CONTENTS.

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Dialogue I. p. 1.
Introduction. Definition. The sun and planets. A globe defined. Sun’s distance and magnitude. Planets, what; their names, periods, and distances from the sun; their magnitudes, compared with the earth; called inferior and superior, why. Comets; derivation of the name. Solar system; why so called.
Dialogue II. p. 10.
Different systems explained. Planets appear like stars; they shine by reflection; how known from stars; they never twinkle, why. Stars shine with their own native light; their inconceivable distance; are suns, the centers of other systems. Plurality of worlds.
Dialogue III. p. 20.
The earth has the appearance of a star to Venus. Remote objects appear at equal distances from us. Our earth is a moon to the moon. The orb of the moon visible soon after the change; her disc and bulk compared with the earth; her mean distance. Sun’s disc compared with hers. Our sun a star, if seen from a planet of another system. Stars as far from each other as the nearest is to us. Stars distinguished by their apparent magnitude. The Milky Way innumerable stars. Number of stars visible at one time to the naked eye.
Dialogue IV. p. 29.
Stars divided into constellations; necessary for ascertaining the situation of the planets, and of the stars with each other. Planets motion regular if seen from the sun; irregular as seen from the earth, the motion being sometimes direct, sometimes retrograde; at others they appear stationary. Superior and inferior conjunction, and opposition, what. Venus has the different phases of the moon. Planets, how distinguished from each other.
Dialogue V. p. 39.
Ecliptic, what. Inclination of the orbits of the planets. Nodes of the planets, what. A plane, what. Planets move in unbounded space. Mercury and Venus seen on the sun’s disc. Number of signs in the zodiac. Zodiac, what. A degree, what. Names of the signs. Number of degrees in each sign. Sun’s place in the ecliptic. Table of signs, their characters, &c. To find the sun’s place in the ecliptic for any day in the year.
Dialogue VI. p. 50.
The orbits of the planets are not true circles, but somewhat elliptical. Perihelion, aphelion, and mean distance, what. Attraction, what. Laws of attraction. Attraction of gravitation, its effects. Simple motion rectilineal. Attractive or centripetal, and projectile or centrifugal forces, what.
Dialogue VII. p. 61.
Bodies moving in circles have a tendency to fly off. Planets kept in their orbits by the joint action of the centripetal and centrifugal forces; they describe equal areas in equal times. Orbits of the comets very elliptical. The earth in its perihelion in December. Equation of time. Center of gravity, what; sun and planets move round it. Sun the center of the system.
Dialogue VIII. p. 73.
The earth revolves on its axis. Cause of day and night. The motion of the earth so uniform as not to be perceived. The apparent motion of the sun caused by the earth’s motion on its axis. An objection to the earth’s motion answered. The sun and some of the planets revolve on their axes. Atmosphere, what; cause of twilight. Horizon, what; the sun and moon appear largest near the horizon, why; they appear above the horizon when below it; caused by refraction; proved by experiment.
Dialogue IX. p. 87.
Inclination of the earth’s axis. An angle, what. The poles, what. Equinoctial, what. Earth’s parallelism described. The axis of the earth points to the same parts of the heavens. Equator, ecliptic, polar circles, and meridians, explained. Difference of time between places lying under different meridians. Longitude, what. How to reduce longitude to time, and time to longitude. Latitude, what.
Dialogue X. p. 101.
The seasons. Vernal and autumnal equinoxes. Days and nights always equal, if the axis of the earth were perpendicular to the plane of its orbit. Seasons occasioned by the inclination of the earth’s axis. Seasons continued. Days and nights equal at all times under the equator. The sun above the horizon of the poles six months; and six months below them alternately, so that they have but one day and one night in the year; the longest day under the polar circles is twenty-four hours. The sun rises on different points of the compass at different seasons of the year. Twilight in the polar regions of long duration. We are nearest the sun in winter, yet it is our coldest season, why. The earth divided into zones; proved to be globular, but is not a true sphere.
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