us and
later astronomers, is found to be distant 29 and about 5/6 diameters of the
Earth from the Earth's centre. The Moon's revolution with respect to the
Sun takes place in 29-1/2 days and 44 minutes of time. We reckon the motion
with respect to the sun, not the periodic motion, {232} just as a day is
one entire revolution of the Earth with respect to the Sun, not one
periodick revolution; because the Sun is the cause of lunar as of
terrestrial motion: also, because (on the hypotheses of later observers)
the synodical month is truly periodic, on account of the Earth's motion in
a great orbit. The proportion of diameters to circumferences is the same.
And the concentrick orbit of the Moon contains twice over 29 and 1/2 great
circles of the Earth & a little more. The Moon & the Earth, then, agree
together in a double proportion of motion; & the Earth moves in the space
of twenty-four hours, in its diurnal motion; because the Moon has a motion
proportional to the Earth, but the Earth a motion agreeing with the lunar
motion in a nearly double proportion. There is some difference in details,
because the distances of the stars in details have not been examined
sufficiently exactly, nor are mathematicians as yet agreed about them. The
Earth therefore revolves in a space of 24 hours, as the Moon in her monthly
course, by a magnetick confederation of both stars, the globes being
forwarded in their movement by the Sun, according to the proportion of
their orbits, as Aristotle allows, _de Coelo_, bk. ii., chap. 10. "It
happens" (he says) "that the motions are performed through a proportion
existing between them severally, namely, at the same intervals in which
some are swifter, others slower," But it is more agreeable to the relation
between the Moon and the Earth, that that harmony of motion should be due
to the fact that they are bodies rather near together, and very like each
other in nature and substance, and that the Moon has more evident effects
upon the Earth than the rest of the stars, the Sun excepted; also because
the Moon alone of all the planets conducts her revolutions, directly
(however diverse even), with reference to the Earth's centre, and is
especially akin to the Earth, and bound to it as with chains. This, then,
is the true symmetry and harmony between the motions of the Earth and the
Moon; not that old oft-besung harmony of coelestial motions, which assumes
that the nearer any sphaere is to the _Primum Mobile_