how caused, 139

Metals found in the sun, 140

Physical constitution of the sun,. . 140

Photosphere, 141

Exterior atmosphere or chromosphere,. . : 141

Cause of the spots, 142

Nature of the faculae,. 142

Solar disturbances — their influence

on the magnetism of the earth,. . 143

Chapter II. The Moon

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, 14"

From full moon to the third quarter 147

From the third quarter to new

moon, 147

What the phases prove 150

Sidereal month, 150

Physical aspects of the moon,. . . , 152

Lunar mountains, 153

The heights measured, , . 164

Vlll

CONTENTS.

Page.

Names and heights of the lunar

mountains, 156

Lunar craters, 157

Lunar volcanoes, 159

Lunar atmosphere, 160

Lunar heat, 161

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

Page Increased apparent si* 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 by the

moon, 177

Of the penumbra, 177

Duration of a lunar eclipse, 173

Red light of the disk, 179

Earliest observations of lunar eclipses, 180

Eclipses of the sun, 180

Porm 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 I Universal gravitation discovered,. . . 194

Its variation, 193 | Universal gravitation defined, 197

Chapter V

the planets.

Table of the elements of the planets, 193

Their distances, 198

Kepler's law of distances, 200

Bode's law of distances, 201

Of their magnitudes, 201

Division of the planets with reference

to the earth 202

Inferior planets, 202

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

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

Value of the solar parallax, 217

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

Juno, 225

Vesta, 226

Later discoveries, 228

Peculiarities of the Asteroids, 229

Their size, 229

Obliquity and eccentricity 229

CONTENTS.

IX

Page.

Orbits 229

Probable number of the Asteroids,. . 229

Origin of the Asteroids, 230

Jupiter, 231

Periodic time — distance, 231

Diameter — apparent — real, 231

Ellipticity— bulk, 231

Physical aspect of Jupiter— Belts, . . 231

Rotation, 232

Velocity or rotation 233

Mass— Density 233

Satellites of Jupiter— their discovery, 233 Their magnitudes, diameters, distances, and periods of revolution,. 234 Kepler's laws— applicable to the satellites — their rotation, 234

Transits and eclipses of the satellites, 235

Velocity of light, 236

Saturn 287

Distance — Periodical revolution and

inclination of orbit, 238

Form— diameter, 238

Bulk, diameter, intensity of light, . 238

Physical aspect — atmosphere, 238

Rotation and inclination of its axis,. 239

Ring of Saturn— its discovery, 239

Form — constitution, 240

Rotation — Position— Inclination to

the ecliptic,. 240

Phases of the ring, 241

Vanishing of the ring— Three causes, 242 Divisions of the ring,. 243

Page.

Dimensions of the rings,. 244

Satellites op Saturn, 245

Mimas, 245

Enceladus, 245

Tethys 245

Rhea, 246

Titan, 246

Hyperion, 247

Japetus, 247

Diameter of the satellites, 247

Ancient observations of Saturn, 247

Uranus or Herschel, 248

Aspect— Diameter— Mass— Density, . 248

Rotation , 248

Distance — Inclination of Orbit — Periodic time, 249

Satellites of Uranus, 249

intensity of light, 250

Neptune, 250

History of its discovery , 250

Name, — Diameter, — Mass, — Density, 251 Orbit, — Inclination of orbit, — Distance, — Periodic time, 251

Intensity of light, 251

Has Neptune a ring, 251

The satellite of Neptune, 252

Real and apparent motions of the

planets, .'. 252

Causes of the apparent motions, 263

Apparent motions explained, 253

The planets at times stationary,. ... 254

Chapter VI. Comets

Their constitution, 254

Number of comets,. 255

Splendor and siie, 257

Velocity, 257

Temperature, 258

Comets shine by reflected light, 258

Orbits, — Perihelion distances, 269

Inclination of the orbits,— Direction

of motion, 260

Elements,— Identity 260

Halley's comet, 261

Encke's comet, 262

Biela's comet, 262

Faye's comet, 263

D 1 Arrest's comet, 263

Wennecke's comet, 263

Physical nature of comets, 265

Collision with the earth, 265

Chapter VII. Meteoric Bodies

Shooting stars, 266

Height and Velocity, 266

How they are inflamed, 268

Meteoric showers, 268

Meteorites,. . . 270

Weston meteorite 270

Ohio meteorite, 271

Height and velocity of meteorites,. . 271

Size of meteorites 271

Meteoric iron, 272

Theory of meteoric bodies 273

November stream, 273

August ring, 273

Meteoric Phenomena, 274

Chapter VIII. Tides

Tides defined, 274 | Why high tides occur on opposite

Cause of the tides 276 | sides of the globe, 276

CONTENTS.

Page.

Why low tides occur on opposite

sides of 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, 282

Page.

Actual heights of the tide, 283

Derivative tides, 284

Nature and velocity of the tide wave, 284

Cotidal lines, 284

Tides in| lakes, and seas connected with the ocean, 285

Chapter IX. Terrestrial Longitude

Longitude ascertained by four meth- I By eclipses, 287

ods 286 By the electric telegraph, 288

By chronometers 286 | By the lunar method 288