the motion of the one follows that of the other*(l he Moon ob- 
ferving the Earth as the Center of its periodic^ motion : ) may 1 
m y well enough be looked upon as one Body ,. or rather one Ag* 
gr agate of Bodies , which have one common center of Gravity $ which i 
Center ( according to the known Laws of Staticks ) is in a i 
ftreight Line connecting' their refpedttve Centers, fo divided S 
as that its parts be in reciprocal proportion to the Gravities of* 
the two Bodies, As for Example } Suppofe the Magnitude ' 
Cand;therefore,prob&bly 3 the Gravityjof the Moon to be about : 
an Qne and four tieth partoi that of the Earths ( and thereabouts i 
Heveliuj in his Selenography page 203. doth out of Tycho, efti- 
mate the proportion s and an exact certainty is not neceflary 
to our prefent bufinefle . ) And the diftance of the Moons Cen- 
ter from the Center of the Earth, to be abou x. fifty fix / 'emidiame - 
ters of the Earth, (as thereabouts he doth there eftimate it, in 2 
its middle diftance ; and we need not be now very accurate in 
determining the numbers ; wherein Aftronomers are not yet t 
very well agreed . ) The diftance of the Common Center of 
Gravity of the two Bodies, will be from that of the Earth, a- 
bout a two and fonrtieth part of fifty fix Semidiameters 5 that ? 
is, about if or f of a Semidiameter ^ that is about f of a Se- 
midiameter of the Earthy above its furface, in the Air, direii- 
ly between the Earth and Moon. 
Now fuppofing the Earth and Moon, joyntly as one Body,, 
carried about by the Sun in the great Orb of the Annua! moti- 
on ; this motion is to be eftimated, ('according to the Laws of 
Staticks , in other cafes,) by the motion of the common Cen- 
ter of Gravity of both Bodies. Eor we ule in Staticks , to efti-* 
mate a Body, or Aggregate of Bodies, to be moved upwards, , 
downwards, or otherwife, fo much as its Common Center of! 
Gravity is fo moved, howfoever the parts may change places > 
amongft themfelves. 
And accordingly, the Line of the Annual motion, ( whether r 
Circular or Elliptical s of which I am not here to difpute, ) will 1 
be deferibed, not by the Center of the Earth ( as we common- * 
ly eftimate it, making the Earth a Primary and the Moon a Se-- 
eondary Planet, ) nor by the Center of the Moon, ( as they r 
would do, who make the Moon the Primary and the Earth a 1 
Secondary r 
