In a broad, general way, the moon's surface, if compared with the earth's, differs in having no water. Our extensive oceans are replaced there by smooth, level plains which were at first thought to be seas and so named. There are ten or twelve of them in all. Then we find mountain ranges, so numerous on the earth, relatively few on the moon. Those that exist are named, in part, for terrestrial mountain ranges, as the Alps, Caucasus, and the Apennines.
But the nearly circular crater, a relatively rare formation on the earth, is seen dotted all over the moon in every size, from a fraction of a mile in diameter up to sixty, seventy, and in extreme cases a hundred miles. No mere description of plains and mountains and craters affords an adequate idea of the moon's surface as it actually is; a telescopic view is necessary, or some of the modern photographs which give an even better notion of the moon than any telescopic view. Many of the lunar craters are without doubt volcanic in origin, others seem to be ruins of molten lakes. Many thousands of the smaller ones appear as if formed by a violent pelting of the surface when semi-plastic, perhaps by enormous showers of meteoric matter. More than 30,000 craters cover the half of the lunar surface visible from the earth, and hundreds of them are named for philosophers and astronomers.
Measurement of the height of lunar mountains has been made in numerous instances, especially when their shadows fall on plains or surfaces that are nearly level, so that the length of the shadow can be measured. In general, the height of lunar peaks is greater than that of terrestrial peaks, owing probably to the lesser surface gravity on the moon. About forty lunar peaks are higher than Mont Blanc.
Most astronomers regard it as certain that no changes ever take place on the moon; probably no very conspicuous changes ever do. Some, however, have made out a fair case for comparatively recent changes in surface detail. Extreme caution is necessary in drawing conclusions, because the varying changes of illumination from one phase to another are themselves sufficient to cause the appearance of change. At intervals of a double lunation, equal to fifty-nine days, one and one-half hours, the terminator goes very nearly through the same objects, so that the circumstances of illumination are comparable. In Mare Serenitatis the little crater named Linné was announced to have disappeared about a half century ago; subsequently it became visible again and other minor changes were reported, perhaps due to falling in of the walls of the crater.
If one were to visit the moon, he must needs take air and water along with him, as well as other sustenance. No atmosphere means no diffused light; we could see nothing unless the sun's direct rays were shining upon it. Anyone stepping into the shadow of a lunar crag would become wholly invisible. No sound, however loud, could be heard; sound in fact would become impossible. A rock might roll down the wall of a lunar crater, but there would be no noise; though we should know what had happened by the tremor produced. So slight is gravity there that a good ball player might bat a baseball half a mile or more. Looking upward, all the stars would be appreciably brighter than here, and visible perpetually in the daytime as well as at night.
If one were to go to the opposite side of the moon, he would lose sight of the earth until he came back to the side which is always turned toward the earth. Even then the earth would never rise and set at any given place, as the moon does to us, but would remain all the time at about the same height above the lunar horizon. The earth would go through all the phases that the moon shows to us here, full earth occurring there when it is new moon here. Our globe would appear to be nearly four times broader than the moon seems to us. Its white polar caps of ice and snow, its dark oceans, and the vast cloud areas would be very conspicuous. Faint stars, the zodiacal light, and the filmy solar corona would be visible, probably even close up to the sun's edge; but although his rays might shine upon the lunar rocks without intermission for a fortnight, probably they would still be too cold to touch with safety. On the side of the moon turned away from the sun, the temperature of the moon's surface would fall to that of space, or many hundred degrees below zero.
CHAPTER XXIX
ECLIPSES OF THE MOON
Of all the weird happenings of the nighttime sky, eclipses of the moon are the most impressive. Rarely is there a year without one. What is the cause? Simply the earth getting in between sun and moon, and thereby shutting off the sunlight which at all other times enables us to see the moon. As the earth is a dark body it must cast a black shadow on the side away from the sun, and it is the moon's passing into this shadow or some part of it that causes a lunar eclipse.