A Popular History of Astronomy During the Nineteenth Century / Fourth Edition

by Agnes M. Clerke

Melloni was the first to get undeniable heating effects from moonlight. His experiments, made on Mount Vesuvius early in 1846,[943] were repeated with like result by Zantedeschi at Venice four years later. A rough measure of the intensity of those effects was arrived at by Piazzi Smyth at Guajara, on the Peak of Teneriffe, in 1856. At a distance of fifteen feet from the thermomultiplier, a Price's candle was found to radiate just twice as much heat as the full moon.[944] Then, after thirteen years, in 1869-72, an exact and extensive series of observations on the subject were made by the present Earl of Rosse. The lunar radiations, from the first to the last quarter, displayed, when concentrated with the Parsonstown three-foot mirror, appreciable thermal energy, increasing with the phase, and largely due to "dark heat," distinguished from the quicker-vibrating sort by inability to traverse a plate of glass. This was supposed to indicate an actual heating of the surface, during the long lunar day of 300 hours, to about 500° F.[945] (corrected later to 197°),[946] the moon thus acting as a direct radiator no less than as a reflector of heat. But the conclusion was very imperfectly borne out by Dr. Boeddicker's observations with the same instrument and apparatus during the total lunar eclipse of October 4, 1884.[947] This initial opportunity of measuring the heat phases of an eclipsed moon was used with the remarkable result of showing that the heat disappeared almost completely, though not quite simultaneously, with the light. Confirmatory evidence of the extraordinary promptitude with which our satellite parts with heat already to some extent appropriated, was afforded by Professor Langley's bolometric observations at Allegheny of the partial eclipse of September 23, 1885.[948] Yet it is certain that the moon sends us a perceptible quantity of heat on its own account, besides simply throwing back solar radiations. For in February, 1885, Professor Langley succeeded, after many fruitless attempts, in getting measures of a "lunar heat-spectrum." The incredible delicacy of the operation may be judged of from the statement that the sum-total of the thermal energy dispersed by his rock-salt prisms was insufficient to raise a thermometer fully exposed to it one-thousandth of a degree Centigrade! The singular fact was, however, elicited that this almost evanescent spectrum is made up of two superposed spectra, one due to reflection, the other, with a maximum far down in the infra-red, to radiation.[949] The corresponding temperature of the moon's sunlit surface Professor Langley considers to be about that of freezing water.[950] Repeated experiments having failed to get any thermal effects from the dark part of the moon, it was inferred that our satellite "has no internal heat sensible at the surface"; so that the radiations from the lunar soil giving the low maximum in the heat-spectrum, "must be due purely to solar heat which has been absorbed and almost immediately re-radiated." Professor Langley's explorations of the terra incognita of immensely long wave-lengths where lie the unseen heat-emissions from the earth into space, led him to the discovery that these, contrary to the received opinion, are in good part transmissible by our atmosphere, although they are completely intercepted by glass. Another important result of the Allegheny work was the abolition of the anomalous notion of the "temperature of space," fixed by Pouillet at -140° C. For space in itself can have no temperature, and stellar radiation is a negligible quantity. Thus, it is safe to assume "that a perfect thermometer suspended in space at the distance of the earth or moon from the sun, but shielded from its rays, would sensibly indicate the absolute zero,"[951] ordinarily placed at -273° C.

A "Prize Essay on the Distribution of the Moon's Heat" (The Hague), 1891, by Mr. Frank W. Very, who had taken an active part in Professor Langley's long-sustained inquiry, embodies the fruits of its continuation. They show the lunar disc to be tolerably uniform in thermal power. The brighter parts are also indeed hotter, but not much. The traces perceived of a slight retention of heat by the substances forming the lunar surface, agreed well with the Parsonstown observations of the total eclipse of the moon, January 28, 1888.[952] For they brought out an unmistakable divergence between the heat and light phases. A curious decrease of heat previous to the first touch of the earth's shadow upon the lunar globe remains unexplained, unless it be admissible to suppose the terrestrial atmosphere capable of absorbing heat at an elevation of 190 miles. The probable range of temperature on the moon was discussed by Professor Very in 1898.[953] He concluded it to be very wide. Hotter than boiling water under the sun's vertical rays, the arid surface of our dependent globe must, he found, cool in the 14-day lunar night to about the temperature of liquid air.

Although that fundamental part of astronomy known as "celestial mechanics" lies outside the scope of this work, and we therefore pass over in silence the immense labours of Plana, Damoiseau, Hansen, Delaunay, G. W. Hill, and Airy in reconciling the observed and calculated motions of the moon, there is one slight but significant discrepancy which is of such importance to the physical history of the solar system, that some brief mention must be made of it.

Halley discovered in 1693, by examining the records of ancient eclipses, that the moon was going faster then than 2,000 years previously—so much faster, as to have got ahead of the place in the sky she would otherwise have occupied, by about two of her own diameters. It was one of Laplace's highest triumphs to have found an explanation of this puzzling fact. He showed, in 1787, that it was due to a very slow change in the ovalness of the earth's orbit, tending, during the present age of the world, to render it more nearly circular. The pull of the sun upon the moon is thereby lessened; the counter-pull of the earth gets the upper hand; and our satellite, drawn nearer to us by something less than an inch each year,[954] proportionately quickens her pace. Many thousands of years hence the process will be reversed; the terrestrial orbit will close in at the sides, the lunar orbit will open out under the growing stress of solar gravity, and our celestial chronometer will lose instead of gaining time.

This is all quite true as Laplace put it; but it is not enough. Adams, the virtual discoverer of Neptune, found with surprise in 1853 that the received account of the matter was "essentially incomplete," and explained, when the requisite correction was introduced, only half the observed acceleration.[955] What was to be done with the remaining half? Here Delaunay, the eminent French mathematical astronomer, unhappily drowned at Cherbourg in 1872 by the capsizing of a pleasure-boat, came to the rescue.[956]

It is obvious to anyone who considers the subject a little attentively, that the tides must act to some extent as a friction-brake upon the rotating earth. In other words, they must bring about an almost infinitely slow lengthening of the day. For the two masses of water piled up by lunar influence on the hither and farther sides of our globe, strive, as it were, to detach themselves from the unity of the terrestrial spheroid, and to follow the movements of the moon. The moon, accordingly, holds them against the whirling earth, which revolves like a shaft in a fixed collar, slowly losing motion and gaining heat, eventually dissipated through space.[957] This must go on (so far as we can see) until the periods of the earth's rotation and of the moon's revolution coincide. Nay, the process will be continued—should our oceans survive so long—by the feebler tide-raising power of the sun, ceasing only when day and night cease to alternate, when one side of our planet is plunged in perpetual darkness and the other seared by unchanging light.

Here, then, we have the secret of the moon's turning always the same face towards the earth. It is that in primeval times, when the moon was liquid or plastic, an earth-raised tidal wave rapidly and forcibly reduced her rotation to its present exact agreement with her period of revolution. This was divined by Kant[958] nearly a century before the necessity for such a mode of action presented itself to any other thinker. In a weekly paper published at Königsberg in 1754, the modern doctrine of "tidal friction" was clearly outlined by him, both as regards its effects actually in progress on the rotation of the earth, and as regards its effects already consummated on the rotation of the moon—the whole forming a preliminary attempt at what he called a "natural history" of the heavens. His sagacious suggestion, however, remained entirely unnoticed until revived—it would seem independently—by Julius Robert Mayer in 1848;[959] while similar, and probably original, conclusions were reached by William Ferrel of Allensville, Kentucky, in 1858.[960]

Delaunay was not then the inventor or discoverer of tidal friction; he merely displayed it as an effective cause of change. He showed reason for believing that its action in checking the earth's rotation, far from being, as Ferrel had supposed, completely neutralised by the contraction of the globe through cooling, was a fact to be reckoned with in computing the movements, as well as in speculating on the history, of the heavenly bodies. The outstanding acceleration of the moon was thus at once explained. It was explained as apparent only—the reflection of a real lengthening, by one second in 100,000 years, of the day. But on this point the last word has not yet been spoken.

Professor Newcomb undertook in 1870 the onerous task of investigating the errors of Hansen's Lunar Tables as compared with observations prior to 1750. The results, published in 1878,[961] proved somewhat perplexing. They tend, in general, to reduce the amount of acceleration left unaccounted for by Laplace's gravitational theory, and proportionately to diminish the importance of the part played by tidal friction. But, in order to bring about this diminution, and at the same time conciliate Alexandrian and Arabian observations, it is necessary to reject as total the ancient solar eclipses known as those of Thales and Larissa. This may be a necessary, but it must be admitted to be a hazardous expedient. Its upshot was to indicate a possibility that the observed and calculated values of the moon's acceleration might after all prove to be identical; and the small outstanding discrepancy was still further diminished by Tisserand's investigation, differently conducted, of the same Arabian eclipses discussed by Newcomb.[962] The necessity of having recourse to a lengthening day is then less pressing than it seemed some time ago; and the effect, if perceptible in the moon's motion, should, M. Tisserand remarked, be proportionately so in the motions of all the other heavenly bodies. The presence of the apparent general acceleration that should ensue can be tested with most promise of success, according to the same authority, by delicate comparisons of past and future transits of Mercury.