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HISTORY OF PHYSICAL ASTRONOMY.'

July 8, 1842. If it be assumed that the atmospheric light diminisheswith such rapidity from the sun’s limb that the light of the solar atmo-sphere forms an aliquot fraction of it at a very small distance from the limb,it may then he inferred, as in that case, that the visibility of the planet’s,limb arises from the obscurity of its surface, when contrasted with thebrightness of the ground upon which it appears projected. Such a supposi-tion, however, is by no means probable. The faint ring that was seen bysome observers around the planet’s limb when it had wholly enteredupon the sun’s disk, has been supposed to indicate the existence of anatmosphere about the planet, but the various statements respecting itsappearance are not sufficiently consistent with each other to warrantsuch a conclusion, although at the same time it seems to be justified byother considerations, to which allusion has already been made in a formerchapter.

Sometimes the planets are eclipsed in consequence of the moon passingbetween them and the earth. The earliest recorded phenomenon of thisnature is an occultation of Mars by the moon, which Aristotle makesmention of in one of his works. He states that when the moon was half-full, the planet entered behind the limb on the unenlightened side, andsubsequently emerged on the bright side *. Kepler calculated the dateof this occultation, and found that it occurred on the night of the 4th ofApril in the year 357 a.c. f. The rapid and tortuous motion of the moonproduces occasional occultations of this nature, but they are not of muchimportance to the astronomer. It was expected that the circumstancesaccompanying such occultationshvould serve to throw light upon the much-disputed question of the existence of a lunar atmosphere, but no suchadvantage has hitherto been derived from them.

The occultation of a star by the moon is a phenomenon which, althoughof frequent occurrence, never fails to jirove interesting. When the darklimb of the moon comes up to the star, the occultation is invariably foundto take place instantaneously, whence it is manifest that the apparentmagnitude of the star must be excessively small. The effect in such casesis most striking when the whole body of the moon is invisible, as occa-sionally happens when she is totally eclipsed. A phenomenon of thiskind was witnessed by Wargentin during the lunar eclipse of May 18,1761, to which allusion has already been made. Previous to the totaldisappearance of the moon, he perceived a star near her eastern limb,which seemed to he about to undergo occultation. He therefore followedit with great attention until at length, at 10 h 52™ 39 s , when even thefaintest trace of the moon had ceased to be visible, it vanished in less thanthe twinkling of an eye j.

Occultations of stars by the moon serve to fix the apparent position ofthe latter body with great accuracy, and in consequence they have provedvery serviceable in correcting the elements of her motion. For a similarreason they are of great use in determining the differences of longitude ofplaces on the earth’s surface.

A singular phenomenon of a physical nature is sometimes observed onthe occasion of an occultation of a star. When the limb of the moon hascome up to the star, the latter appears to advance upon the moon’s disk,continuing visible in this manner for several seconds previous to its oc-

* De Casio., lib. ii., cap. 12. f Ad. Vitellionem Paralipomena, p. 307.

\ “ Oculi ictu citius” ( Phil. Trans,, 1761, p. 210).