NOTES. 



441 



The consecutive return of an 

 Fig. 18, 



NOTE 69, p. 17. Tropical revolution. 

 object to the same tropic or equinox. 



NOTE 70, p. 17. The orbit only bulges, 

 $c. In fig. 18 the effect of the variation 

 in the excentricity is shown where Pp A 

 is the elliptical orbit at any given instant ; 

 after a time it will take the form Pp' A, in 

 consequence of the decrease in the excen- 

 tricity C S ; then the forms Pp" A, Pp"' A, 

 &c., consecutively from the same cause ; 

 and, as the major axis P A always retains 

 the same length, the orbit approaches more 

 and more nearly to the circular form. But, 

 after this has gone on for some thousands of 

 years, the orbit contracts again, and becomes 

 more and more elliptical. 



NOTE 71, pp. 18, 19. The ecliptic is the apparent path of the sun in 

 the heavens. See note 46. 



NOTE 72, p. 18. This force tends to pull, fyc. The force in question, 

 acting in the direction p m, fig. 13, pulls the planet p towards the plan 

 N m n, or pushes it farther above it, giving the planet a tendency to move 

 in an orbit above or below its undisturbed orbit Npn, which alters the 

 angle p N m, and makes the node N and the line of nodes N n change their 

 positions. 



NOTE 73, p. 18. Motion of the nodes. Let S, fig. 19, be the sun ; 

 S N n the plane of the ecliptic ; P the disturbing body ; and p a planet 

 moving in its orbit p n, of which p n is so small a part that it is repre* 

 sented as a straight line. The plane S np of this orbit cuts the plane of the 

 ecliptic in the straight line S n. Suppose the disturbing force begins to, act 

 on p, so as to draw the planet into the arc pp' ; then, instead of moving in 

 the orbit p n, it will tend to move in the orbit pp' n', whose plane cuts the 

 ecliptic in the straight line S n'. If the disturbing force acts again upon 



Fig. 19. 



the body when at jp', so as to draw it into the arc p'p", the planet will 



U 3 



