261. The Moon generally appears as large as the Sun; for the Angle
_vkA_, under which the Moon is seen from the Earth, is the same with the
Angle _LkM_, under which the Sun is seen from it. And therefore the Moon
may hide the Sun’s whole Disc from us, as she sometimes does in solar
Eclipses. The reason why she does not eclipse the Sun at every Change
shall be explained afterwards. If the Moon were farther from the Earth
as at _a_, she could never hide the whole of the Sun from us; for then
she would appear under the Angle _NkO_, eclipsing only that part of the
Sun which lies between _N_ and _O_: were she still further from the
Earth, as at _X_, she would appear under the small Angle _TkW_, like a
spot on the Sun, hiding only the part _TW_ from our sight.
[Sidenote: A proof of the Moon’s turning round her Axis.]
262. The Moon turns round her Axis in the time that she goes round her
orbit; which is evident from hence, that a spectator at rest, without
the periphery of the Moon’s orbit, would see all her sides turned
regularly towards him in that time. She turns round her Axis from any
Star to the same Star again in 27 days 8 hours; from the Sun to the Sun
again in 29-1/2 days: the former is the length of her sidereal day, and
the latter the length of her solar day. A body moving round the Sun
would have a solar day in every revolution, without turning on it’s
Axis; the same as if it had kept all the while at rest, and the Sun
moved round it: but without turning round it’s Axis it could never have
one sidereal day, because it would always keep the same side towards any
given Star.
[Sidenote: Her periodical and synodical Revolution.]
263. If the Earth had no annual motion, the Moon would go round it so as
to compleat a Lunation, a sidereal, and a solar day, all in the same
time. But, because the Earth goes forward in it’s orbit while the Moon
goes round the Earth in her orbit, the Moon must go as much more than
round her orbit from Change to Change in compleating a solar day as the
Earth has gone forward in it’s orbit during that time, _i. e._ almost a
twelfth part of a Circle.
[Sidenote: Familiarly represented.
A Table shewing the times that the hour and minute hands of a
watch are in conjunction.
A machine for shewing the motions of the Sun and Moon.