826 Astronomy. [Lecture 20. 



planet; they therefore call that the first satel- 

 lite which is nearest the planet, the second sa- 

 tellite that which is nearest to the former, &c. 



From the continual changes of their phases 

 or appearances, it is evident that these secondary 

 planets are also opaque bodies like the planets 

 themselves, and shine only by means of the 

 borrowed light which they receive from the 

 sun. 



The angles under which the orbits of Jupiter's 

 moons are seen from the earth, at their mean 

 distance from Jupiter, are as follow : the first 

 8' 35"; the second 6' 14"; the third 9 58"; and 

 the fourth 17' 30". And their distances from 

 Jupiter, .measured by his semi-diameter, are 

 thus : the first 5 2-3ds ; the second 9 ; the third 

 1 4 23-60ths ; and the fourth 25 1 8-60ths. This 

 planet, seen from its nearest moon, would appear 

 a thousand times as large as our moon does to 

 us ; waxing and waning in all its monthly shapes 

 every 42^ hours. 



Jupiter's three nearest moons fall into his sha- 

 dow, and are eclipsed in every revolution ; but 

 the orbit of the fourth moon is so much inclined, 

 that it passes by its opposition to Jupiter, with- 

 out falling into his shadow, two years in every 

 six. By these eclipses astronomers have not only 

 discovered that the sun's light takes up eight 

 minutes of time in coming to us ; but they have 

 determined the longitudes of places on this earth 

 with considerable certainty, and with much 



