380 SPECIAL RESULTS IN THE URANOLOGICAL 



his satellites, the mass of the planet alone, without the satel- 

 lites, is 10 48-oagi <> r only about -oVo-th less. 



Comparisons of magnitude, distance, and excentricity, 

 with the satellites of other planets or systems, have already 

 been given in an earlier part of the present volume (p. 338 

 to 340). The intensity of the light of the four satellites 

 of Jupiter is not proportional to their volume ; for, gene- 

 rally speaking, the 3d and the 1st, the ratio of whose magni- 

 tudes according to their diameters is as 8 : 5, appear the 

 brightest, and the 2d, which is the smallest and densest of 

 the satellites, is usually brighter than the larger 4th, which 

 is considered the faintest of all. Casual or temporary fluc- 

 tuations in the intensity of light of the satellites, which have 

 also been remarked, have been ascribed sometimes to alte- 

 rations in their surfaces, and sometimes to obscurations in 

 their atmospheres ( 619 ). They, however, all appear to re- 

 flect a more intense light than the central planet 

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

 satellites, therefore, in their movement from east to west, 

 appear to enter the eastern margin of Jupiter, and, passing 

 in front of the planet's disk, successively cover, to our eyes, 

 different portions of it, they can be recognised in their 

 passage, even with not very high magnifying powers, as they 

 detach themselves " in bright" from the disk. It becomes 

 more difficult to distinguish the satellites as they approach 

 the centre of the planet's disk. From this early-observed 

 phenomenon, Pound, Newton's and Bradley 's friend, had 

 already inferred that the disk of Jupiter is less bright 

 towards the margin than at the centre. Arago believes that 

 this statement, which has been repeated by Messier, pre- 

 sents difficulties which require to be solved by new and more 



