y, the Gravity) of the Moon to
be about an _One and fourtieth part_ of that of the Earth; (and thereabouts
_Hevelius_ in his _Selenography_ page 203. doth out of _Tycho_, estimate
the proportion; and an exact certainty is not necessary to our present
businesse.) And the distance of the Moons Center from the Center of the
Earth, to be about _fifty six Semidiameters_ of the Earth, (as thereabouts
he doth there estimate it, in its middle distance; and we need not be now
very accurate in determining the numbers; wherein Astronomers are not yet
very well agreed.) The distance of the Common Center of Gravity of the two
Bodies, will be from that of the Earth, about a two and fourtieth part of
fifty six Semidiameters; that is, about 56/42 or 4/3 of a Semidiameter;
that is about 1/3 of a Semidiameter of the Earth, above its surface, in the
Air, directly between the Earth and Moon.

Now supposing the Earth and Moon, joyntly as one Body, carried about by the
Sun in the great Orb of the _Annual_ motion; this motion is to be
estimated, (according to the Laws of _Staticks_, in other cases,) by the
motion of the common Center of Gravity of both Bodies. For we use in
_Staticks_, to estimate a Body, or Aggregate of Bodies, to be moved
upwards, downwards, or otherwise, so much as its Common Center of Gravity
is so moved, howsoever the parts may change places amongst themselves.

And accordingly, the Line of the _Annual_ motion, (whether _Circular_ or
_Elliptical_; of which I am not here to dispute,) will be described, not by
the Center of the Earth (as we commonly estimate it, making the Earth a
Primary and the Moon a Secondary Planet,) nor by the Center of the Moon,
(as they would do, who make the Moon the Primary and the Earth a {273}
Secondary Planet, against which we were before disputing:) But by the
_Common Center of Gravity of the Bodies, Earth and Moon_, as one Aggregate.

[Sidenote: See Fig. 2. and 3.]

Now supposing A B C D E to be a part of the great Orb of the _Annual_
motion, described by the Common Center of Gravity, in so long time as from
a _Full-Moon_ at A to the next _New-Moon_ at E; (which, though an Arch of a
_Circle_ or _Ellipse_, whose Center we suppose at a due distance below it;
yet being put about 1/25 of the whole, may well enough be here represented
by a streight Line:) the Center of the Earth at T, and that of the Moon at
L, must each of them (supposing their common Center of Gravity to keep the
Line A E) be