NOTES. 



445 



and equator seen edgewise, then the orbits of his four satellites seen edge- 

 wise will have the positions J 1 , J 2, J 3, J 4. These are extremely near 

 to one another, for the angle E J is only 3 5' 30". 



NOTE 88, p. 28. 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 evenings of the 3rd, 4th, 5th, and 6th of January, 1835, 

 the satellites had the configurations given in fig. 23, where is Jupiter, 

 and 1, 2, 3, 4, are the first, second, third, and fourth satellites. The 



Fig. 23. 



Jan. 



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. 28. 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. 29. Displacement of Jupiter's orbit. The action of the 

 planets occasions secular variations in the position of Jupiter's orbit J 0, 

 fig. 22, without affecting the plane of his equator J E. Again, the sun 

 and satellites themselves, by attracting the protuberant matter at Jupitei-'s 

 equator, change the position of the plane J E without affecting J 0. Both 

 of these cause perturbations in the motions of the satellites. 



NOTE 91, p. 29. Precession, with regard to Jupiter, is a retrograde 

 motion of the point where the lines J 0, J E, intersect fig. 22. 



NOTE 92, p. 30. Synodic motion 

 of a satellite. Its motion during the 

 interval between two of its consecutive 

 eclipses. 



NOTE 93, p. 30. 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 the earth, then Jupiter is in 

 opposition when at 0, and in conjunc- 

 tion when at C. In these positions the 

 three bodies are in the same straight 

 line. 



NOTE 94, p. 30. Eclipses of the 



Fig. 24. 



