516 



COSMOS. 



time, a picture in miniature of the Solar System. This view, 

 rapidly diffused at that time, as well as the discovery, shortly 

 afterwards, of the phases of Venus (February, 1610), con- 

 tributed greatly to the general introduction of the Copernican 

 system. The quadruple group of satellites of Jupiter, is the 

 only one of the exterior principal planets which has not been 

 increased by any new discovery, during a period of nearly 

 two centuries and a half, since the epoch of their first disco- 

 very by Simon Marius on the 29th of December, 1609. 



The following table contains the periods of sidereal revolts 

 tion of the satellites of Jupiter, their mean distances expressed 

 in diameters of the primary, their diameters in geographical 

 iniles, and their masses as parts of the mass of Jupiter. 



If ToTT-TTg expresses the mass of Jupiter with his satel- 

 lites, then his mass without the satellites is to48-oT5' only 

 about -g-Q-Vo smaller. 



The comparisons of the magnitudes, distances, and eccentri- 

 cities with other satellite systems, has already been given 

 (Cosmos, vol. iv. pp. 426-457). The Iwninous intensity of 

 Jupiter's satellites is various, and not in proportion to their 

 volume, since, as a general rule, the third and. \hefirst, whose 

 relation of magnitude is as 8 : 5, appear the brightest. The 

 smallest and densest of all the second is generally brighter 

 than the larger fourth, which is ordinarily called the least 

 luminou*. Accidental (temporary) fluctuations in the luminous 



