322 cosmos. 



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 * during which the doctrines 

 of Copernicus, regarding the movement of the earth, and the 

 immobility of the sun, have been promulgated without hin- 

 derance, because it is deemed allowable to dispute concerning 

 natural things, and to elucidate the works of God ; and now 

 that neio 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 sys- 

 tem 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 Ger- 

 many, t 



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

 rable epoch in the history and the vicissitudes of astronomy.^ 

 The occultations 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 explanation of the aber- 

 ratio7i-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 Romer 

 and Bradley have been justly termed " the keystone of the 

 Copernican system," the perceptible evidence of the transla- 

 tory motion of the earth. 



Galileo had also early perceived (September, 1612) the im- 

 portance of the occultations of Jupiter's satellites for geograph- 

 ical determinations of longitude on land. He proposed this 

 method, first to the Spanish court in 1616, and afterward to 

 the States-General of Holland, with a view of its being ap- 

 plied to nautical purposes, $ little aware, as it would appear, 



* 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 FreiheiT von Breitschwert, Keppler , s Leben, s. 36. 



\ Sir John Herschel, Astron., s. 465. 



Galilei, Opere, t. ii. (Longitudine per via de 1 Pianeti Medicei), p. 

 435-506; Nelii, Vita, vol. ii., p. 656-688; Venturi, Memorie e I ettere 

 di G. Galilei, Part i., p. 177. As early as 1612, or scarcely tw_ years 

 after the discovery of Jupiter's satellites, Galileo boasted, somewhat 

 prematurely indeed, of having completed tables of those secondary sat- 

 ellites " to within 1' of time." A long diplomatic correspondence was 

 carried on with the Spanish embassador in 1616, and with the Dutch 



