180 ESSAYS ann OBSERVATIONS 
tion lately difcovered by Mr. Bradley, which, 
on this account, lay fo long concealed from 
aftronomers. Of the fame.kind is the mo- 
on of the fixed ftars, that ought to arife from 
the parallax of the earth’s orbit. And of this 
kind I fufpect the variation of the obliquity 
of the ecliptic to be. 
To proceed therefore to what I take to be 
the caufe of this motion, Sir I/aac Newton 
has found, that the fun and planets gravitate 
all towards each other mutually; that it is 
neither the fun (according to the Copernt- 
can fyftem), nor the earth (according to Pfo- 
Jomy), that is the center of the fyftem, or 
fixed point; but the center of gravity of the 
whole fyftem. That the fun therefore moves 
about this center: and that when ‘Fupvter 
and Saturn, the two biggeft planets, are in 
the fame right line, on the fame fide of the 
fun; the center of the fun will be almoft 2 
diameter of the fun, diftant from this fixed 
point. Hence, tho’ we fuppofe the earth to 
move always in the fame plane, the fun will 
appear to have different declinations at the 
time of the folftice: and, as the obliquity of 
the ecliptic is determined always from the 
declination of the fun at the folftice, it will 
therefore 
