Now, the thermopile cannot, of course, discriminate directly between the two portions of the lunar heat; but to some extent it does enable us to do so indirectly, since they vary in quite a different way with the moon's age. The simple reflected heat must follow the same law as moonlight, and come to its maximum at full moon. The radiated heat, on the other hand, will reach its maximum when the average temperature of that part of the moon's surface turned toward the earth is highest; and this must be some time after full moon, for the same sort of reasons that make the hottest part of a summer's day come two or three hours after noon.
The conclusion early reached by Lord Rosse was that nearly all the lunar heat belonged to the second category—dark heat radiated from the moon's warmed surface, the reflected portion being comparatively small—and he estimated that the temperature of the hottest parts of the moon's surface must run as high as 500° F.; well up toward the boiling-point of mercury. Since the lunar day is a whole month long, and there are never any clouds in the lunar sky, it is easy to imagine that along toward two or three o'clock in the lunar afternoon (if I may use the expression), the weather gets pretty hot; for when the sun stands in the lunar sky as it does at Boston at two P.M., it has been shining continuously for more than two hundred hours. On the other hand, the coldest parts of the moon's surface, when the sun has only just risen after a night of three hundred and forty hours, must have a temperature more than a hundred degrees below zero.
Lord Rosse's later observations modified his conclusions, to some extent, showing that he had at first underestimated the percentage of simple reflected heat, but without causing him to make any radical change in his ideas as to the maximum heat of the moon's surface.
For some time, however, there has been a growing skepticism among astronomers, relating not so much to the correctness of his measures as to the computations by which he inferred the high percentage of obscure radiated beat compared with the reflected heat, and so deduced the high temperature of lunar noon.
Professor Langley, who is now engaged in investigating the subject, finds himself compelled to believe that the lunar surface never gets even comfortably warm—because it has no blanket. It receives heat, it is true, from the sun, and probably some twenty-five or thirty per cent. more than the earth, since there are no clouds and no air to absorb a large proportion of the incident rays; but, at the same time, there is nothing to retain the heat, and prevent the radiation into space as soon as the surface begins to warm. We have not yet the data to determine exactly how much the temperature of the lunar rocks would have to be raised above the absolute zero (-273° C. or -459° F.) in order that they might throw off into space as much heat in a second as they would get from the sun in a second. But Professor Langley's observations, made on Mount Whitney at an elevation of fifteen thousand feet, when the barometer stood at seventeen inches (indicating that about fifty-seven per cent. of the air was still above him), showed that rocks exposed to the perpendicular rays of the sun were not heated to any such extent as those at the base of the mountain similarly exposed; and the difference was so great as to make it almost certain that a mass of rock not covered by a reasonably dense atmosphere could never attain a temperature of even 200° or 300° F. under solar radiation, however long continued.
It must, in fact, be considered at present extremely doubtful whether any portion of the moon's surface ever reaches a temperature as high as -100°.
The subject, undoubtedly, needs further investigation, and it is now receiving it. Professor Langley is at work upon it with new and specially constructed apparatus, including a "bolometer" so sensitive that, whereas previous experimenters have thought themselves fortunate if they could get deflections of ten or twelve galvanometric divisions to work with, he easily obtains three or four hundred. We have no time or space here to describe Professor Langley's "bolometer;" it must suffice to say that it seems to stand to the thermopile much as that does to the thermometer. There is good reason to believe that its inventor will be able to advance our knowledge of the subject by a long and important step; and it is no breach of confidence to add that so far, although the research is not near completion yet, everything seems to confirm the belief that the radiated heat of the moon, instead of forming the principal part of the heat we get from her, is relatively almost insignificant, and that the lunar surface now never experiences a thaw under any circumstances.
Since the superstition as to the moon's influence upon the wind and weather is so widespread and deep seated, a word on that subject may be in order. In the first place, since the total heat received from the moon, even according to the highest determination (that of Smyth), is not so much as 0.00001 of that received from the sun, and since the only hold the moon has on the earth's weather is through the heat she sends us (I ignore here the utterly insignificant atmospheric tide), it follows necessarily that her influence must be very trifling. In the next place, all carefully collated observations show that it is so, and not only trifling, but generally absolutely insensible.
For example, different investigators have examined the question of nocturnal cloudiness at the time of full moon, there being a prevalent belief that the full moon "eats up" light clouds. On comparing thirty or forty years' observations at each of several stations (Greenwich. Paris, etc.), it is found that there is no ground for the belief. And so in almost every case of imagined lunar meteorological influence. As to the coincidence of weather changes with changes of the moon, it is enough to say that the idea is absolutely inconsistent with that progressive movement of the "weather" across the country from west to east, with which the Signal Service has now made us all so familiar.
Princeton, April 12, 1884.