A NEW ASTRONOMY 221 



examination of the planets with a more powerful telescope. On the 

 7th of January, 1610, at one o'clock in the morning, when he directed 

 his telescope to Jupiter, he observed three stars near the body of the 

 planet, two being to the east and one to the west of him. They were 

 all in a straight line, and parallel to the ecliptic and appeared brighter 

 than other stars of the same magnitude. Believing them to be fixed 

 stars, he paid no great attention to their distances from Jupiter and 

 from one another. On the 8th of January, however, when, from some 

 cause or other, he had been led to observe the stars again, he found a 

 very different arrangement of them ; all the three were on the west 

 side of Jupiter, nearer one another than before and almost at equal 

 distances. Though he had not turned his attention to the extraordi- 

 nary fact of the mutual approach of the stars, yet he began to con- 

 sider how Jupiter could be found to the east of the three stars, when 

 but the day before he had been to the west of two of them. The only 

 explanation which he could give of this fact was that the motion of 

 Jupiter was direct, contrary to the astronomical calculations and 

 that he had got before these two stars by his own motion. 



In this dilemma between the testimony of his senses and the results 

 of calculation, he waited for the following night with the utmost 

 anxiety, but his hopes were disappointed, for the heavens were wholly 

 veiled in clouds. On the 10th, two only of the stars appeared, and 

 both on the east side of the planet. As it was obviously impossible 

 that Jupiter could have advanced from west to east on the 8th of 

 January, and from east to west on the 10th, Galileo was forced to 

 conclude that the phenomenon which he had observed arose from the 

 motion of the stars, and he set himself to observe diligently their change 

 of place. On the llth there were still only two stars, and both to the 

 east of Jupiter, but the more eastern star was now twice as large as 

 the other one, though on the preceding night they had been per- 

 fectly equal. 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 revolve 

 around Jupiter, in the same manner as Venus and Mercury revolve 

 around the sun.' On the 12th of January he again observed them in 

 new positions, and of different magnitudes ; and on the 13th he 

 discovered a fourth star, which completed the four secondary planets 

 with which Jupiter is surrounded. Brewster. 



His results were published in ' The Sidereal Messenger,' announc- 



