SO Astronomy. [Lecture 24. 



ence of the meridians of these different places, and 

 consequently their relative longitudes. There are 

 two points to be principally attended to in the 

 eclipses of Jupiter's satellites ; viz. the immersion 

 of the satellite in the shadow, and its emersion. 

 While the earth is at a certain distance from the 

 point in which Jupiter is in opposition to the sun, 

 but before that opposition, the spectator will see 

 the immersion of the satellite into Jupiter^s sha- 

 dow ; but if it be the first satellite, on account 

 of its nearness to Jupiter, the emersion is never 

 seen in this relative state of the several bodies, the 

 satellite being behind Jupiter's body at the said 

 emersion. The other three satellites may have 

 both their immersions and emersions visible in the 

 same eclipse; the possibility of this, however, 

 depends upon the position of the earth. When 

 the earth, after the opposition, reaches a certain 

 point, the emersions of the first satellite become 

 visible, while the immersions of that satellite are 

 no longer to be seen : both immersions and emer^ 

 sions of the other three satellites may, in this 

 case, be visible ; in like manner as in the former 

 position of the several bodies. Before the opposi- 

 tions of Jupiter to the sun, the immersions and 

 emersions happen to the west of Jupiter ; after 

 opposition they happen to the east. If an astro- 

 nomical telescope be used, which reverses objects, 

 the appearances will, of course, be just the con- 

 trary, 



