The New Student's Reference Work/Moon
Moon, a satellite of the earth and our nearest neighbor in the stellar universe. Its distance from the earth varies from 221,614 to 252,972 miles. Its apparent mean diameter is 31′ 7″ so that its real diameter is 2,163 miles, and its volume only 1/49 that of the earth. The moon’s mass, however, is only about 1/80 of the earth’s, which makes the acceleration of gravity at its surface only 1/6 that at the surface of the earth. Professor Young illustrates this by saying that “a man on the moon could jump six times as high as he could on the earth and could throw a stone six times as far.” The absence of any atmosphere or water on the surface of the moon has been proved by the moon’s appearance in the telescope, by the spectroscope and by the absence of refraction in the occultation of stars. The moon, like the sun, moves constantly toward the east among the stars; but it gains 12° 11.4′ daily on the sun. Accordingly the moon requires (360° / 12° 11.4′) days to gain one complete revolution on the sun. This length of time, which is 29d 12h 44m 2.7s is called one month. This is also exactly the time required for one rotation of the moon upon her own axis. The consequence is that she always keeps the same side toward the earth. The other side of the moon is something that no inhabitant of the earth has ever seen. The reason why the period of the moon’s rotation is exactly one month is a matter which is thoroughly understood — namely, tidal friction — but is too advanced for discussion in this place. The various phases which the moon presents will be clear from the accompanying figure which represents the earth and the moon’s orbit, illuminated by a sun at a great distance above the top of the page. When the moon lies exactly in the direction of the sun we say it is “new.” In this position we see none of its illuminated hemisphere; but as the moon moves away from the sun’s direction we see more and more of the illuminated portion. At the end of one week, half of the bright surface is seen by an observer on the earth, and we speak of this as a “half moon.” A week later we see the complete, illuminated hemisphere and call it “full moon.” The moon now begins to wane and passes through these same phases, in reverse order, until the next “new moon.” The moon has in all ages been and still is the subject of many superstitions. Witness such words as moon-struck and lunacy.