108 A CENTURY'S PROGRESS IN ASTRONOMY. 



satellite was found by Barnard to revolve round 

 Jupiter in 11 hours 57 minutes at a mean dis- 

 tance of 112,000 miles. 



Although the existence of other satellites of 

 Jupiter was predicted by Sir Robert Stawell Ball 

 (born 1840) soon after the discovery of the fifth, 

 much surprise was created by the announcement, 

 in January 1905, that a sixth satellite had been 

 discovered by Perrine, who, in the following 

 month, announced the discovery of a seventh. 

 These discoveries were made by photography, 

 the objects being very faint. The periods of 

 revolution were found to be 242 days and 200 

 days for the sixth and seventh satellites re- 

 spectively, the mean distances being 6,968,000 

 and 6,136,000 miles. It is possible that they 

 may belong to a zone of asteroidal satellites. 

 In fact, the fifth moon may belong to a similar 

 zone, so that Jupiter may have two asteroidal 

 zones ; but this is anticipating future discovery. 



A particular charm has always attached itself 

 to the study of Saturn, the ringed planet. The 

 magnificent system of rings has for two and a 

 half centuries been the object of wonder and 

 admiration in the Solar System, and accordingly 

 they have been exhaustively studied by many 

 eminent observers. While observing the two 

 bright rings of Saturn on June 10, 1838, Galle 



