_Lunar Changes._—The question as to whether physical changes are occurring in the surface-formations of our satellite is one which offers attractive inducements to telescopic observers. Though the Moon appears to have passed the active state, it is very possible that trivial alterations continue to affect some of her features. In April 1787 Sir W. Herschel wrote:—“I perceive three volcanoes in different places of the dark part of the new Moon. Two of them are already nearly extinct, or otherwise in a state of going to break out; the third shows an eruption of fire or luminous matter.” Schröter, however, was correctly of opinion that these appearances were due to reflected light from the Earth falling upon elevated spots of the Moon having unusual capacity to return it. Schröter himself thought he detected sudden changes in 1791. He says that, on the 30th of December, at 5^h P.M., with a 7-foot reflector magnifying 161 times, he perceived the commencement of a small crater on the S.W. declivity of the volcanic mountain in the Mare Crisium, having a shadow of at least 2′ 5″. On the 11th of January, 1792, at 5^h 20^m P.M., on looking at the place again he could see neither the new crater nor its shadow. In this case the disappearance was doubtless an apparent one, merely due to the reversed illumination under which the object was examined in the interval of 12 days.
Many other observers besides Herschel have been struck with the brightness of certain spots situated in the opaque region of the lunar disk; but there is no doubt the cause has been uniformly one and the same, viz. the highly reflective properties of some of the mountains (notably of one named Aristarchus), which are distinctly visible as luminous spots amid the relatively dark regions surrounding them. They afford no certain evidence of existing volcanic energy, and in the light of modern researches such an idea cannot be entertained.
On June 10, 1866, Temple noticed a remarkable light appearance, agreeing with the position of Aristarchus, upon the dark side of the Moon, faintly illuminated by earthshine. The object did not exhibit a faint white light analogous to that of other craters in the dark side, but it was star-like, diffused, in colour reddish yellow, and evidently dissimilar to other bright spots. He wrote, in reference to this matter:—“Of course I am far from surmising a still active chemical outbreak, as such an outbreak supposes water and an atmosphere, both of which are universally allowed not to exist on the Moon, so that the crater-forming process can only be thought of as a dry, chemical, although warm one.”