5 



SATELLITES. ( Vince, Play fair.) 



1. Of Jupiter. 



Jupiter's satellites were discovered by Galileo in 1610. The times of 

 their rotation are the same with the periodic times round the primary,, 

 Occupations happen to the first and second satellites at every revolu- 

 tion j the third very rarely escapes an occultation, but the fourth more 

 frequently, by reason of its distance. The three first are eclipsed in 

 every revolution, the fourth not always. In the first satellite we never 

 can see both the immersion and emersion j the other three satellites may 

 have both visible, but it depends on the position of the earth. The first sa- 

 tellite is the most proper for finding the longitude, its tables being the most 

 correct. The observer should be settled at his telescope three minutes 

 before the expected time of an immersion of the first satellite, six or 

 eight minutes before that of the second or third ; and at least a quarter 

 of an hour before that of the fourth. If the longitude be different from 

 that of Greenwich, allowance must be made for it. The telescopes pro- 

 per for observing these eclipses are reflecting ones of 18 inches or 2 feet, 

 or the 46 inch achromatic with three object glasses. 



There is a singular analogy between the three first satellites, discover- 

 ed by Laplace, viz. that if m' t m" t m'" t are the mean motions of the 1st, 

 2d, and 3d, satellites of Jupiter, 



^' -j- 2m'" ~3m". 



Also if L', L" L'", are the mean longitudes of these satellites, 

 L' 3 L" + 2 L'" - 1800. 



The last equation shews that the three satellites can never be eclipsed 

 at the same time. 



Table of the Satellites. 



