Arthur Zagat - The Golden Age of Science Fiction Volume IX

“The moon’s distance from the earth is generally given as being about 238,000 miles, but this is its mean distance. When farthest away from our world its distance is about 260,000 miles, but at its nearest it is less than 220,000 miles distant. This difference of course arises from the eccentricity of the moon’s orbit, and it explains why we sometimes see the moon a trifle larger than it appears at other times. By this I mean that it really is seen larger, because it is closer to us. But you have no doubt often noticed that when the moon is near the horizon it seems to be very large indeed. This apparent increase of size is, however, an illusion, owing to our unconsciously comparing it with the apparent size of terrestrial objects.

“The surface of the moon shows evidence of very violent volcanic action having occurred in every part of it, and astronomers in the past were much puzzled to account for the excessive volcanic energy which was indicated by what they saw, as such a small globe as the moon would not, in the ordinary course of events, have ever possessed sufficient heat to have developed such violent action. A theory of later years has, however, provided a reasonable explanation. It is that the moon was at one time a part of the same mass as the earth, which became separated from it before the earth had quite cooled down and solidified into its present form, and was then gradually driven farther and farther away from the earth by natural forces. It was therefore originally as hot as the rest of the mass which formed the earth, but being formed into a smaller globe of much less gravity—only one-sixth of that of the earth—volcanic action of the same intensity as that on the earth would have a much more far-reaching effect. A force which on the earth would project volcanic lava and scoriæ a distance of three miles would, on the moon, project it a distance of eighteen miles. This accounts for the very high mountains we see on the moon, some of which are comparatively, for the size of the globe, much higher than those on the earth. It also accounts for the vast size of the lunar craters, ring-plains, and ring-mountains.

“These latter are formations quite unknown upon our earth, but on the moon they are numbered by hundreds of all sizes, from a few miles up to one hundred and fifty miles in diameter. They are large plains, roughly circular in shape, and surrounded by mountains; in a few cases the ring is in some parts a double range of mountains. Sometimes the plain (or ‘floor,’ as it is termed) is many thousands of feet below the general level of the lunar surface; in a few cases it is raised considerably above it, and in one or two instances, instead of being flat, the floor is convex. Some of the mountain rings are comparatively low, but in other cases the mountains are fifteen to twenty thousand feet in height, or even higher. Frequently a mountain rises near the centre of the floor, some rings containing more than one such mountain, whilst others have none at all.

[Illustration: Drawn by M. Wicks Plate II IDEAL VIEW OF LUNAR SCENERY

As there is no atmosphere on the moon, the sky is a dense black, and the stars shine brilliantly in the daytime. The view is a typical one, showing numerous craters and cracks, and a small ring-mountain with terracing. Ring-mountains and plains vary from a few miles to 150 miles diameter, some mountains being nearly 20,000 feet in height.]

“There are numerous instances where one mountain ring has overlapped or cut into another, thus indicating that it was a later formation; and in many cases the mountains are ‘terraced,’ [3] Those who have seen the undercliff in the Isle of Wight will be able to form some idea of the terraces of the lunar ring-mountains, as they are very similar formations. as it is termed, either owing to a series of landslips or to the rise and fall of a sea of lava, which cooled as it sank down, thus forming terraces. Small craters abound all over the surface of the moon and on the floors of the rings; cracks in the lunar surface are also numerous.

“As regards the lunar mountains, it may truly be said that we have a fairly accurate knowledge of peaks and mountains which would either be too precipitous to be climbed, or quite inaccessible to us, if we could actually land upon the moon; and the whole visible surface has been more carefully and thoroughly mapped out and studied than is the case with many parts of our own earth.

“If the moon has any atmosphere it must be so very attenuated indeed that human beings could not possibly live in it at all; but nothing has yet been detected which would enable us to say positively that any atmosphere does exist there, although there have been some indications observed which support the supposition that there may be an extremely thin air.

“Nor does it appear possible that there is any water upon its surface at the present; in fact, many astronomers are of opinion that the moon never did have any water upon it. Personally, from a study of many of the formations as seen through the telescope, it seems to me quite impossible that they could owe their existence in their present state to anything but the action of water. They present much the same appearance as formations on our own earth which we know have been fashioned by that means. There is no water upon the moon now, I think, though several large depressions are still called oceans, seas, lakes, or marshes, because at one time they were believed to be such. Probably in some of those places, if not in all, water existed millions of years ago; but ages since they must have lost it either by evaporation or by absorption into the soil.

“I will not say any more just now, but as we pass above the lunar surface I will point out a few of the natural features that may be of interest to you.”

M’Allister here paid me the compliment of saying, “Well, Professor, I always thought astronomy was a very dry and difficult subject; but your remarks were really very interesting, and quite easy to understand. There is only one thing that seemed to me rather strange as coming from a scientific man, and I would like you to explain that.”

“Certainly; if there is anything you do not quite understand, you have only to ask and I will try to clear the matter up,” I answered. “What is it you wish to know?”

“Well,” he answered, “I noticed that when you were speaking about the distance of the moon you always said it was about so far away. Why didn’t you tell us the exact distance? I’m not a scientific man by any means, but if any one were to ask me the length of a connecting rod on one of my machines I should say ‘25 inches,’ not ‘about 25 inches,’ for that would not do for a practical man!”

“It’s like this, M’Allister,” I said. “You measure things with a two-foot rule, which is something you can actually handle, and you know it is made according to a standard measure and must contain exactly 24 inches. If, however, your rule was 24-1/4 inches long, yet still divided into twenty-four equal parts, you could measure work with it just the same, but would know that every measurement was just a little bit out. If you had no possible means of obtaining another rule, you would have to put up with a little inexactitude.

“That is just the position in which astronomers are placed; they have to put up with a measure which they know is not perfectly accurate, yet it is the best which can be secured.

“Their two-foot rule, so to speak, may be the distance from the earth to the sun, or the length of the whole diameter of the earth’s orbit, and these cannot be handled like your rule; and although we know the measurements of these are nearly correct, they are not quite so. Yet the distances of the moon, planets, stars, &c., have to be measured by these rules, so it is clear we can only know those distances with a near approximation to accuracy.