the motioti of the one follows that of the otherj( 1 he Moou ob. 
ferving the Earth as the Center of its periodick, motion; ) may 
Well cnongh be looked upon as Body, ov v^thzr one Agj^ret 
gate of Bodies , which have one commen center of Gravity ; which 
Center ( according to the known Laws of Staticl^j ) is in a 
ftreight Line connc<Sting their refpedtive Centers, lo divided 
as that its parts be in reciprocal proportion to the Gravities of 
thc twQ Bodies. As for Example , Suppofethe Magnitude 
(and ther^fqre.proba^ the Moon to be about 
an One ank, fom tieth part of that oi the Earth >, ( and thereabouts 
Hevelius in his Selenography page 205. doth out of Tycho, elli- 
macethe proportions and an exa<9: certainty is not ncceflary 
to ourprefentbuiSnelTe;). And the diftance of the Moons Cen- 
ter from, the Center of the Earth,to be about fifty fix femidiame^ 
terj of the Earth> ( as thereabouts he doth there eftimate it^ in 
its middle diftance j and we need not be now very!^ accurate in 
detci mining the numbers ; wherein Aftronomers arc not yet 
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 fourtieth part of fifty fix Semidiameters ^ that 
is, about 4» or t of a SemidiametCT ^ that is about t ofa Se- 
midiameter of the Earthj above its (urface, in the Air, direct- 
ly between the Earth antl Moon. 
Now fuppofiing the Earth and .Moon, Joyntly as one Body, 
carried about by the Sun in the great Orb of the Annual ixxoiu 
on ; this motion is to be eftimated, ("according to the Laws of 
Statick^, in other cafes,) by the motion of the common Cen- 
ter of gravity of both Bodies. For we ufe in Statieks, to efti- 
rnate a Bpdy, or Aggregate of Bodief, to be movred upwards, 
downwards, or other wife, fo much as its Common Center of 
Gravity is fo moved, howfoever the parts may change places 
amongft thcmfelves. 
. And accordingly, the Line of the Annual motion, ( whether 
Circular ox EUiptical i of which I arn not here to difpute, ) will 
bedefcribed, notby the Center; of the Earth ( as we common- 
ly eftimate it; makingthe Earth a Primary and the Moon a Se- 
condary Planet, ) nor by the Center of the Moon, ( as they 
would do, who make the Moon the Primary and the Earth a 
Secondary 
