NOTES. 405 



NOTE 85, p. 25. Conf/rvration. The relative position of the planets 

 with regard to one another, to the sun, and to the plane of the ecliptic. 



NOTE 86, p. 26. In the same manner that the eccentricity of an ellipti- 

 cal orLit may be increased or diminished by the action of the disturbing 

 forces, so a circular orbit may acquire less or more ellipticity from the 

 same cause: It is thus that the forms of the orbit of the first and second 

 satellites of Jupiter oscillate between circles and ellipses differing very 

 little from Circles. 



NOTE 87, p. 27. The plane of Jupiter's equator is the imaginary plane 

 passing through his center at right angles to his axis of rotation ; and 

 corresponds to the plane qEQe, in fig. 1. The satellites move very 

 nearly in the plane of Jupiter's equator, for if J be Jupiter, fig. 22, Pp his 



axis of rotation, eQ, his equatorial diameter, which is 6000 miles longer 

 than Pp, and if JO 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. 27. 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 3d, 4th, 5th, and 6th of January, 1835, 

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



Fig. 23. 

 J! - Wi Ea 



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

 lite is supposed to be moving in a direction from the figure toward the 

 point. On the sixth evening th 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. ^.Displacement of Jupiter's orbit. The action of the 

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

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

 and satellites themselves, by attracting the protul>erant matter at Jupiter's 

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

 of these cause jerturbations in the motions of the satellites. 



NOTE 91, p. 26. Precession, with regard to Jupiter, is a retrograde 

 notion of the point where the lines JO, J E, intersect fig. 22, 



