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Art. VIII.-On the Moon's Figure, Rotation, and Surface.
the Rev. JAMES Glasgow, D.D.
Presented 10th October 1861,
You were, perhaps, too young to have paid much attention to a discussion, displaying more of vanity than science, which went the round of newspapers some four or five years ago.* The Moon's rotation was positively denied ; and it was urged that, because she always keeps the same face next the earth, therefore she does not rotate ; while the obvious and mathematically certain inference is, therefore she does rotate, though, as I propose to show, her rotation depends on a different cause from that of the earth. It was said the moon is fixed in relation to the earth, as if attached by a wire, or as if she were a part of the earth. But as every part of the earth rotates, so, if the moon were a part of the earth, she would share the earth's rotation. It was said this would constitute rotation about the earth's centre; but the word rotate properly applies only to a wheel-like motion within the body.
The moon's rotation is round her own centre, which, however, is constantly shifting her position or describing the lunar orbit.
To show this,-let a (Pl. IV. fig. 1) be the centre of the earth, B that of the moon, and c that of the moon after she has revolved to some distance in her orbit. Produce AB, AC, to DE. Draw the tangent da, which, after the revolution of the centre B to c, takes the position Eg. Now, as the line ad revolves to ac, the diameter id, being still directed to the earth, takes the position ke, and dr takes the position Eg, which produced cuts du in F. The moon therefore has rotated by the angle HFE, or the central angle a.
Among the questions which have been raised rather than settled by astronomers, one relates to the presence or absence of fluid matter as part of the moon's superficial mass. The telescopic observations of former times led astronomers to map out the disk of the moon, denominating the duskier parts seas, and assigning them names. This was too hasty ; but some later astronomers have been perhaps equally hasty in the opposite direction. They have asserted that the moon contains neither water nor air. The facts are that clouds do not appear, and that when the moon is about to cause an occultation of a star, no atmosphere is detected. But from these facts the true conclusion is no more than that the quantity of air is, if existent, not sufficient to be detected by present telescopic means, Its existence may appear improbable to some; but more cannot be scientifically alleged, and late astronomers are not unanimous in supposing so much. The telescopic observations of Schroster are thought by some of them to have proved the existence of a very rare atmosphere.
* Some queries by this juvenile friend respecting the asteroids, became the immediate occasion of my putting into a written form my previous ideas, as referred to in p. 126. The first copy having been addressed and sent to him, the epistolary forın, though perhaps not the most eligible, is retained. It scarcely appears afterwards, and affects not the mode of proof,
Perhaps none has gone nearer to a negation of Auids on the moon than Sir J. Herschel in his volume on Astronomy in Lardner's Cyclopædia. He says (p. 229), "what is, moreover, extremely singular in the geology of the moon is, that, although nothing having the character of seas can be traced, yet there are regions perfectly level, and apparently of a decidedly alluvial character. The moon has no clouds nor any other indications of an atmosphere. Were there any, it could not fail to be perceived in the occultation of stars, and the phenomena of solar eclipses.” But in the same and next page, he asserts that decisive marks of volcanic stratification, arising from successive deposits of ejected matter, may be clearly traced with powerful telescopes.” Now with volcanic action we usually associate ignition and combustion; and we know chemically, that the general supporter of these is oxygen ; and, though chlorine may also support some combustion, as of metals, yet that equally with oxygen supposes an atmosphere of some gas or gases. It is also true that volcanoes, without ignition, as those of mud and water, are found on the earth; yet I must ask, what conception can we form of volcanic action, without any of those agents,
fire, gases, or water ? But it is admitted on all hands that the surface of the moon is honeycombed with volcanoes which, if not now, were formerly active. The appearances attributed to fire are as strenuously explained away as those supposed to prove the presence of air. It is indeed correct to say an atmosphere may have formerly existed ; but it is surely no absurd idea to suppose that some portion of it may yet remain. He also says (p. 231), • Whatever moisture may exist on its surface must be constantly transferred by distillation in vacuo, &c. The consequence must be absolute aridity below the vertical sun,