A compendium of astronomy

Front matter

Telescopic wonders of the Starry Heavens.

1. Great Cluster of Stars in Hercules.

2. Whirlpool Nebula of Lord Ross.

Preface

The extensive patronage which this work has enjoyed, both as a private manual, and as a text-book in the schools, lays the author under peculiar obligation to render it deserving of public favor. He has therefore, with much care and pains, prepared this revised edition, using his best endeavors to present to the learner, in a short compass, a clear, faithful, and comprehensive outline of the noble science of Astronomy. The earlier portions of the work, treating as they do of subjects which are in their nature of a fixed character, such as definitions and the doctrine of the sphere, have appeared to him susceptible of little improvement, and accordingly have been suffered to remain unchanged ; but the latter portions, relating to the Planets, Comets, Fixed Stars, and Nebulae, have required to be entirely rewritten, in order to embrace those numerous and grand discoveries with which astronomy has been enriched within a few years past.

To render difficult subjects plain and intelligible to the young, has constituted with him the leading object of a life sedulously devoted to the instruction of youth, through the several gradations of the common-school, the academy, and the university. He would not, however, encourage any one to suppose, that he can make any valuable attainments in this profound science, without diligent study and close reflection. If any book on astronomy is very easy, it is because it is very superficial, and contains little worth knowing. The riches of this mine lie deep ;

IV PREFACE.

and no one can acquire them who is either incompetent or unwilling to dive beneath the surface.

The author would beg leave to direct the attention of teachers to the improvements introduced into the present edition for learning the Constellations. Although the diagrams here given will not supersede the necessity of resorting to the Celestial globe, or to maps of the stars, yet as a starting-point they will be found greatly to facilitate the study of the nocturnal heavens, and to afford to the young learner such plain and conspicuous landmarks that he will be able afterwards, with little assistance, to travel successively from constellation to constellation, until he becomes entirely familiar with every portion of the starry firmament.

Yale College, April, 1855.

ANALYSIS.

These Outlines the author has found very valuable as a basis for public examinations. Instead of being interrogated in the usual way by individual questions, the student is assigned at random (or by lot) some portion of the Analysis, which, after a little time for collecting his thoughts, he is called upon to expand; and the fulness and accuracy with which he performs this process, determine the mark accorded to him on the scale of merit.

Preliminary Observations.

PAGE

Astronomy— defined 1

Descriptive, Physical, and Practical. 1 History. Astronomy of the Ancients 1

Astronomy of the Greeks 2

Pythagoras, Ilipparchus, Ptolemy. . . 2

Astronomy of the Middle Ages 3

Copernicus, Tycho Brahe, Kepler,

Galileo 3

Astronomy of the Moderns 3

Sir Isaac Newton, La Place 3

Astrology — defined 3

Natural Astrology, Judicial Astrology 3

Copernican System — briefly stated . 4

Figure and Dimensions of the Earth, and Doctrine of the Sphere.

Figure of the Earth 5

Proofs of its being globular 5

Illustrations by Figs. 1 and 2 6

Exact figure of the earth 6

Diameter, Circumference 7

Doctrine of the Sphere— defined . . 8

Great and small circles 9

Axis of a circle, pole, secondary 9

Horizon — sensible and rational 11

Zenith and Nadir 11

Vertical Circles, Meridian 12

Prime Vertical, Altitude, Azimuth,

Amplitude 12

Axis of the earth, Poles of the earth

and heavens 12

Equator, Hour Circles, Latitude, Longitude 13

Ecliptic, Equinoxes, Solstices, Signs

of the Zodiac 14

Colures — equinoctial and solstitial ... 15

Eight Ascension, Declination 16

Celestial Latitude and Longitude 16

PAGE

Parallels of Latitude, Tropics, Polar

Circles 16

Elevation of the Pole in degrees 17

Elevation of the Equator 17

Zones — Torrid,Temperate, and Frigid 17

Zodiac 17

Huw to represent the Circles of the

Sphere by an apple 17

Projection of the Sphere— Fig. 5 19

Diurnal Revolution.

Circles of diurnal revolution 21

Sidereal Day, Eight Sphere, Parallel

Sphere 22

Oblique Sphere— Fig. 6 24

Circle of Perpetual Apparition 25

Circle of Perpetual Occultation 26

Artificial Globes— described 2S

Hour Circles, Hour Index, Quadrant

of Altitude 30

To rectify the globe for any place ... 31 Problems on the Terrestrial Globe . . 31 To find the Latitude and Longitude

of a place 31

To find a place, the Latitude and Longitude being given 31

To find the bearing and distance of

two places 32

To determine the difference of time

in different places 32

The hour being given at any place, to tell what hour it is in any other

part df the Avorld '. 32

To find what people live directly under us 32

To find what people of the southern hemisphere live directly opposite

to us 32

To find the Antipodes 33

To rectify the globe for the Sun's place 33

ANALYSIS.

PAGE

The latitude of a place being given, to find the time of the Sun's rising

and setting 34

Froblems on the Celestial Globe 84

To find the right ascension and declination 34

To represent the appearance of the

stars at any time 84

To find the altitude and azimuth of a

star 35

To find the angular distance of two

stars 35

To find the sun's meridian altitude. . 35

Parallax, Refraction, and Twilight.

Parallax— defined, Fig. 7 86

Horizontal Parallax— its importance. 37

Refraction— defined, Fig. 8 33

Its' amount at different altitudes 40

Effects of refraction upon the sun and

moon when near the horizon 41

Twilight— defined, Fig. 9 42

Its appearance in different latitudes . 43

Its uses 44

Time.

Time defined 44

Sidereal day, Solar day 45

Apparent time, Mean time 46

Astronomical day, Equation of Time 46

Clocks, how regulated 46

The Calendar 47

Astronomical year, Civil year 47

Bissextile or Leap-year 48

Rule for Leap-year 49

How the common year begins and

ends 50

Astronomical Instruments.

When first used 51

Angular measurement illustrated ... 52

Telescope, principle, Fig. 11 53

Eefractors and Eeflectors 55

Transit Instrument, Fig. 12 56

Its use, Noon mark 57

Astronomical Clock 59

To what kind of time adapted 59

Altitude and Azimuth Instrument, 60

Sextant, Fig. 14 63

Figure and Density of the Earth.

Spheroidal figure, Fig. 15 63

How measured by arcs of the meridian 66

By the Pendulum 67

Difference in the polar and equatorial

diameters 67

Earth's ellipticity 67

Density of the earth 68

How estimated, Fig. 16 68

PAGE

Sun — Solar Spots — Zodiacal Light.

Sun — figure, distance, diameter, size,

density 70

Solar Spots— described, Fig. 17 72

Part of the sun's disk occupied by

them 72

Period of their revolution— Extent. . 74

Zodiacal Light— described, Fig. 20 . 76

Earth's Annual Motion — Seasons

— Figure of the Earth's Orbit.

Annual motion — illustrated, Fig. 21. 79

Obliquity of the Ecliptic SI

Apparent motion of the Sun 82

Dimensions of the Earth's orbit 83

Seasons — cause of the change of seasons 84

Illustration by Fig. 22 85

Consequences had the ecliptic been

perpendicular to the equator 86

Figure of the Earth's Orbit, Fig. 23 8S How the variations of the distances

from the sun are found .- 89

Universal Gravitation.

Tendency of all matter to all other

matter 91

Illustration by Fig. 24 92

Law of gravity in'thr ee parts 93

Law of falling bodies 94

First law of motion 94

Universal Gravitation defined 96

Illustrated by Fig. 25 97

Kepler's Laws 98

First law, figure of the planetary orbits 99

Second law, spaces described by the

radius vector 101

Third law, relations between times

and distances 101

Motion in an elliptical orbit 102

Illustrated by Figs. 28, 29, 30 104

Precession of the Equinoxes 107

Its annual amount 107

Tropical year 109

The Moon.

Distances, diameter, terminator 110

Proofs of mountains and valleys 111

Names of the lunar spots 112

Heights of lunar mountains 113

Forms of lunar mountains and valleys 114

Lunar atmosphere 117

Whether there is water in the moon. 117

Whether inhabitants US

Phases of the moon 120

Syzygies, quadratures, octants 121

Phases illustrated by Fig. 81 121

Revolutions of the moon 122

ANALYSIS.

Vll

P-AGE

Inclination of the orbit 123

Why the moon runs high and low .. 124

Moon's revolution on her axis 125

Moon's three librations 126

"Whether the earth carries the moon

round the sun 128

Causes of both motions explained. . . 129

Causes of the lunar irregularities 130

Figure of the moons orbit 132

Backward motion of the nodes s 133

Synodical revolution of the node 133

The Saros explained 134

Metonic Cycle 134

Revolution of the apsides 135

Periodical and secular irregularities . 135

Eclipses.

When an eclipse of the moon happens 137

When an eclipse of the sun happens. 137

Illustration by Fig. 32 13S

Representation, Figs. 33. 34 139

Why the moon's surface is visible. . . 142

How eclipses are foretold 143

Nature of eclipses explained 144

Annular eclipses 146

Longitude and Tides.

How difference of longitude is found 150

Mode by chronometers 151

By eclipses 152

By the lunar method * . 153

Tides defined 155

High, low, spring, neap tides 155

Cause of tides explained 156

Influence of the declination of the sun

and moon 160

Tides of rivers, bays, and lakes 162

Atmospheric tide 166

Planets.

Origin of the name 167

Planets long known 167

Planets recently discovered 167

Number of Planets and Asteroids . . . 168 Distances — Dimensions of the system 169 Mean distances, how determined by

Kepler's law 170

Magnitudes — diameters in miles 170

Periodic times 171

Inferior Planets, Mercury and Ye-

nus 172

Motions illustrated by Fig. 40 173

Inferior and superior conjunction . . . 173

Synodical revolution 173

Direct and retrograde motions 174

When the inferior planets are stationary 175

Phases of Mercury- and Yenus 176

Eccentricity and inclination of their

orbits 177

When brightest 177

Revolutions on their axes 178

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! Yenus as the evening and morning

I star 178

Position every eight years 178

Transits of the inferior planets 179

Transits of Mercury ISO

Transits of Yenus 181

Sun's Hor. Parallax by Transits of

Yenus 1S2

Superior Planets 183

Mars — distance, color 1S3

Changes in apparent size 184

Phases, revolution on his axis 185

I Jupiter — size, telescopic appearance.. 186

Belts, satellites 1S7

| Eclipses of the satellites. Fig. 44 139

j Longitude by these eclipses 191

Discovery of the progressive motion

of light 193

Saturn, telescopic appearance 194

Dimensions of his system 194

Saturn's Rings. Fig. 46 196

Saturn's Satellites, number and appearances 200

Uranus — size, distance, discovery ... 201

Satellites, number and motions 202

Uniformity of direction in the planetary motions 202

Neptune — size, distance, periodic time 203

History of its discovery 203

Asteroids, history of their discovery 204

Number and names 206

Distances, periodic times, size 206

Motions of the Planetary System.

Two methods of considering them . . . 208

Conception of absolute space 20S

Motions of the planets as seen from

Illustrated by the motions of Mercury 210 Inadequate representations by diagrams and orreries 211

Apparent motions of the planets 212

Of the Inferior planets by Fig. 40. . . 213

Of Superior planets by Fig. 47 213

Masses of the Planets 216

Comparative density 216

Stability of the Solar System 217

Causes of disturbance 217

How the perturbations were discovered 218

Extreme minuteness of some of them 219 Provisions for the stability of the system 220

Numerical arrangement of the planets 221

Comets and Meteoric Showers,

Comet described, Fig. 43 221

Number — Six most remarkable 223

Magnitude and brightness 224

Periods, distances, light 225

Mass, proofs of its smallness 227

Orbits and Motions 229

Elements 230

Vlll

ANALYSIS.

PARK

How the return is predicted 23.2

Halley's Comet 232

Encke's Comet 233

Proofs of a resisting medium 234

Comet of 1843 235

Physical nature of comets 236

Dangers from them 237

Meteoric shower of Nov. 18, 1833. ... 238 Conclusions respecting them 240

The Constellations.

Fixed Stars— Classes 242

Catalogues of the Stars 244

Aries, "Taurus 246

Gemini 247

Cancer, Leo 24S

Virgo, Libra, Scorpio 249

Sagittarius, Capricornus, Aquarius . . 250

Pisces, Little Bear 251

Great Bear 252

Draco 253

Cepheus, Cassiopeia 254

Camelopard, Andromeda, Perseus,

Auriga 255

Bootes, Crown, Hercules 256

Lyre, Swan 257

Little Fox, Eagle, Antinous 258

Dolphin, Pegasus, Ophiuchus 258

PAGE

Whale, Orion, Hare, Canis Major and

Minor 260

Monoceros, Hydra 261

Lesson for September 261

Lesson for December 2G2

Lesson for March 263

Lesson for June 264

Double, Temporary, and Variable Stars, and Nebulae.

Great Telescopes , 265

Double Stars, Number 267

Multiple Stars 268

Temporary Stars, Variable Stars . . . 269

Clusters 270

Nebulas, 271

Nebula of Hercules 272

Nebulous Stars 274

Planetary Nebulae 275

Galaxy 275

Binary Stars ' 277

Proper motions of the stars 278

Motion of the Solar System 278

Distances of the Stars 281

Distance of 61 Cygni 2S2

Amount of its parallax 282

Nature of the Stars 284-

Copernican System 286

COMPENDIUM OF ASTRONOMY.

PRELIMINARY OBSERVATIONS.

1. Astronomy is that science which treats of the heavenly bodies.

More particularly, its object is to teach what is known respecting the Sun, Moon, Planets, Comets, and Fixed Stars ; and also to explain the methods by which this knowledge is acquired.

Astronomy is sometimes divided into Descriptive, Physical, and Practical. Descriptive Astronomy respects facts ; Physical Astronomy, causes; Practical Astronomy, the means of investigating the facts, whether by instruments, or by calculation. It is the province of Descriptive Astronomy to observe, classify, and record, all the phenomena of the heavenly bodies, whether pertaining to those bodies individually, or resulting from their motions and mutual relations. It is the part of Physical Astronomy to explain the causes of these phenomena by investigating and applying the general laws on which they depend ; especially by tracing out all the consequences of the law of universal gravitation. Practical Astronomy lends its aid to both the other departments.

2. Astronomy is the most ancient of all the sciences. At a period of very high antiquity, it was cultivated in Egypt, in Chaldea, and in India. Such knowledge of the heavenly bodies as could be acquired by close and long continued observation, without the aid of instru-

1 , Define Astronomy. What does it teach 1 Name the three parts into which it is divided. What does Descriptive Astronomy respect ? What does Physical Astronomy ? What does Practical Astronomy ? What is the peculiar province of each ?

merits, was diligently amassed ; and tables of the celestial motions were constructed, which could be used in predicting eclipses, and other astronomical phenomena.

About 500 years before the Christain era, Pythagoras, of Greece, taught astronomy at the celebrated school at Crotona, (a Greek town on the southeastern coast of Italy.) and exhibited more correct views of the nature of the celestial motions, than were entertained by any other astronomer of the ancient world. His views, however, were not generally adopted, but lay neglected for nearly 2000 years, when they were revived and established by Copernicus and Galileo. The most celebrated astronomical school of antiquity, was at Alexandria in Egypt, which was established and sustained by the Ptolemies, (Egyptian princes,) 300 years before the Christian era. The employment of instruments for measuring angles, and bringing in trigonometrical calculations to aid the naked powers of observation, gave to the Alexandrian astronomers great advantages over all their predecessors.

The most able astronomer of the Alexandrian school was Hipparchus, who was distinguished above all the ancients for the accuracy of his astronomical measurements and determinations. The knowledge of astronomy possessed by the Alexandrian school, and recorded in the Almagest, or great work of Ptolemy, constituted the chief of what was known of our science during the middle ages, until the fifteenth and sixteenth centuries, when the labors of Copernicus of Prussia, Tycho Brake

2. Trace the history of Astronomy. Among what ancient nations was it cultivated ? What kind of knowledge of the heavenly bodies was amassed ? Who was Pythagoras? When and where did he live ? Where was his school ? How correct were his views 1 Were they generally adopted ? Give an account of the Alexandrian school. When was it established and by whom ? What gave it great advantages over all its predecessors ? Give some account of Hipparchus — of Ptolemy — of Copernicus — of Tycho Brahe — of Kepler — of Galileo — o! Newton — of La Place. Specify the respective labors of each.

of Denmark, Kepler of Germany, and Galileo of Italy, laid the solid foundations of modern astronomy. Copernicus expounded the true system of the world, or the arrangement and motions of the heavenly bodies ; Tycho Brahe carried the use of instruments, and the art of astronomical observation, to a far higher degree of accuracy than had ever been done before ; Kepler discovered the v great laws which regulate the movements of the planets ; and Galileo, having first enjoyed the aid of the telescope, made innumerable discoveries in the solar system. Near the beginning of the eighteenth century, Sir Isaac Newton discovered, in the law of universal gravitation, tbo great principle mat explains the causes of all celestial phenomena ; and recently, La Place has more fully completed what Newton begun, having followed out all the consequences of the law of universal gravitation, in his great work, the Mecanique Celeste.

3. Among the ancients, astronomy was studied chiefly as subsidiary to astrology. Astrology was the art of dv vining future events by the stars. It was of two kinds, natural and judicial. Natural Astrology, aimed at predicting remarkable occurrences in the natural world, as eathquakes, volcanoes, tempests, and pestilential diseases. Judicial Astrology, aimed at foretelling the fates of individuals, or of empires.

4. Astronomers of every age, have been distinguished for their persevering industry, and their great love of accuracy. They have uniformly aspired to an exactness in their inquiries, far beyond what is aimed at in most geographical investigations, satisfied with nothing short of numerical accuracy wherever this is attainable ; and years of toilsome observation, or laborious calculation, have been spent with the hope of attaining a few se-

3. Define Astrology. What was Natural and what Judicial Astrology ?

4. What is said of the industry and accuracy of astronomers 1 Can this science be taught by artificial aids alone ?

PRELIMINARY OBSERVATIONS.

conds nearer to the truth. Moreover, a severe but delightful labor is imposed on all, who would arrive at a clear and satisfactory knowledge of the subject of astronomy. Diagrams, artificial globes, orreries, and familiar comparisons and illustrations, proposed by the author or the instructor, may afford essential aid to the learner, but nothing can convey to him a perfect comprehension of the celestial motions, without much diligent study and reflection.

5. In this treatise, we shall for the present assume the Copernican system as the true system of the world, postponing the discussion of the evidence on which it rests to a late period, when the learner has been made extensively acquainted with astronomical facts. This system maintains (1,) That the apparent diurnal revolution of the heavenly bodies, from east to west, is owing to the real revolution of the earth on its own axis from west to east, in the same time ; and (2,) That the sun is the center around which the earth and planets all revolve from west to east, contrary to the opinion that the earth is the center of motion of the sun and planets.

5. What system is assumed as the true system of the world ? Specify the two leading points in the Copernican system.