Distance and Magnitude.—From repeated observations of the moon's horizontal par allax (q.v.), and of the occultations by her of the fixed stars, her mean distance* from the earth has been estimated at 237,600 m., and as her angular diameter averages 31' 26", her actual diameter is 2,153 m., or a little less than Allis of the earth's diameter. IIer vol ume is therefore about nth of that of the earth, and her density being only .577 (that of the earth being taken as unity), her mass is only -hill of the earth's mass; consequently, the force of gravity at her surface is so much less than it is at the surface of the earth, that a body which weighs 1000 lbs. here, would at the moon weigh only 163 lbs.
Orbit.—The moon revolves round the earth in an elliptic orbit, with the earth in the focus; the eccentricity of the ellipse being equal to .05491 of half its major axis, or more than 31- times that of the earth's orbit. The plane of her orbit does not coincide with the ecliptic, but is inclined to it at an angle of 5° 8' 47.9', and intersects it in two oppo site points, which are called the flocks (q.v.). The point at which the moon is nearest to the earth is called her perigee, and that at which she is furthest from it her apogee, and the line joining these two points is called the line of apsidcs. Were time moon's orbit a true ellipse, which, to various irregularities known as perturbations, it is not, the lunar theory would be exceedingly simple; but these perturbations, which, in the case of the planets, produce a sensible variation in their orbit only after many revolutions, cause, in the case Of the moon, it distinct and well-marked deviation from her previous course in a single revolution. The retrogradation of her nodes along the ecliptic causes a continual change in the plane of 'her orbit, so that if, during one revolution round the earth, she occults certain stars, at the next revolution site will pass to one side of them, and will remove further and further from them in each successive revolution. A little considera tion will show that by this continual change of her orbit, the moon will, in course of lime, pass over or occult every star situated within 5°24'30' of the ecliptic. The motion of the nodes is so rapid that they perform a complete circuit of the orbit in 6793.39 mean solar days, or 18.6 years. Another important change in the moon's orbit is the revolution of the line of apsides, by which the perigee and apogee are continually chang ing their position relative to the earth and sun. This revolution is more than twice as
rapid as that of the nodes, being performed in 3232.57 mean solar days, or 8.85 solar years. As this motion is common to all the heavenly bodies, its nature and origin will be treated of under the head of perturbations (q.v.). Its effect upon tine moon is to pro duce a variation in her distance from time earth, independent of that produced by her elliptic motion.
Eclipses.—As the moon in her course passes the sun at the commencement of every (synodic) month, and by the middle of the month has placed the earth between herself and the sun, it is evident that if she moved in the plane of the ecliptic, there would be either a total or an annular eclipse of the sun at the commencement, and a total eclipse of the sneers in the middle of every month. The inclination of her orbit allowing her to pass the sun 5° 9' to the n. or s. of his track, prevents such a frequent occurrence of eclipses. If the moon, when in conjnuction, is at either of her nodal points, and at the same time near her perigee, a total eclipse of the sun takes place; but if near her apogee, the eclipse is only annular, for at that time her apparent diameter is less than the sun's. If also, at her conjunction, her latitude n. or s. is less than the suns of her semi diameter and of that of the sun, a partial eclipse takes place, and is greater the nearer the moon is to her node. These partial eclipses are seldom seen from all parts of the earth's illuminated surface, but are confined to a portion of it, which is greater or less according to the extent of the eclipse. Lunar eclipses, which occur when the moon is in opposition (i.e., at full moon), are seen equally from all parts of the earth's surface which are turned towards her. The conical shadow of the earth which is projected into space on the side opposite to the sun, is in length equal to about times the moon's mean distance, and a section of it at the moon's distance is 1° 23' in diameter. If, then, the moon, which is never more than in diameter, happens to be at or near her node, a total eclipse will take place, and in no case can it be annular, as is sometimes the case with those of the sun. Even during total eclipses; the moon is seldom quite invisible, but generally shines with a faint copper-colored light. See ECLIPSES.