104 THE UNIVERSE A VAST SYSTEM OF PARTS forty or fifty miles in diameter. The mountain chains and the crater walls rise three, four, and even five miles. The cause of these lunar craters is uncertain. Perhaps they were formed by huge meteors. Fig. 25. Lunar crater Theophilus. (Photograph from the Yerkes Observatory, reprinted by permission of the Chicago University Press.) Small craters have been formed in the earth's crust at least twice in fairly recent times, and possibly some craters of previous times have been obliterated by the erosive agents which are lacking on the moon. The unequal distribution of the craters and their concentration near the mountain ranges lead many astronomers to attribute their formation to ancient volcanic activity. The density of the moon is only 60 per cent of the density of the earth and its mass only 1/82 as great. Its gravitational attraction is 1/6 that of the earth. We are sure that the moon has no atmosphere because there is no refraction of light as stars appear from behind it. The masses of atmospheric particles and their velocities are known, and calculations based upon Newton's laws show that the moon could not hold such particles to its surface. The absence of erosion and spectroscopic evidence also indicate that these calculations are correct. Inasmuch as the moon has no atmosphere, the side facing the sun must be much hotter than the Sahara desert, while the other side must be much colder than our Perhaps man may contrive to make a journey coldest polar regions. to the moon in the future ; but its temperature extremes, complete lack of life, and oppressive silence would lead one to terminate his visit as soon as possible. With no atmosphere, man's voice and ears would be useless because sound would be impossible, and he would have to carry along his own supply of oxygen for life itself. The light from the moon is, of course, reflected sunlight. The different phases of the moon (that is, the periodic change of shape from crescent to full and back again every twenty-nine days) depend upon the relative positions of the earth, moon, and sun, as shown in the diagram. It has already been noted that the planets closer to the sun than the earth pass through similar phases for the same reason.
THE PLANETS AND THEIR MOTIONS 105 To a man on the moon the earth would be bright because of the fact that it reflects sunlight just as the other planets do. This earthshine is in turn reflected back to the earth, so that when only a part of the moon receiving direct sunlight is visible, the rest of the side of the moon facing the earth becomes faintly visible because of reflected earthshine. The moon always turns the same face toward us, which means that the moon rotates on its axis once while it revolves once around the earth. Eclipses No Longer Are a Source of Terror. Modern man no longer is terrified by eclipses but looks upon their appearance at the precise place and time predicted by astronomers many years before as evidence of the true value of the scientific method. Fig. 26. Earth-lit "old moon in the new moon's arms." (Photograph from the Yerkes Observatory, reprinted by permission of the Chicago University Press.) It is possible to predict to within a few hundred yards where the shadow of the moon will be located on the earth at any given second of time during the next few years. O O € Full Moon '. O' € First Quarter EARTH New Moon Light from the Sun c Last Quarter Fig. 27. The phases of the moon.