ASTRONOMY. 181 



progressive, or in the same direction with the mo- 

 tion of the planets round the Sun. 



fL The stars which thus circulate round another 

 planet, are called Satellites ; and also Secondary 

 Planets ; those that revolve immediately about the 

 Sun, being distinguished by the name of Primary. 



The Moon is therefore a secondary planet \ but has 

 been treated of before the primary planets, because 

 her motions are conspicuous, and very useful for 

 explaining the general phenomena of the heavens. 

 The satellites of Jupiter were discovered by GA- 

 ULEO, in 1610. Nuncius Sydeiius Florentine, 

 1610. 



180. The distances of the [different satellites 

 from Jupiter, are nearly in proportion to the 

 angles which these distances subtend from the 

 Earth, when the satellites are at their greatest 

 elongations. 



a. The distances are found, on this principle, to be 

 nearly in the ratio of the numbers 6, 9, 15, and 26. 

 The satellites are called First, Second, &c. in the 

 order of the distances, beginning from that which 

 is nearest to Jupiter. 



b. The moments of greatest elongation, and of the be- 

 ginnings and ends of eclipses, are almost the only 

 times at which the satellites have been observed ; 

 and it is thence, that with much ingenuity, and in- 



credible 



