[ 417 1 
his revolution about the earth, or M inftead of Ti 
Laftly, the lunar orbit, inftead of the ecliptic. Like- 
wife, inftead of A, which [Art. 8.) was equal to 
2 S n r, 7 L a a 
— , we mult put B = x 
Km i — 2 a 
17. We lhail find then, by Art. 12. that the arc 
defcribed by the pole P of the earth parallel to the 
plane of the lunar orbit, whilft the moon makes one 
quarter of a revolution about the earth, is Bb x — x — \ 
2r 4, 
which gives the arc defcribed by the mean motion in 
an inftant, d /, equal to Bby.-—xdt = Bby. — a ■ 
2 r 2 rr 
x d t‘j putting, inftead of g, its value — . 
18. We fliall find alfo, by Art. 14. that the varia- 
tion of the inclination of the earth’s axe to the plane 
of the lunar orbit, when the moon is at its point of 
ftation po degrees diftant from the node of the equa- 
M 
tor with the lunar orbit, is i B x x a a, whereby 
2 irr 
the angle of inclination of the earth’s axe to the plane 
of the lunar orbit is increafed when the moon is at its 
point of ftation. 
19. We therefore ftiould have nothing to add, if 
the plane of the lunar orbit were the fame as that of 
the ecliptic, or if it always retained the fame pc- 
fition : But as this plane continually varies its pofition, 
and its pole deferibes a fmall circle about the pole of 
the ecliptic, from whence it is always about 5 degrees 
diftant j it ftiil remains that we examine what is the 
G g g pre- 
