MOON, the single satellite attendant on the earth. Its diameter is 2,160 miles, that of the earth (which is 7,918) being nearly four times as great. Its superficial extent is about a 13th part of the earth's surface; its bulk is 1-45 that of the earth, but as the earth is relatively heavier, its weight is about 80 times that of the moon. As the moon revolves around the earth it manifests phases. After absence for a few nights it reappears as a deli cate crescent of white light in the W. sky after sunset. Night after night it moves farther to the E., the illuminated portion of its disk continually increasing till the moon becomes full and rises about sunset. When the light of the moon has again so diminished that it is in its last quarter, it is seen high in the heavens in the morning. When it becomes full, the sun and the earth are so nearly in a straight line that the moon narrowly escapes being eclipsed; when new moon is again reached, the sun is nearly under going similar obscuration (see ECLIPSE) . The moon shines only by the light of the sun reflected from its surface. To equal the brilliance of the sun 600,000 full moons would be required. The moon appears at all times nearly of the same size, showing that its orbit cannot be far from circular. Its average distance
is 240,000 miles, varying at times between 220,000 and 260,000, but the ordinary fluctuations do not exceed 13,000 miles on either side of the mean value. The moon performs a complete revolution around the earth in 27 days, 7 hrs., 23 min., and 11.461 secs. This is called its sidereal period. The lunar month is longer than the sidereal period by 2 days, 5 hrs., 51.41 secs., because of the ad vance of the earth in the orbit between two successive conjunctions of the moon. As the moon revolves on its own axis nearly in the same time as it completes its orbit round the earth, it presents to us at all times nearly the same side of its surface. No clouds appear on it; apparently there is no water to send them forth or an atmosphere in which they may float. The whole surface is studded with volcanoes, apparently extinct. Their craters are broad, beyond anything exist ent on the earth. Tycho is 50 miles across, so is Aristotle, Theophilus is 64, and Petavius 78. Some are 16,000 and 17,000 feet deep. From the absence of an atmosphere the moon must be uninhabit able by any life analogous to that with which we are acquainted.