MOON, TuE, the satellite of the earth, revolving round the earth from w. to e. in a p-riod of one month (q.v.), and in consequence accompanying the earth in its motion round the sun. As the moon, to an observer on the earth, advances more than 13° to the e. daily, whilst the corresponding advance of the stun is barely 1°, her progress among the stars is much more notable than that of the latter. This rapid angular motion, the continual and regular variation of her illuminated surface, and her large apparent size (being nearly equal to that of the sun), have rendered the moon an object of general inter est; while her importance as the principal nocturnal substitute for the sun, and her special value to navigators and geographers, in the determination of longitudes (see LAT riUDE and LONGITUDE), have rendered the lunar theory the object of the most thorough and careful investaration.
Phases of the :Moon —The first peculiarity about the moon that strikes a casual observer is the constant and regular change of her illuminated surface from a thin cres cent to a circle, and vice versa, and a corresponding change in the time of her appearance above the horizon. These changes depend upon the position of the moon relative to the earth and the sun, for it is only the half of the moon facing the .sun that is illu minated by hiS rays, and the whole of this illuminated portion can only be seen from the earth when the sun, earth, and moon are in a straight line (the line of syzygies), and the earth is between_ the stun and moon. When the moon Itt in the lino• of syzygies, but between the earth and the sun, no part of her illuminated disk can be seen from the earth. In the former case, the moon is said to be full, and in the latter, new. A few hours after "new moon," the moon appears a little to the east of the sun as a thin crescent, with the horns pointing toward the east, and as she increases her angular distance from the sun at tlin rate of about 12' daily, the crescent of light becomes broader, till, after the lapse of a little more than seven days, at which time she is 90° in advance of the sun, she presents the appearance of a semicircle of light. The moon is then said to have completed her
first quarter. Continuing her course, she becomes "gibbous" (q.v.); and at the 15th or 16th day from new moon, attains a position 180° in advance of the sun, and now pre sents the appearance known as fall moon. From this point she begins to approach the sun, again appearing gibbons, and after a third period of more than seven days, reaches a point 90° w. of him, and enters her last quarter. Here, again, she appears as a semi circle of light, the illuminated portion being that which was not illuminated at the end of the first quarter. The moon now rapidly approaching the sun, resumes the crescent form, but this time with the horns pointing westward, the crescent becoming thinner and thinner, till the moon reaches the position of new moon, and disappears. From "full moon" to "new moon," the moon is said to be waning; and from "new moon" to "full moon," waxing. The earth as seen from the moon presents similar phases, and has, con sequently, at the time of new moon, the appearance of a round illuminated disk, and at full moon is invisible. This explains the peculiar phenomenon occasionally observed when the moon is near the sun (either before or after new moon), of the part of the moon's face which is unilluminated by the sun appearing faintly visible, owing to the reflection upon it of strong earth-light. This phenomenon is designated by the Scottish peaskntry as "the new bone wi' the auld mune in her aims." At new moon, the moon of course comes above the horizon about the same time as the sun, and sets with him, but rises each day about 50 minutes later than on the day previous, and at the end of the first quarter, rises at midday, and sets at midnight, continuing to lag behind the sun. When at the full, she rises about sunset, and sets about sunrise, and at the commence ment of her last quarter, she rises at midnight, and sets at midday.