Popular Science Monthly/Volume 3/August 1873/Lunar Temperatures
POETS have so long sung of the cold, chaste Moon, pallid with weariness of her long watch upon the Earth (according to the image used alike by Wordsworth and Shelley), that it seems strange to learn from science that the full moon is so intensely hot that no creature known to us could long endure contact with her heated surface. Such is the latest news which science has brought us respecting our satellite. The news is not altogether unexpected; in fact, reasoning had shown, long before the fact had been demonstrated, that it must be so. The astronomer knows that the surface of the moon is exposed during the long lunar day, lasting a fortnight of our terrestrial time, to the rays of a sun as powerful as that which gives us our daily heat. Without an atmosphere to temper the sun's heat as ours does—not, indeed, by impeding the passage of the solar rays, but by bearing aloft the cloud-veil which the sun raises from our oceans—the moon's surface must become intensely hot long before the middle of the lunar day. Undoubtedly the want of an atmosphere causes the moon's heat to be rapidly radiated away into space. It is our atmosphere which causes a steady heat to prevail on our earth. And at the summits of lofty mountains, where the atmosphere is rare, although the mid-day heat is intense, yet so rapidly does the heat pass away that snow crowns forever the mountain-heights. Yet, although the moon's heat must pass away even more rapidly, this does not prevent the heating of the moon's actual surface, any more than the rarity of the air prevents the Alpine traveller from feeling the action of the sun's direct heat even when the air in shadow is icily cold. Accordingly, Sir John Herschel long since pointed out that the moon's surface must be heated at lunar mid-day—or rather, at the time of lunar mid-heat, corresponding to about two o'clock in our afternoon—to a degree probably surpassing the heat of boiling water.
Such, in point of fact, has now been proved to be the case. The Earl of Rosse has shown, by experiments which need not here be described, that the moon not only reflects heat to the earth (which, of course, must be the case), but that she gives out heat by which she has been herself warmed. The distinction may not perhaps appear clear at first sight to every reader, but it may easily be explained and illustrated. If, on a bright summer's day, we take a piece of smooth, but not too well polished, metal, and by means of it reflect the sun's light upon the face, a sensation of heat will be experienced; this is reflected sun-heat: but if we wait while so holding the metal until the plate has become quite hot under the solar rays, we shall recognize a sensation of heat from the mere proximity of the plate to the face, even when the plate is so held as not to reflect sun-heat. We can in succession try—first, reflected heat alone, before the metal has grown hot; next, the heat which the metal gives out of itself when warmed by the sun's rays; and, lastly, the two kinds of heat together, when the metal is caused to reflect sun-heat, and also (being held near the face) to give out a sensible quantity of its own warmth. What Lord Rosse has done has been to show that the full moon sends earthward both kinds of heat; she reflects solar heat just as she reflects solar light, and she also gives out the heat by which her own surface has been warmed.
It may perhaps occur to the reader to inquire how much heat we actually obtain from the full moon. There is a simple way of viewing the matter. If the full moon were exactly as hot as boiling water, we should receive from her just as much heat (leaving the effect of our atmosphere out of account) as we should receive from a small globe as hot as boiling water, and at such a distance as to look just as large as the moon does. Or a disk of metal would serve equally well. Now, the experiment may be easily tried. A bronze halfpenny is exactly one inch in diameter, and as the moon's average distance is about 111 times her own diameter, a halfpenny at a distance of 111 inches, or three yards and three inches, looks just as large as the moon. Now let a halfpenny be put in boiling water for a while, so that it becomes as hot as the water; then that coin taken quickly and set three yards from the observer will give out, for the few moments that its heat remains appreciably that of boiling water, as much heat to the observer as he receives from the full moon supposed to be as hot as boiling water. Or a globe of thin metal, one inch in diameter and full of water at boiling-heat, would serve as a more constant artificial moon in respect of heat-supply. It need not be thought remarkable, then, if the heat given out by the full moon is not easily measured, or even recognized. Imagine how little the cold of a winter's day would be relieved by the presence, in a room no otherwise warmed, of a one-inch globe of boiling water, three yards away! And, by-the-way, we are here reminded of an estimate by Prof. C. B. Smyth, resulting from observations made on the moon's heat during his Teneriffe experiments. He found the heat equal to that emitted by the hand at a distance of three feet.
But, after all, the most interesting results flowing from the recent researches are those which relate to the moon herself. We cannot but speculate on the condition of a world so strangely circumstanced that a cold more bitter than that of our arctic nights alternates with a heat exceeding that of boiling water. It is strange to think that the calm-looking moon is exposed to such extraordinary vicissitudes. There can scarcely be life in any part of the moon—unless it be underground life, bike that of the Modoc Indians (we commend this idea specially to the more ardent advocates of Brewsterian ideas respecting other worlds than ours). And yet there must be a singularly active mechanical process at work in yonder orb. The moon's substance must expand and contract marvellously as the alternate waves of heat and cold pass over it. The material of that crater-covered surface must be positively crumbling away under the effects of these expansions and contractions. The most plastic terrestrial substances could not long endure such processes, and it seems altogether unlikely that any part of the moon's crust is at all plastic. Can we wonder if, from time to time, astronomers tell us of apparent changes in the moon—a wall sinking here, or a crater vanishing elsewhere? The wonder rather is, that the steep and lofty lunar mountains have not been shaken long since to their very foundations.
Our moon presents, in fact, a strange problem for our investigation. It is gratifying to us terrestrials to regard her as a mere satellite of the earth, but in reality she deserves rather to be regarded as companion planet. She follows a path round the sun which so nearly resembles that pursued by the earth, in shape as well as in extent, that, if the two paths were traced down on a quarto sheet, it would not be easy to distinguish one from the other. Our earth is simply the largest, while the moon is the smallest of that inner family of worlds over which the sun bears special sway, nor does Mercury exceed the moon to so great a degree in mass and in volume as the earth or Venus exceeds Mercury. Yet the moon, with her surface of 14,000,000 square miles, seems to be beyond a doubt a mere desert waste, without air or water, exposed to alternations of heat and cold which no living creature we are acquainted with could endure; and notwithstanding her position as an important member of the solar system, as well as the undoubted fact that in her motions she obeys the sun in preference to the earth, she has nevertheless been so far coerced by the earth's influence as to be compelled to turn always the same face toward her larger companion orb, so that not a ray from the earth ever falls upon fully 5,000,000 square miles of the farther lunar hemisphere. A waste of matter here, we might say, and a waste of all the energy which is represented by the moon's motions, did we not remember that we can see but a little way into the plan of Creation, and that what appears to us waste may in reality be an essential and important part of the great scheme of Nature.—Spectator.