MEMOIR OF GALILEO. 89 



Jupiter's satellites. When he first observed them ; two were 

 on the east side, and one on the west side of the planet, all 

 in a straight line, parallel to the ecliptic, and much brighter 

 than fixed stars of their magnitude. He regarded them at 

 first as fixed stars ; but, on chancing to direct his attention 

 to them again on the 8th of January, he found all the three 

 to be on the west side of Jupiter, and nearer each other. 

 Disregarding the circumstance of these stars having ap- 

 proached each other, he considered how Jupiter could be to 

 the east of them, when the day before he had been to the 

 west of two of them ; and the conclusion he came to was, 

 " that the motion of Jupiter was direct contrary to the as- 

 tronomical calculations, and that he had got before these 

 two stars by his own motion." On the 10th, however, an- 

 other observation showed him only two stars, and both on 

 the east side of Jupiter. It was evident that the planet 

 could not have moved from west to east on the 8th of Jan- 

 uary, and two days after have moved from east to west. 

 Under these circumstances he came to the conclusion, that 

 the different appearances arose from the motion of the stars 

 themselves. On the llth, there were two stars on the east 

 side of Jupiter, but the one was twice the size of the other. 

 " This fact threw a new light upon Galileo's difficulties, and 

 he immediately drew the conclusion which he considered to 

 be indubitable, 'that there were in the heavens three stars, 

 which revolved round Jupiter in the same manner as Venus 

 and Mercury revolve round the sun.' "* On the 13th, Ga- 

 lileo discovered the fourth satellite of Jupiter. Having 

 made these discoveries, he named them the Medician stars, 

 in honor of his patron, Cosmo de Medici, grand-duke of 

 Tuscany, and published an account of them in a work en- 

 titled the "Sidereal Messenger." 



These discoveries, the fruits of the newly discovered tel- 



* " Martyrs of Science/' by Sir D. Brewster. 



