MOON; The, from the earliest times, has been the chief object in the sky to attract the attention of the human race. The con tinual change in its appearance from day to day, its value as a light giver at night, the apparent irregularity of its motions, the curious markings on its face, its connection with the ebb and flow of the ocean, and many a real or supposed influence in terrestrial affairs, have always caused it to be a subject of speculation and inquiry among philosophers of every age. Yet it is only within the last 300 years that the various theories and superstitions connected with it have been put to the test, and that order has been evolved from the chaos of fact and fancy. Even at the present time many a belief, tested by science and found wanting, finds sup port in otherwise enlightened communities.
Size, Weight, The moon is a nearly spherical body with a diameter of 2,163 miles, a little more than a quarter of that of the earth, moving at an average distance from the earth of 239,000 miles. Its surface is therefore 1/28 and its volume Ma that of the earth. It is, however, less dense in the ratio of 10 to 16, or, on the average, its density is the same as that of the rocks on the earth's surface, so that it would re quire the materials contained in 81 moons to form our globe. Its smaller size and mass cause gravity at the surface to be only 16 of the terres trial attraction: the same exertion which would lift a given weight here would raise a weight six times as great there, and a body, instead of falling 16 feet in the first second, would fall only 23 feet. It moves so as to always turn the same face to the earth and therefore rotates on its axis in the same time that it takes to go round the earth — about 27 days. The rotation of the moon about its axis and its mo tion round the earth is neither quite uniform or circular, as will be seen later; consequently extra portions of the eastern and western faces come successively into view. Further, the moon's axis is not quite perpendicular to the plane of its orbit, so that the north and south caps are in turn a little inclined toward the earth. These motions are called the librations
of the moon, and they permit us to see rather more than half (about three-fifths) the surface.
Light and Like the earth, the moon possesses no light of its own, but re ceives all from the sun, and its day— the inter val from sunrise to sunrise — is a month. At full moon it sends to us about one 600,000th part of that given by the midday sun. The sur face is not nearly so white as its concentrated light would seem to indicate; its brightest por tions are nearly of the shade of salt and its darkest that of slate, the average being the color of gray weathered sandstone. The long sun shine for two weeks and the absence of sun for the following two weeks must cause immense variations of temperature. But Professor Langley of Washington estimates that the tem perature of the surface, even at the hottest, probably never rises above the freezing point of water; the heat is radiated out nearly as fast as it is received. In the long night the temperature must fall to something like 200° below zero. In spite of this small amount of heat, Professor Langley, by inventing a very sensitive instrument called a bolometer which would detect the heat from a candle a mile away, succeeded in detecting heat rays coming from the moon. His results depend partly on the many fruitless attempts which have been made to find evidences of the existence of an at mosphere. If air is present its pressure cannot exceed 1-750 that at the earth's surface and it is probably much less, as no refraction has ever been observed when the moon's limb passes over a star. A similar argument applies to the exist ence of water in any quantity, and no clouds have been certainly seen. It is possible that water —probably in the form of ice — may exist at the bottom of some of the deeper craters, but the low temperature would scarcely permit it to be liquified even when the sun was shining full on it.