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TREATISE ON ASTRONOMY.

SIR JOHN F. W. HERSCHEL, KNT GUELP.

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CORRESPONDENT OF THE ROYAL ACADEMY OF SCIENCES OF PARIS,

AND OTHER FOREIGN SCIENTIFIC INSTITUTIONS,

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London: ITRIJSHED BY LONGMAN, REES, ORME, BROWN, GREEN & LONGMAN, PATERNOSTER ROW, AND JOHN TAYLOR, UPPER GOWER STREET,

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CHAP. L 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 forming an idea of the Distance of the Stars. - Definitions.

CHAP. II.

of the Nature of Astronomical Instruments and Observations in general.

- Of Sidereal and Solar Time. Of the Measurement of Time. — Clocks, Chronometers, the Transit Instrument. — Of the Measurement of Angular Intervals. — Application of the Telescope to Instruments de. stined to that Purpose. Of the Mural Circle. -Determination of Polar and Horizontal Points. — The Level. - Plumb Line.-Artificial Horizon. - Collimator.-Of Compound Instruments with Co-ordinate Circles, the Equatorial. - Altitude and Azimuth Instrument. Of the Sextant and Reflecting Circle. - Principle of Repetition. -

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CHAP. 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 Rotation. -Trade Winds. - Determination of Geographical Positions. — Of Latitudes. - Of Longitudes. - Conduct of a Trigonometrical Survey.- Of Maps. - Projections of the Sphere. - Measurement of Heights by the Barometer.

. . . 107

CHAP. 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. - Celestial Latii tudes and Longitudes. - Precession of the Equinoxes. - Nutation. Aberration. Uranographical Problems.

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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.- Equable 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.

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CHAP, 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 Excentricity and Inclina. tion. — Motion of the Nodes of its Orbit. - Occultations. - Solar Eclipses.- Phases of the Moon. Its synodical Period.-Lunar Eclips - 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.

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CHAP. VII. Of Terrestrial Gravity. Of the Law of universal Gravitation. - Paths of

Projectiles, apparent-real. - The Moon retained in her Orbit by Gra. vity. - 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. 232

CHAP. VIII.

OF THE SOLAR SYSTEM. Apparent Motions of the Planets. — Their Stations and Retrogradations.

-The Sun their natural Center of Motion. - Inferior Planets. — Their Phases, Periods, &c. - Dimensions and Form of their Orbits. - Transits across the Sun. - Superior Planets, their Distances, Periods, &c. - Kepler's Laws and their Interpretation.- Elliptic Elements of a Planet's Orbit. - Its Heliocentric and Geocentric Place. -Bode's Law of Planetary Distances. The four Ultra-Zodiacal Planets. - Physical Peculiarities observable in each of the Planets.

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CHAP. IX.

OF THE SATELLITES. Of the Moon, as a Satellite of the Earth. - General Proximity of Satellites

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 Jupi. ter's Satellites. — Their Eclipses, &c. - Velocity of Light discovered by their Means. Satellites of Saturn-Of Uranus.

. 288

CONTENTS.

CHAP. 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. - Subject to the general Laws of Planetary Motion - Elements of their Orbits. Peri. odic Return of certain Comets. – Halley's. - Encke's. - Biela's. - Di. mensions of Comets. – Their Resistance by the Ether, gradual Decay, and possible Dispersion in Space.

Page 300

CHAP, XI.

OF PERTURBATIONS. 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 the Obliquity of the Ecliptic. - Precession of the Equi. noxes. -Nutation. - Theorem respecting forced Vibrations. Of the Tides. - Variation of Elements of the Planet's Orbits -- Periodic and Secular. - Disturbing Forces considered as Tangential and Radial. Effects of Tangential Force : - 1st, in Circular Orbits ; 2d, in Elliptic. Compensations effected. — Case of near Commensurability of Mean Motions. — The great Inequality of Jupiter and Saturn explained. - The long Inequality of Venus and the Earth. - Lunar Variation. - Effect 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 secular Variations of the Excentricities and Perihelia. - Motion of the Lunar Apsides. - Lagrange's Theorem of the Stability of the Excentricities. -Nutation of the Lunar Orbit. Perturbations of Jupiter's Satellites.

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CHAP. 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. - Tem. porary 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 Nebulæ. - Nebulous Stars. - Annular and Planetary Nebulæ. - Zodiacal Light.

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