318 EPOCHS IN THE HISTORY OF THE CONTEMPLATION OF 



The occultations of the satellites, or their entrance into the 

 shadow of Jupiter, led to the knowledge of the velocitj of 

 light (1675), and through this, in 1727, to the explanation 

 of the " aberration-ellipse" of the fixed stars, in which the 

 orbit of the earth, in her annual revolution round the sun, 

 is, as it were, reflected on the celestial vault. These disco- 

 veries of Romer's and Bradley's have justly been termed the 

 " key-stone of the Copernican system;" the visible demon- 

 stration of the earth's movement of translation. 



The importance of the occultations of Jupiter's satellites 

 for geographical determinations of longitude on land was 

 early perceived by Galileo (Sept. 1612). He proposed this 

 method of determining longitudes, first to the Court of 

 Spain (1616), and subsequently to the States General of 

 Holland ; he proposed it, indeed, as a method available at 

 sea, ( 488 ) apparently little aware of the insuperable difficul- 

 ties which oppose its practical application on the unstable 

 ocean. He wished either to go himself, or to send his son 

 Vicenzio, to Spain, with a hundred telescopes which he 

 should prepare; requiring for recompense "una Croce di 

 S. Jago," and an annual pension of 4000 crowns; a small 

 sum, he says, as at first, in Cardinal Borgia's house, he had 

 been led to expect 6000 ducats a year. 



The discovery of Jupiter's satellites was soon after fol- 

 lowed by the observation of Saturn as a triple star, " pla- 

 neta tergeminus." As early as November 1610, Galileo 

 wrote to Kepler that "Saturn consists of three heavenly 

 "bodies in contact with each other." In this observation 

 there was the germ of the discovery of Saturn's ring. He- 

 velius described, in 1656, the variations in the form of 

 Saturn, the unequal opening of the " handles/' and their 



