186 OUTLINES OF NATURAL PHILOSOPHY. 



. From analogy we may conclude, with considera- 

 ble probability, that the orbits of all the satellites 

 are either circles or ellipses. 



< Dr HERSCHEL has observed, that the same satel- 

 lite is more luminous at one time than another, 

 and that the period of these changes is for each sa- 

 tellite the same with the time of its revolution a- 

 bout Jupiter; hence he has inferred, that the sa- 

 tellite also revolves on its axis, in the same time 

 that it revolves about Jupiter. 



This is also the law of the Moon's rotation, 1 32, 



190. The beginning or end of an eclipse of a 

 satellite, marks the same instant of absolute time 

 to all the inhabitants of the Earth, and may 

 therefore be employed for finding the longi- 

 tude, in the same way with an eclipse of the 

 Moon, 67, and 145. 



a. The immersion of a satellite into the shadow of 

 Jupiter, and its emersion from it, are instants more 

 precisely defined than the beginning or end of a 

 lunar eclipse ; and therefore the longitude is more 

 accurately found by the former. The comparison 

 may either be made between the observation and 

 the Tables ; or between it and another observa- 

 tion made under a known meridian. 



b. The first satellite is the most proper to be observ- 

 ed, its motions being best known, and its eclipses 

 recurring most frequently. 



c. Astronomers, 



