Page:Popular Science Monthly Volume 34.djvu/170

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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,