OUTLINES OF NATURAL PHILOSOPHY. 



285. The third satellite moves on a fixed plane 

 that is between the equator and the orbit of Ju- 

 piter, and is inclined to that plane at an angle 

 of 12'.20", its nodes making a tropical revolu- 

 tion, (retrograde), in 141.739 years. 



The equator of Jupiter is inclined to the plane of his 

 orbit, at an angle of 3.5 / .27 // ; the fixed planes on 

 which the planes of the orbits move, are determi- 

 ned by theory, and probably could never be dis- 

 covered by observation alone. 



286. The orbit of the third satellite is eccen- 

 tric ; but appears to have two distinct equations 

 of the centre ; one which really arises from its 

 own eccentricity ; and another, which theory 

 shews to be an emanation from the equation of 

 the centre of the fourth satellite. The first 

 equation is referable to an apsis, which has an 

 annual motion of 2 36' 39" forward in respect 

 of the fixed stars ; the 2d equation is referable 

 to the apsides of the 4th satellite. 



These two equations may be considered as forming 

 one equation of the centre, referable to an apsis 

 that has an irregular motion. The two equations 

 coincided in 1682, and the sum of their maxima 

 was 13' 16". In 1777, the equations were oppo- 

 sed, and their difference was 5' 6". 



Observation 



