Earth has one satellite, our Moon. Nothing else can Give us a better idea of the general status of cosmic conditions than a survey of what is known about the Moon. It is the celestial body nearest to us, being something of the order of a quarter of a million miles away. It ls about 2,100 miles id diameter, and it is bathed constantly in brilliant sunshine, all of which offers us exceptional opportunities for observation. It therefore pays us to take a good look around in the case of the Moon.
First glance at Figure 3. It is a reproduction of a Photograph made by G.W. Ritchey at Yerkes Observatory with a 40-inch telescope. As we contemplate this Illustration and those which follow, we experience a feeling of awe, for these are pictures of the actual detailed appearance of the surface of a neighboring celestial body. They offer us the nearest thing to actually visiting it. They show us all we would see if we were to take up a position some 2,500 miles above the lunar surface and look down upon it. In the photographs the image is inverted, north being at the bottom, south at the top. The 'idea that such pictures as these would be available in the twentieth century would have filled the men and women of Christopher Columbus' day with a great wonder. The most conspicuous thing that we notice at first in Figures 3 and 4 is the enormous number of so-called "craters." There are 30,000 of them that have been ob- served. The surface in some places is seen to be peppered with them. They vary in size from something of the order of 150 miles in diameter to the smallest visible speck. There appears to be an absence of them in some areas, while in others they actually overlap each other. What caused all these queer craters is not positively settled. Whether they are what is left of volcanic eruptions or whether they are points of impact of meteors we are not sure. They are all substantially circular, which is significant. The largest appear to be older or more weathered-looking than the smallest. All seem to have a rim around their edges of the nature of a "burr" such as is noticed around the edges of a hole made by a projectile in armor plate. This burr is not like the rims of volcanic eruptions on Earth, there being little evidence of large quantities of lava having flowed down the sides. But the force of gravity is immensely less on Moon than it is on Earth, and mountain sides should be very much steeper than they are here. The fact that the craters are all circular and that stray stellar matter naturally would cool into spheres tempted early astronomers toward the theory that the craters are the circular scars left by spherical meteors.
Our Earth is known to have at least three major scars from meteors. One is in Arizona, another is in Siberia, and the third is in Africa. Were it not for the erosion that thousands of centuries of rain has caused and the covering that vegetation has afforded, Earth might present a surface when viewed from an elevation of z,5oo miles not wholly unlike that of Moon. Some argue that the absence of craters on certain flat areas suggest the possibility that the latter were once seas of water which received the impacts and protected the solid surface below. This theory, however, is not accepted by modern astronomers. The characteristic flat bottom of the crater suggests the liquefaction of the meteors as a result of their terrific impact. The tendency of liquid or partially liquid material is to level off. It must be remembered that Moon has no atmosphere and that no meteor would burn up when approaching it as happens to those approaching Earth. Passage at cosmic speeds through our atmosphere melts all but the ones so massive that unmelted material is left when the surface of Earth is reached. But most of our astronomers insist that there are reasons for believing that the craters are volcanic and even that some of them are still discharging gas.
But let us go along to some of the other interesting "sights." In Figure 3 we have at A what has been named Mare Tranquilitatis. At B we have Mare Serenitatis. At C we have part of Mare Imbrium. Connecting it with Mare Serenitatis is a narrow strait flanked by what have been named the Caucasus Mountains, at D, and the Apennine Mountains, at E. This latter mountain chain runs along a curve and ends near one of the great lunar craters which has been named Copernicus, at F. Above Copernicus is another mare named Mare Nubium, at G. At H notice a faint straight line, which is called the Straight Wall. It is 65 miles long and probably is a cliff facing east about 500 feet high. It is thought to be a geologic fault such as we have on Earth. Above Mare Nubium, at I, is the crater Tycho. At f is the huge crater Clavius, 142 miles in diameter. Its rim is raised only slightly above the surrounding plain but is some two miles higher than the depressed floor. At K is the crater Longomontanus, 90 miles in diameter.
The so-called mares, or seas, are always lower than the surrounding surface. Few astronomers believe now that they are sea bottoms. They are darker in color than the surrounding surface and show shades of yellow and green. It has been suggested that these color tints may be some kind of vegetation. It is pointed out that the necessary moisture to support vegetation might come from moist vapors still being discharged from some of the craters. Others insist it is more reasonable to believe that the colors come from areas of volcanic ash, which always has some tint. We must preserve open minds on this matter until more positive evidence is obtained, which surely it will be in due time.
Turning now to Figure 5 we see the strait between Mare Imbrium and Mare Serenitatis at C. At Q is a small, round, bright crater named Linnrus which is thought to be still active and discharging vapors and moisture. At L is the crater Plinius in Mare Tranquilitatis. Just below Plinius, at P, is a cleft consisting of many small craters close together. This cleft appears to be easy to see at times and at other times very difficult. Its changing visibility is thought to be caused by gases and water vapor still discharging irregularly from craters and fissures. It ail suggests the last expiring gasps of a body close to final death.
Look now at Figure 7. At N is the crater Theophilus. It is 64 miles across. In places its rim is 3/2 miles above its floor. The taller of the central mountain peaks is shown by the length of its shadow to be 6,ooo feet high, or over a mile. W. H. Pickering maintains that the valleys seen on the lower side of this central peak were caused by ice erosion. He believes that the very bright portions of the peaks are snow.
Figure 8, considered to be one of the impressive pictures of the Moon, shows the great crater Copernicus. It is 56 miles in diameter. The peaks on the west side indicated by the pointed shadow must be two miles above the floor. The surrounding terrain is extraordinary, with its network of bright streaks, its hills and ridges. The most remarkable feature of Copernicus are the "rays" or bright streaks. Their discontinuity is evident. They differ from the rays around Tycho shown in Figure 9 in that they are yellowish instead of white. They are also less straight. What they signify is not yet clearly understood. All of these craters, it will be noticed, are normal to the surface. Were they meteor scars, some of them certainly would show evidence of at least a few meteors having come in at an angle. On the other hand, volcanic craters would always be normal to the surface.
Turn now to Figure 9. This centers upon the crater Tycho and is particularly interesting because the mysterious rays or streaks which we see so often are especially visible. Tycho is about equal to Copernicus, being 54 miles in diameter. It seems to have a dark ring which is absent in Copernicus. The rays or streaks which radiate from it are immensely intriguing. One long ray may be traced across a mare known as Mare Nectaris at O. The longest may be traced across Mare Serenitatis at B. It has been traced even farther on other photographs.
These rays do not appear to radiate always from the center of a crater. All of the most conspicuous originate outside the rim, which is a queer thing. Two of the very bright ones run parallel to each other for hundreds of miles, which is another queer performance. These characteristics lead astronomers away from the meteor-splash explanation. They believe there is a better evidence suggesting that the rays are cracks in the surface. These cracks are thought to be part of the volcanic effects, as are the craters themselves. It is reasoned that eruptions which opened up craters over 50 miles in diameter, and in some cases over 100 miles in diameter, must have been of such proportions that the surrounding surface for many miles around may easily have been split or cracked. A crater 100 miles in diameter would vent prodigious quantities of gas and molten matter. To need such a vent the pressure must have been tremendous. This pressure is supposed to have cracked the surface, the cracks radiating from the approximate location of the crater.
A singular thing about these rays, and one which bothered for years, is the fact that they become almost invisible when the illumination is oblique. Only when the Sun shines from nearly overhead do we see them clearly. W. H. Pickering, one of our foremost American astronomers, concluded after thinking about it for years that the rays are cracks or crevices filled with white snow. When the light is oblique, it does not reach much of the snow down in the crevices, but when it is overhead, it illuminates all of the snow and we are able to discern the crevice.
A condition which is hard for us Earthians to realize is that sunlight and shadow are very different on Moon than on Earth, due to the fact that Moon substantially has no atmosphere. On Moon the Sun shines with a terrific steely-blue glare in an inky-black sky. A11 the stars are as bright as at midnight. There is no diffusion of light which an atmosphere would afford. Stepping from sunshine into shadow means stepping from an in- tense glare into pitch blackness. One's experience would be like stepping from the fiercely hot rays of a steel-blue Sun, where no breeze ever blows and no clouds ever temper the light and heat, to a freezing, silent, and black cold.
The lunar day is 14 of our days. During the long nights the temperature must go down to unimaginable levels, probably 300 to 400 degrees below zero Fahrenheit. The days accumulate enough heat in excess of what is lost by radiation to raise the temperature of the soil to something like 40 degrees Fahrenheit. The snow that Pickering believes he sees is untold ages old, if it is snow. It fell when Moon had an atmosphere, and that was a long, long time ago.
If Pickering is right and it is snow, then it follows that there must be ice. If ice and snow exist, there may be a little moisture straying around in a frozen state which a peculiar form of vegetation might get hold of. If vegetation exists, it represents the last remnant of life. If Moon's is an example of what Earth's experience is to be, some tough and virile plant life will be the last survivor on Earth. But instead of being a bleak and jagged cinder with no evidence of animal life, Earth will probably offer innumerable works in imperishable stone that will evidence man's long residence. No observer has suggested a single bit of evidence of any sign of life ever having been on Moon that was higher than a crude vegetation.
From Moon, Earth would appear a great, bright orb four times as large as Moon appears to us and of somewhat greater light value. Sun would be the same in size but would be a fiercely bright, sharply defined disk in a black sky. All the stars would appear as they do to us, except that they would shine night and day, since there is no atmosphere to difluse the Sun's light and obliterate all lesser lights.
One of the intensely impressive sights in connection with Moon is an occultation of a bright fixed star. When the occultation is just short of being total, the fixed star is seen to pop in and out of visibility. The reason for this popping or snapping in and out is that the fixed star is so far away that it has no visible area and is a mere point of light. The mountains and valleys on Moon, therefore, require practically no time to cut the light ofi or to let it pass. The slightest movement does one or the other, and so we see the star shining brightly one instant and gone in the next and back again in the next.
Those who are inclined toward the unraveling of mysteries and the reconciling of complicated evidence will derive a lot of pleasure and will familiarize themselves with the geography of Moon by comparing Figure 4 with Figure 3 and locating in Figure 4 the great crater Clavius, the craters Tycho and Longomontanus, the Straight Wall in Mare Nubium, all as they are marked on Figure 3. Still another opportunity is to locate on Figure 6 the Mare Imbrium, the strait between it and Mare Serenitatis, and the crater almost in the latter with the small bright spot which is supposed to be emitting fumes. Anyone doing this once will thereafter know what is what when he looks at the Moon.
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