THE UNIVERSE. DISCOVERIES IN THE CELESTIAL SPACES- 317 



names of the members of the family of the Medicean ruling 

 house, Catharina, Maria, Cosimo the elder, and Cosimo the 

 younger. 



The knowledge of Jupiter's satellites and of thp phases 

 of Venus was most influential in confirming and extending 

 the Copernican system. The little world composed of the 

 planet Jupiter and his satellites (Mundus Jovialis) offered 

 to the intellectual eye a perfect image of the great solar and 

 planetary system. It was recognised that the satellites of 

 Jupiter obeyed the laws discovered by Kepler, and, in the 

 first place, that the squares of their periods of revolution 

 were in the ratio of the cnbes of their mean distances from 

 the central planet. This led Kepler, in the Harmonices 

 Mundi, to exclaim with the confidence and courage which 

 belongs to intellectual freedom, addressing himself to those 

 whose voices'bore sway beyond the Alps : "Eighty years ( 485 ) 

 have elapsed, during which the Copernican doctrine of the 

 motion of the earth and the immobility of the sun has been 

 taught unhindered, because it was held permissible to dispute 

 concerning natural things, and to throw light upon the 

 works of God ; and now, when new documents have been 

 discovered for the proof of this doctrine, documents which 

 were unknown to the (ecclesiastical) judges, the promulga- 

 tion of the true system of the fabric of the universe is by 

 you prohibited !" This prohibition or ban, a consequence 

 of the ancient feud between ecclesiastical authorities and 

 natural science, had been already experienced by Kepler 

 even in Protestant Germany. ( 486 ) 



The discovery of Jupiter's satellites marks a memorable 

 epoch in the history of astronomy, and in the permanent 

 establishment of the principles upon which it is founded. ( 487 ) 



