NOTES. 



401 



plane of the ecliptic, N m n, than it would otherwise do. The action of 

 the disturbing forces is admirably explained in a work on gravitation by 

 Professor Airy, of Cambridge. 



NOTE 64, pp. 16, 69. Perihelion. Fig. 10, P, the point of an orbit 

 nearest the sun. 



NOTE 65, p. 16. Aphelion. Fig. 10, A, the point of an orbit farthest 

 from the sun. 



NOTE 66, pp. 16, ib., 17. In fig. 15 the central force is greater than the 

 exact law of gravity ; therefore the curvature Ppa is greater than Pp A 

 the real ellipse ; hence the planet p comes lo the point a, called the aphe- 

 lion, sooner than if It moved in the orbit Pp A, which makes the line 

 PSA advance to a. In fig. 16, on the contrary, the curvature P p a is 

 Fig. 15. Fig. 16. 



less than in the true ellipse, so that the planet p must move through 

 more than the arc Pp A, or 180, before it comes to the aphelion a, which 

 causes the greater axis P S A to recede to a. 



NOTE 67, pp. 16, 17. Motion of apsides. 

 Let PSA, fig. 17, be the position of the 

 elliptical orbit of a planet at any time ; 

 then, by the action of the disturbing 

 forces, it successively takes the position 

 P' S A', P" S A", &c., till by this direct 

 motion it has accomplished a revolution, 

 and then it begins again ; so that the 

 motion is perpetual. 



NOTE 68, p. J6. Sidereal revolution. 

 The consecutive return of an object to 

 the same star. 



NOTE 69, p. 16. Tropical revolution. 

 object to the same tropic or equinox. 



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

 &-c. In fig- 18 the effect or the varia- 

 tion in the eccentricity is shown, where 

 Pp A is the elliptical Orbit at any given 

 instant: after a time it will take the 

 form P p' A, in consequence of the 

 decrease in the eccentricity CS ; then 

 the form? Pp" A.Pp'" A,"&c., conse- 

 cutively from the same cause, and as * 

 the mHjor axis P A always retains the 

 name length, the orbit approaches more 

 nd more- nearly to the circular form. 

 But after this has pone on for some 

 thousands of years, the orbit contracts 

 aeain, and become* more and more 

 elliptical. 



26 L L2 



--..U-" 



K 



The consecutive return of an 



Fig. 18. 



