CHAPTER VII.

OF THE MOON. -ITS SIDEREAL PERIOD_ITS APPARENT DIAMETER.

-ITS PARALLAX, DISTANCE, 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 POS-SIBLE.-THEY PROVE THE MOON TO BE AN OPAKE SOLID-ITS

LIGHT DERIVED FROM THE SUN.-ITS PHASES.-SYNODIC REVO-LUTION OR LUNAR MONTH_OF ECLIPSES MORE PARTICULARLY.

-THEIR PHENOMENA. —THEIR PERIODICAL RECURRENCE_PHY-SICAL CONSTITUTION OF THE MOON_ITS MOUNTAINS AND OTHER

SUPERFICIAL FEATURES.—INDICATIONS OF FORMER VOLCANIC

ACTIVITY.-ITS ATMOSPHERE.-CLIMATE_RADIATION OF HEAT

FROM ITS SURFACE.-ROTATION ON ITS OWN AXIS.-LIBRATION.

-APPEARANCE OF THE EARTH FROM IT.

(401.) The moon, like the sun, appears to advance among thestars with a movement contrary to the general diurnal motion ofthe heavens, but much more rapid, so as to be very readily per-ceived (as we have before observed) by a few hours’ cursory atten-tion on any moonlight night. By this continual advance, which,though sometimes quicker, sometimes slower, is never intermittedor reversed, it makes the tour of the heavens in a mean or averageperiod of 27 d 7 h 43™ ll s -5, returning, in that time, to a positionamong the stars nearly coincident with that it had before, andwhich would be exactly so, but for reasons presently to be stated.

(402.) The moon, then, like the sun, apparently describes anorbit round the earth, and this orbit cannot be very different froma circle, because the apparent angular diameter of the full moonis not liable to any great extent of variation.

(403.) The distance of the moon from the earth is concludedfrom its horizontal parallax, which may be found either directly,