SATURN. 517 



intensity have, as already remarked, been ascribed some- 

 times to changes of the surface, sometimes to obscurations 

 in the atmosphere of the satellites. 78 They all appear more- 

 over to reflect a more intense light than the primary. When 

 the Earth is situated between Jupiter and the Sun, and the 

 satellites therefore moving from east to west, apparently 

 enter on the eastern edge of Jupiter, they hide from us, in 

 their passage, successive portions of the disc of their primary, 

 and can be perceived with telescopes of moderate power, since 

 they stand out in luminous relief from the disc. The visibi- 

 lity of the satellite is attended with more difficulty the nearer 

 it approaches the centre of the primary. From this pheno- 

 menon, which was early observed, Pound, Newton's and 

 Bradley's friend, inferred that the disc was less luminous near 

 the edge than at the centre. Arago considers that this 

 assumption, renewed by Messier, involves difficulties which 

 can only be solved by new and more delicate observations. 

 Jupiter was seen without any satellites by Molyneux in 

 November, 1681 ; by Sir William Herschel on the 23rd of 

 May, 1802; and, lastly, by Griesbach, on the 27th of Sep- 

 tember, 1843. Such a non- visibility of the satellites has 

 reference however to the space without the disc of Jupiter, 

 and is not inconsistent with the theorem that all the four 

 satellites cannot be eclipsed at one time. 



SATURN. 



The period of sidereal or true revolution of Saturn is 29 

 years, 166d. 23h. 16m. 32s. His mean diameter is 62,028 

 geographical miles, equal to 9,022 terrestrial diameters. The 

 period of rotation deduced from the observation of some dark 

 spots (knot-like condensations of the bands) upon the sur- 



75 Sir John Herschel, Outlines, 540. 

 Q 2 



