The New Astronomy

by S. P. Langley

FIG. 70.—VESUVIUS AND NEIGHBORHOOD OF NAPLES.

To see this more clearly, we may consider that in general we find that the early stages of cosmical life are characterized by great heat; a remark of the truth of which the sun itself furnishes the first and most obvious illustration. Then come periods which we appear to have seen exemplified in Jupiter, where the planet is surrounded by volumes of steam-like vapor, through which we may almost believe we recognize the dull glow of not yet extinguished fires; then times like those which our earth passed through before it became the abode of man; and then the times in which human history begins. But if this process of the gradual loss of heat go on indefinitely, we must yet come to still another era, when the planet has grown too cold to support life, as it was before too hot; and this condition, in the light of some very recent investigations, it seems probable we have now before us on the moon.

We have, it is true, been taught until very lately that the side of the moon turned sunward would grow hotter and hotter in the long lunar day, till it reached a temperature of two hundred to three hundred degrees Fahrenheit, and that in the equally long lunar night it would fall as much as this below zero. But the evidence which was supposed to support this conclusion as to the heat of the lunar day is not supported by recent experiments of the writer; and if these be trustworthy, certain facts appear to him to show that the temperature of the moon’s surface, even under full perpetual sunshine, must be low,—and this because of the absence of air there to keep the stored sun-heat from being radiated away again into space.

As we ascend the highest terrestrial mountains, and get partly above our own protecting blanket of air, things do not grow hotter and hotter, but colder and colder; and it seems contrary to the teachings of common sense to believe that if we could ascend higher yet, where the air ceases altogether, we should not find that it grew colder still. But this last condition (of airlessness) is the one which does prevail beyond a doubt in the moon, on whose whole surface, then, there must be (unless there are sources of internal heat of which we know nothing) conditions of temperature which are an exaggeration of those we experience on the summit of a very lofty mountain, where we have the curious result that the skin may be burned under the solar rays, while we are shivering at the same time in what the thermometer shows is an arctic cold.

We have heard of this often; but a personal experience so impressed the fact on me that I will relate it for the benefit of the reader, who may wish to realize to himself the actual conditions which probably exist in the airless lunar mountains and plains we are looking at. He cannot go there; but he may go if he pleases, as I have done, to the waterless, shadeless waste which stretches at the eastern slope of the Sierra Nevadas (a chain almost as high and steep as the lunar Apennines), and live some part of July and August in this desert, where the thermometer rises occasionally to one hundred and ten degrees in the shade, and his face is tanned till it can tan no more, and he appears to himself to have experienced the utmost in this way that the sun can do.

The sky is cloudless, and the air so clear that all idea of the real distance and size of things is lost. The mountains, which rise in tremendous precipices above him, seem like moss-covered rocks close at hand, on the tops of which, here and there, a white cloth has been dropped; but the “moss” is great primeval forests, and the white cloths large isolated snow-fields, tantalizing the dweller in the burning desert with their delusive nearness. When I climbed the mountains, at an altitude of ten thousand feet I already found the coolness delicious, but at the same time (by the strange effect I have been speaking of) the skin began to burn, as though the seasoning in the desert counted for nothing at all; and as the air grew thinner and thinner while I mounted still higher and higher, though the thermometer fell, every part of the person exposed to the solar rays presented the appearance of a recent severe burn from an actual fire,—and a really severe burn it was, as I can testify,—and yet all the while around us, under this burning sun and cloudless sky, reigned a perpetual winter which made it hard to believe that torrid summer still lay below. The thinner the air, then, the colder it grows, even where we are exposed to the sun, and the lower becomes the reading of the thermometer. Now, by means of suitable apparatus, it was sought by the writer to determine, while at this elevation of fifteen thousand feet, how great the fall of temperature would be if the thin air there could be removed altogether; and the result was that the thermometer would under such circumstances fall, at any rate, below zero in the full sunshine.

FIG. 71.—PTOLEMY AND ARZACHEL.

Of course, all this applies indirectly to the moon, above whose surface (if these inferences be correct) the mercury in the bulb of a thermometer would probably freeze and never melt again during the lunar day (and still less during the lunar night),—a conclusion which has been reached through other means by Mr. Ericsson,—and whose surface itself cannot be very greatly warmer. Other and direct measures of the lunar heat are still in progress while this is being written, but their probable result seems to be already indicated: it is that the moon’s surface, even in perpetual sunshine, must be forever cold. Just how cold, is still doubtful; and it is not yet certain whether ice, if once formed there, could ever melt.