364 Neighbourhood of the Lunar Spot, Mare Crisium. 



at some times much brighter and more uniform than at others ; 

 this, he says, may partly arise from a different angle of reflec- 

 tion, but is probably in part also atmospheric, resembling the 

 periodical changes of weather in some portions of our globe. 

 For, he argues, if reflection alone were concerned, why should 

 the change extend uniformly over the whole breadth of the 

 surface, instead of advancing across it by degrees ? and why 

 should not many other similar surfaces be similarly affected ? 

 while, on the contrary, as a general rule, the greater spots pre- 

 serve their colour invariable, whether bright or grey ; of which 

 he alleges Gojyernicus and Plato as instances, and the large 

 spots in the immediate vicinity of Cleomedes itself. He has 

 also called attention to similar variations in the immediate 

 neighbourhood of Cleomedes, especially between it and the 

 Ma/re Qrisi/um, where black shadows and grey patches among 

 the mountains exhibited to him a strange inconstancy of aspect, 

 even under similar angles of incidence and reflection of light. 

 As to all this, Beer and Miidler would be at issue with their 

 predecessor, and would resolve the whole affair into the effects 

 of varied illumination. It is very probable that there may have 

 been more in this than Schroter has allowed for ; and as the 

 question affects the labours of future selenographers, and may 

 prove perplexing to the uninitiated student, it may be well to 

 give some attention to it in this place. 



That apparent change of position in the lunar spots which is 

 termed Libration, arises, as is well known, from two independent 

 causes, and the whole effect is a combination of the results of each. 

 Libration in longitude — which causes sometimes more of the E., 

 sometimes more of the W., limb to come into sight, and throws 

 tin mean apparent centre of the moon at one time into the E., 

 at another into the W. hemisphere — arises from the unequable 

 velocity of the moon in its elliptic orbit, combined with its 

 equable rotation on its axis, whence any given point on its sur- 

 will sometimes appear in advance of, sometimes behind, its 

 mean position. This will, of course, alter the perspective pro- 

 jection of all objects in an El. ami \Y. direction, affecting the 

 equator most strongly, flic poles not at all; while, aa regards 

 (lie direction of the incident solar light, ami the corresponding 

 iint of shadow, the result will be the same as if the globe of 



the moon were swung slightly round its axis, backwards or for- 

 wards, as the case may hi', alternately on each side of its normal 

 of mean position. The mil Length of the shadow will thus be 

 a lit I le varied, by turning 1 1 bjecl somewhat toor from the sun, 



and its projected (or apparent) Length, by turning it somewhat to 



or from the eve ■ the former effect being greatest in the centre, 



and Least near the limb, thelaitergreateratadistancoof45 from 



'.■litre than in either I f those two positions. The extreme 



