VENUS, the name of the planet which is nearest to the earth, end, except Mercury, nearest to the nun. The principal point of its physical description Is the distinctness with which its phases are seen through a telescope, in which it exhibits all the changes of appearance which are, to the naked eye. characteristic of nothing but the moon. With regard to these phases, there is a remarkable historical error which we cannot trace higher than Dr. Smith's ' Optics,' but which has been copied by Hutton and others. it is raid," When Copernicus revived the ancient Pythagorie system, asserting that the earth end planets moved round the sun at time centre of their orbits, the Ptolemaic* objected, if this were true, that the phases of Venus should resemble those of the moon. Copernicus replied, Mat some time or ether that resemblance scould be fuusd oat." (Smith's Optics,' p. 415.) Now, first, Coperni cus never answered an objection to his system in the manner implied in the story ; for he literally only lived to lay his hand upon a copy of his own work, and never opened It. [Coeenaners, In Btoo. Div.] Secondly, Gameudi, his biographer, in stating the verification of his system (as be calls it), derived from those phases, never alludes to any cich prophecy ; nor does Galileo in announcing the telescopic discovery of the phases of Venue (a D. 1611), and In giving praise to Copernicus and Kepler for not having abandoned the motion of the earth on account of the difficulty arising out of the apparent want of them, ever appose that Copernicus, or any of his folio's era, haul the slightest Idea of that apparent want arising from impel feetion of vision. Thirdly, Copernicus himself, in mentioning the udficulty, expreesly meets It by the supposition that Mercury and Venus, the " tuella," alluded to in the coining quotation, either shine by their own light, or completely ratnrated with the solar rays: "Non ergo fatemur In stellis opacitatem ease aliquam, lunari similem, seri vet proprlo lumina, vel 'solar' totie inibutes etorporibue fulgare, et idoireo solem non impedire" (lib. i.
cap 101. And the objection itself has nuthiog to do with the differ ence between the system of Copernicus and that of Ptolemy. for it had long before been raised against the latter: all, in fact, who maintained that the orbit of Venus comes between the earth and sun having to meet it In one way or another, whether they made the earth or the sun the centre of their system.
In the theory of the motion of Venue, the most remarkable circum stance is the long inequality discovered some years ago by Mr. Airy, depending upon the earth. [Ortavtvartota) A satellite was at one time suspected to belong to this planet, but no such thing has ever been detected. It may be remarked that the satellite of an inferior planet might not be easy to find, if it were very email ; for when the planet Is nearest the earth,and eiroumatances are most favourable for its discovery in other respects, tho dark side would be turned towards the earth. Though in the earlier period of telescopic observations, spots of various aorta were detected on the disc of Venus, yet the general de scription of its appearance in our day, as given by Sir J. Iferachel. is as follows :—" Although it occasionally attains the diameter of 61", which is larger than that of any other planet, it is yet the most difficult of them all to define with telescopes. The intense lustre of its illuminated part darzlee the sight, and exaggerates every imperfection of the tele scope ; yet we see clearly that its surface is not mottled over with per manent spots like the moon ; we perceive in it neither mountains nor 'shadows, but a uniform brightness, in which sometimes we may indeed fancy obscurer portions, but can seldom or never rest fully satisfied of the feet. The most natural conclusion, from the very rare appearance and want of permanence of the spots, is that we do not see, as in tho moon. the real surface of, this planet, but only its much loaded with clouds, and which may scree to mitigate the otherwise intense glare of its sunshine." Venus, like Mercury, from the orbit being entirely within that of the earth, is never at more than a certain angular distance from' the sun; her greatest angular distance, or elongation, being from 45' to 47' 12'. Her mean apparent diameter Is 16'9, but varies from to The real diameter is .975 of that of the earth, or about 7700 miles, find the volume is '927 of that of the earth. The density is very nearly that of the earth ; but this element, as also the mass, is rather uncertain. The mass of this planet is stated by Laplace at 0.-,th of that of the sun, but later writers give as more probable.