Popular Science Monthly/Volume 34/December 1888/New Light on a Lunar Mystery
|NEW LIGHT ON A LUNAR MYSTERY.|
EVERY possessor of a telescope knows that among the mountains of the moon there are some to which the name of "shining mountains" seems peculiarly applicable. The most celebrated of these is the huge extinct volcano Aristarchus, the slopes of whose crater possess such extraordinary reflective power that it is visible on the night-side of the moon by virtue of the comparatively faint light received from the earth. Another famous bright mountain on the moon is Proclus, which rears its crest high above the eastern shore of the so-called Crisian Sea. With a telescope I have seen Proclus glittering above the brownish plains surrounding it, in the middle of a summer afternoon, when, to the naked eye, the moon appeared as a faint silvery disk, half blended with the blue of the sky. There are other natural features of the moon's surface which shine with extraordinary brightness, the most conspicuous being the systems of long "rays," radiating from such crater-rings as Tycho and Copernicus.
But in addition to these long-known and easily recognized objects, there have occasionally been seen upon the moon certain bright points, which are even more curious and mysterious than the shining mountains. The earliest observation of this kind appears to have been made by Herschel in 1783. It was repeated by him in 1787, when he did not hesitate to report, in a communication to the Royal Society, that he had discovered three lunar volcanoes in a state of eruption. Astronomers have been considerably puzzled ever since to account for Herschel's statement. Nobody could question the accuracy of his observation, so far as the power of his telescope enabled him to carry it. At the same time, few, if any, especially in recent times, were willing to admit that that prince of telescopists had really seen volcanoes in action upon the moon. The complete absence of any evidence that volcanic activity did not cease upon our satellite ages upon ages ago militated too strongly against Herschel's assertion. The general conclusion finally was, that Herschel had been misled by the extraordinary brightness of some of the shining mountains which I have just described. It remained almost the only serious blot upon Herschel's record as an observer. He had described the appearance of the supposed eruption too carefully to admit any question as to his meaning. And yet, it seemed, a mere tyro in astronomical observation could hardly be deceived in such a manner, much less the most famous astronomer of his time.
But just now new light has been thrown upon the mystery, and it comes from that center of astronomical interest, the Lick Observatory. Prof. Holden believes that he has discovered, if not one of the same objects described by Herschel, a phenomenon of the same kind. It is hardly necessary to say that Prof. Holden has not discovered a lunar volcano in action, but the extraordinary appearance that he has seen sufficiently accounts for Herschel's mistake. It will be best to quote the Lick Observatory director's own words from his letter on the subject to "The Observatory," an English astronomical journal:
"I have never been able to understand how Herschel, the keenest of observers, could have been deceived in this observation until the night of July 15th of this year, when I was looking at the moon with the great telescope. At the southern extremity of the Alps, in the dark portion of the disk, not far from the terminator, I saw an illumination of the crest of a high peak which was extraordinarily and incredibly bright. . . . No part of this illumination seemed less bright than a first-magnitude star, and, taken altogether, it was the brightest object I have ever seen in the sky. It was apparently ten times as bright as neighboring portions of the moon's surface. Its yellow light was tinged in places with the purple due to the secondary spectrum of the objective; and, viewed as a whole, it presented the appearance of a vast conflagration—something quite foreign to the brilliant white of the rest of the moon's surface.
"It would have required no stretch of the imagination to have supposed it to be a tremendous eruption of a range of lunar volcanoes. . . . Observations on this and the succeeding nights showed that it was in fact due to a specially brilliant and favorable illumination of a mountain-ridge near the southern termination of the lunar Alps.
"I have now no doubt that the observation of Sir William Herschel referred to similar appearances."
I may add that there are at least two other recorded apparitions of this sort which were seen in that neighborhood, but evidently not in exactly the same place. The first was observed by Schroeter, the German selenographer, in 1788. He saw in the shadow of the great range of the lunar Alps, at the eastern foot of the mountains, a bright point, as brilliant as a fifth-magnitude star, which disappeared after he had watched it for fifteen minutes. Subsequently, when the region where this light appeared had become fully illuminated by the rising sun, Schroeter perceived, where the light had been, a round shadow on the surface of the moon, which was sometimes gray and sometimes black. Nothing more was ever seen of the light, so far as any record informs us, until 1865, when Grover, an English observer, caught sight of it again, under circumstances similar to those of its first apparition, and watched it for half an hour, when it once more disappeared. It should be said that, in the case of Dr. Gerling's observation, referred to by Prof. Holden, a "small, round, isolated, conical mountain" was found in the place where the light had been, on the evening following its appearance. It is altogether probable that the gray or black spot perceived by Schroeter was the shadow of a similar mountain, for it is well known that some of the lunar mountains and hills are hardly visible at all except when lateral illumination indicates their position and form by means of the shadows.
Herschel thought he had seen three active volcanoes. If Prof. Holden's discovery accounts for one of these, it is possible that the observations I have just described may give a clew to the others. The phenomenon seen by Schroeter and Grover was located fifty or sixty miles north of the point where Prof. Holden beheld the extraordinary blaze of light last July, and at a point where the mountains, drawing around a culminating peak, confront with tremendous buttresses the broad level of the Mare Imbrium.
The objection has been made by Messrs. Elger and Williams, two competent English observers, that Herschel's volcanoes can not be identical with the glittering peaks seen by either Holden or Gerling, because the latter were observed close to the line of sunrise, where the morning rays touched them, while the phenomena that attracted Herschel's attention were situated far within that part of the disk where the only light came from the earth. But Prof. Holden does not say that the illumination he witnessed was identical in place with those recorded by Herschel, but simply that it was identical in kind. Besides, it must be remembered that, if these luminous appearances are due to peculiar angles of reflection, a similar effect must be produced whether the reflecting surfaces are presented to the sunlight or only to the earth-shine. The difference would be simply in the degree of brightness of the phenomena.
But while the discovery with the Lick telescope may account for Herschel's mistake, it does not clear up the mystery of the cause of these extraordinary lights. In every case quoted above, the illumination was evidently very much greater than that of Aristarchus, the most brilliant of the shining mountains. Proctor estimated that the reflective power of Aristarchus must be equal to that of new-fallen snow. But the mountain-crest observed by Prof. Holden blazed with a dazzling brilliancy that it would be difficult to account for except upon the theory that nearly all of the sunlight falling upon it was reflected to the observer's eye. Reflection at a particular angle from vast sheets of ice, as smooth as glass, might be suggested as the cause of such a display, but how could ice be there without water or atmosphere? The suggestion that has been offered to account for the brightness of Aristarchus and the "ray" systems, namely, that they are composed of metallic dikes and masses which, for various reasons, have escaped oxidation, is recalled by the phenomenon in question. Upon that view we might have to assume that these luminous points indicated the existence of tremendous crystallized masses, with polished surfaces, throwing back the glare of the sunshine like mirrors. But then we should not be far from the view set forth in Richard Adams Locke's celebrated "Moon Hoax," that some of the glittering eminences on the moon are nothing less than enormous quartz-crystals, whose dimensions are measured by miles instead of inches!
The fact that the apparitions of extraordinary luminosity are confined to comparatively very small areas, and are visible only for a short time and at long intervals, must be taken as an indication that the reflecting surfaces to which they are due must be of such a nature, and so disposed, that they can reflect the sun's light to us only when presented at a particular angle to our line of sight; just as a piece of looking-glass, exposed to the sun at a distance, suddenly darts a piercing ray when the eye comes within the plane of reflection. That these surfaces are the flanks of mountains is in the highest degree probable, and this but serves to heighten the impression of their extraordinary nature.
The rapid appearances and disappearances, and the long periods of invisibility, are readily accounted for by the various librations of the moon, whereby it presents its disk to us at a continually varying angle, as it swims along in its "squirming orbit," under the conflicting attractions of the sun and the earth.