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CHAPTER VIL.

tance, and real diameter. First approximation to its orbit. An ellipse about

the Earth in the focus. Its excentricity and inclination. Motion of its nodes

and apsides. Of occultations and solar eclipses generally. Limits within

which they are possible. They prove the Moon to be an opaque solid. Its

light derived from the Sun. Its phases. Synodic revolution or lunar month.

Of eclipses more particularly. Their phenomena. Their periodical recur-

CHAPTER XI.

OF COMETS.

Great number of recorded comets. The number of those unrecorded probably

much greater. General description of a comet. Comets without tails, or with

more than one. Their extreme tenuity. Their probable structure. Motions

conformable to the law of gravity. Actual dimensions of comets. Periodical

return of several. Halley's comet. Other ancient comets probably periodic.

parallactic inequality. Analogous planetary inequalities. Three cases of

541

II. Synoptic Table of the Elements of the Planetary System

543

III. Synoptic Table of the Elements of the Orbits of the Satellites, so far

as they are known

545

IV. Elements of Periodical Comets at their last appearance

548

Index.

...... 549

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