A

TREATISE ON ASTRONOMY.

BY

SIR JOHN F. W. HERSCHEL, KNT. GUELP.

F.R.S.L. & E. M.R.I.A. F.R.A.S. F.G.S. M.C.U.P.S.

CORRESPONDENT OF THE ROYAL ACADEMY OF SCIENCES OF PARIS, AND OTHER
FOREIGN SCIENTIFIC INSTITUTIONS.

PHILADELPHIA:
CAREY, LEA & BLANCHARD.

"Et quoniam eadem natura cupiditatem ingenuit hominibus veri inveniendi, quod facillimi apparet, cum vacui curis, etiam quid in cœlo fiat, scire avemus: his initiis inducti omnia vera diligimus; id est, fidelia, simplicia, constantia; tum vana, falsa, fàllendia odimus." Cicero, de Fin. Bon. et Mal. ii. 14.

And forasmuch as nature itself has implanted in man a craving after the discovery of truth (which appears most clearly from this, that, when unoppressed by cares, we delight to know even what is going on in the heavens),-led by this instinct, we learn to love all truth for its own sake; that is to say, whatever is faithful, simple, and consistent; while we hold in abhorrence whatever is empty, deceptive, or untrue.

INTRODUCTION

CONTENTS.

Page 7

CHAPTER I.

General Notions-Form and Magnitude of the Earth-Horizon and

its Dip-The Atmosphere-Refraction-Twilight-Appearances

resulting from diurnal Motion-Parallax-First Step towards form-

ing an Idea of the Distance of the Stars-Definitions

CHAPTER II.

Of the Nature of astronomical Instruments and Observations in gene-

ral-Of sidereal and solar Time-Of the Measurement of Time-

Clocks, Chronometers, the Transit Instrument-Of the Measure-

ment of angular Intervals-Application of the Telescope to Instru-

ments destined to that Purpose-Of the Mural Circle-Determina-

tion of polar and horizontal Points-The Level-Plumb Line-

Artificial Horizon-Collimator-Of compound Instruments with

co-ordinate Circles, the Equatorial-Altitude and Azimuth Instru-

ment-Of the Sextant and Reflecting Circle-Principle of Repeti-

tion

14

66

CHAPTER III.

OF GEOGRAPHY.

Of the Figure of the Earth-Its exact Dimensions-Its Form that of

Equilibrium modified by Centrifugal Force-Variation of Gravity

on its Surface-Statical and dynamical Measures of Gravity-The

Pendulum-Gravity to a Spheroid-Other Effects of Earth's Rota-

tion-Trade-winds-Determination of geographical Positions-Of

Latitudes Of Longitudes-Conduct of a trignometrical Survey-

Of Maps-Projections of the Sphere-Measurement of Heights by

the Barometer

CHAPTER IV.

OF URANOGRAPHY.

Construction of celestial Maps and Globes by Observations of right

Ascension and Declination-Celestial Objects distinguished into

fixed and erratic-Of the Constellations-Natural Regions in the

Heavens The Milky Way-The Zodiac-Of the Ecliptic-Celes-

tial Latitudes and Longitudes-Precession of the Equinoxes-Nu-

tation-Aberration-Uranographical Problems

3

105

- 151

CHAPTER V.

OF THE SUN'S MOTION.

Apparent Motion of the Sun not uniform-Its apparent Diameter also

variable-Variation of its Distance concluded-Its apparent Orbit

an Ellipse about the Focus-Law of the angular Velocity-Equa-

ble Description of Areas-Parallax of the Sun-Its Distance and

Magnitude-Copernican Explanation of the Sun's apparent Motion

-Parallelism of the Earth's Axis-The Seasons-Heat received

from the Sun in different Parts of the Orbit

CHAPTER VI.

Of the Moon-Its sidereal Period-Its apparent Diameter-Its Paral-

lax, Distance, and real Diameter-First Approximation to its Orbit

-An Ellipse about the Earth in the Focus-Its Eccentricity and

Inclination-Motion of the Nodes of its Orbit-Occultations-Solar

Eclipses-Phases of the Moon-Its synodical Period-Lunar

Eclipses-Motion of the Apsides of its Orbit-Physical Constitution

of the Moon-Its Mountains-Atmosphere-Rotation on Axis-

Libration-Appearance of the Earth from it

CHAPTER VII.

Page

- 176

Of terrestrial Gravity-Of the Law of universal Gravitation-Paths

of Projectiles; apparent, real-The Moon retained in her Orbit by

Gravity-Its Law of Diminution-Laws of elliptic Motion-Orbit

of the Earth round the Sun in accordance with these Laws-

Masses of the Earth and Sun compared-Density of the Sun-

Force of Gravity at its Surface-Disturbing Effect of the Sun on

the Moon's Motion

203

221

CHAPTER VIII.

OF THE SOLAR SYSTEM.

Apparent Motions of the Planets-Their Stations and Retrograda-

tions-The Sun their natural Centre of Motion-Inferior Planets

-Their Phases, Periods, &c.-Dimensions and Form of their Orbits

-Transits across the Sun-Superior Planets, their Distances, Pe-

riods, &c.-Kepler's Laws and their Interpretation-Elliptic Ele-

ments of a Planet's Orbit-Its heliocentric and geocentric Place-

Bode's Law of Planetary Distances-The four ultra-zodiacal Pla-

nets-Physical Peculiarities observable in each of the Planets - 231

CHAPTER IX.

OF THE SATELLITES.

Of the Moon, as a Satellite of the Earth-General Proximity of Satel-

lites to their Primaries, and consequent Subordination of their

Motions-Masses of the Primaries concluded from the Periods of

their Satellites-Maintenance of Kepler's Laws in the secondary

Systems Of Jupiter's Satellites-Their Eclipses, &c.-Velocity

of Light discovered by their Means-Satellites of Saturn-Of

Uranus.

- 272

CHAPTER X.

OF COMETS.

Great Number of recorded Comets-The number of unrecorded

probably much greater-Description of a Comet-Comets without

Tails-Increase and Decay of their Tails-Their Motions-Sub-

ject to the general Laws of planetary Motion-Elements of their

Orbits-Periodic Return of certain Comets-Halley's-Encke's-

Biela's-Dimensions of Comets-Their Resistance by the Ether,

gradual Decay, and possible Dispersion in Space -

CHAPTER XI.

OF PERTURBATIONS.

Page

- 284

Subject propounded-Superposition of small Motions-Problem of
three Bodies-Estimation of disturbing Forces-Motion of Nodes

-Changes of Inclination-Compensation operated in a whole

Revolution of the Node-Lagrange's Theorem of the Stability of

the Inclinations-Change of Obliquity of the Ecliptic-Precession

of the Equinoxes-Nutation-Theorem respecting forced Vibra-

tions-Of the Tides-Variation of Elements of the Planet's Orbits

-Periodic and secular-Disturbing Forces considered as tangen-

tial and radial-Effects of tangential Force-1st, in circular Orbits;

2d, in elliptic-Compensations effected-Case of near Commen-

surability of mean Motions-The great Inequality of Jupiter and

Saturn explained-The long Inequality of Venus and the Earth-

Lunar Variation-Effects of the radial Force-Mean Effect of the

Period and Dimensions of the disturbed Orbit-Variable Part of

its Effect-Lunar Evection-Secular Acceleration of the Moon's

Motion-Permanence of the Axes and Periods-Theory of the secu-

lar Variations of the Eccentricities and Perihelia-Motion of the

lunar Apsides-Lagrange's Theorem of the Stability of the Ec-

centricities-Nutation of the lunar Orbit-Perturbations of Jupi-

ter's Satellites

294

CHAPTER XII.

OF SIDEREAL ASTRONOMY.

Of the Stars generally-Their distribution into Classes according to

their apparent Magnitudes-Their apparent Distribution over the

Heavens-Of the Milky Way-Annual Parallax-Real Distances,

probable Dimensions, and Nature of the Stars-Variable Stars-

Temporary Stars-Of double Stars-Their Revolution about each

other in elliptic Orbits-Extension of the Law of Gravity to such

Systems Of coloured Stars--Proper Motion of the Sun and Stars

-Systematic Aberration and Parallax-Of compound sidereal

Systems-Clusters of Stars-Of Nebula-Nebulous Stars-Annu-

lar and planetary Nebula-Zodiacal Light

349