704 COSMOS. 



The knowledge of Jupiter's satellite-system, and of the 

 phases of Venus, has exercised the most marked influence on 

 the establishment and general diffusion of the Copernican 

 system. The little world of Jupiter (Mundus Jovialis] pre- 

 sented to the intellectual contemplation of men a perfect 

 image of the large planetary and solar systems. It was 

 recognised, that the secondary planets obeyed the laws dis- 

 covered by Kepler ; and it was now first observed that the 

 squares of their periodic times were as the cubes of the mean 

 distances of the satellites from the primary planets. It was 

 this which led Kepler, in the Harmonices Mundi, to state, 

 with the firm confidence and security of a German spirit of 

 philosophical independence, to those whose opinions bore sway 

 beyond the Alps ; " eighty years have elapsed,* 1 during which 

 the doctrines of Copernicus, regarding the movement of the 

 earth, and the immobility of the sun, have been promulgated 

 without hinderance, because it is deemed allowable to dis- 

 pute concerning natural things, and to elucidate the works of 

 God; and now that new testimony is discovered in proof of 

 the truth of those doctrines testimony which was not known 

 to the spiritual judges ye would prohibit the promulgation of 

 the true system of the structure of the universe !" Such a 

 prohibition a consequence of the old contest between natural 

 science and the Church Kepler had early encountered in 

 Protestant Germany.f 



The discovery of Jupiter's satellites marks an ever-memo- 

 rable epoch in the history and the vicissitudes of astro- 

 nomy. ;{: The occupations of the satellites, or their entrance 

 into Jupiter's shadow, led to a knowledge of the velocity 

 of light (1675), and, through this knowledge, to the expla- 

 nation of the aberration-ellipse of the fixed stars (1727), in 

 which the great orbit of the earth, in its annual course 

 round the sun, is, as it were, reflected on the vault of 

 heaven. These discoveries of Homer and Bradley have 

 been justly termed "the keystone of the Copernican sys- 



* It should be seventy- three years; for the prohibition of the Coper- 

 nican system, by the Congregation of the Index, was promulgated on 

 the 5th of March, 1616. 



f Freiherr von Breitschwert, Keppler's Leben, s. 36. 



Sir John Herschel, Astron., s. 465. 



