4/4 NOTES. 



of Jupiter's equator; for, if J be Jupiter, fig. 22, P p his axis of rotation, e Q 

 his equatorial diameter, which is 6000 miles longer than P p, and if J O 

 and J E be the planes of his orbit and equator seen edgewise, then the 

 orbits of his four satellites seen edgewise will have the positions J 1, J 2, J 3, 

 J 4. These are extremely near to one another, for the angle E J O is only 

 3 5' 30". 



NOTE 88, p. 32. In consequence of the satellites moving so nearly in the 

 plane of Jupiter's equator, when seen from the earth, they appear to be always 

 very nearly in a straight line, however much they may change their positions 

 with regard to one another and to their primary. For example, on the even- 

 ings of the 3rd, 4th, 5th, and 6th of January, 1835, the satellites had the confi- 

 gurations given in fig. 23, where O is Jupiter, and 1, 2, 3, 4, are the first, 



Fig. 23. 



But 



second, third, and fourth satellites. The satellite is supposed to be moving in 

 a direction from the figure towards the point. On the sixth evening the second 

 satellite was seen on the disc of the planet. 



NOTE 89, p. 33. Angular motion or velocity is the swiftness with which a body 

 revolves a sling, for example; or the speed with which the surface of the earth 

 performs its daily rotation about its axis. 



NOTE 90, p. 33. Displacement of Jupiter's orbit. The action of the planets 

 occasions secular variations in the position of Jupiter's orbit J O, fig. 22, with- 

 out affecting the plane of his equator J E. Again, the sun and satellites them- 

 selves, by attracting the protuberant matter at Jupiter's equator, change the 

 position of the plane J E without affecting J O. Both of these cause perturba- 

 tions in the motions of the satellites. 



NOTE 91, p. 33. Precession, with regard to Jupiter, is a retrograde motion of 

 the point where the lines J O, J E, intersect fig. 22. 



NOTE 92, p. 34. Synodic motion of a satellite. Its motion during the interval 

 between two of its two consecutive eclipses. 



NOTE 93, p. 34. Opposition. A body is said to be in opposition when its 

 longitude differs from that of the sun by 180. If S, fig. 24, be the sun, and E 



