152 A Short History of Astronojny [CH. vi. 



the moons or satellites of Jupiter. On January 7th, 1610, 

 Galilei turned his telescope on to Jupiter, and noticed 

 three faint stars which caught his attention on account of 

 their closeness to the planet and their arrangement nearly 

 in a straight line with it. He looked again next night, and 

 noticed that they had changed their positions relatively 

 to Jupiter, but that the change did not seem to be such 

 as could result from Jupiter's own motion, if the new bodies 

 were fixed stars. Two nights later he was able to confirm 

 this conclusion, and to infer that the new bodies were not 

 fixed stars, but moving bodies which accompanied Jupiter 

 in his movements. A fourth body was noticed on 

 January i3th, and the motions of all four were soon recog- 

 nised by Galilei as being motions of revolution round 

 Jupiter as a centre. With characteristic thoroughness he 



Ori. * * O * Occ 



FIG. 54. Jupiter and its satellites as seen on Jan. 7, 1610. 

 From the Sidereus Nuncius. 



watched the motions of the new bodies night after night, 

 and by the date of the publication Of his book had already 

 estimated with very fair accuracy their periods of revolution 

 round Jupiter, which ranged between about 42 hours and 

 17 days; and he continued to watch their motions for 

 years. 



The new bodies were at first called by their discoverer 

 Medicean planets, in honour of his patron Cosmo de 

 Medici, the Grand Duke of Tuscany; but it was evident 

 that bodies revolving round a planet, as the planets them- 

 selves revolved round the sun, formed a new class of bodies 

 distinct from the known planets, and the name of satellite, 

 suggested by Kepler as applicable to the new bodies as 

 well as to the moon, has been generally accepted. 



The discovery of Jupiter's satellites shewed the falsity 

 of the old doctrine that the earth was the only centre of 

 motion ; it tended, moreover, seriously to discredit the 

 infallibility of Aristotle and Ptolemy, who had clearly no 

 knowledge of the existence of such bodies ; and again 

 those who had difficulty in believing that Venus and 



