254. As the Sun can only enlighten that half of the Earth which is at
any moment turned towards him, and being withdrawn from the opposite
half leaves it in darkness; so he likewise doth to the Moon: only with
this difference, that the Earth being surrounded by an Atmosphere, and
the Moon having none, we have twilight after the Sun sets; but the Lunar
Inhabitants have an immediate transition from the brightest Sun-shine to
the blackest darkness § 177. For, let _tkrsw_ be the Earth, and _A_,
_B_, _C_, _D_, _E_, _F_, _G_, _H_ the Moon in eight different parts of
her Orbit. As the Earth turns round its Axis, from west to east, when
any place comes to _t_ the twilight begins there, and when it revolves
from thence to _r_ the Sun _S_ rises; when the place comes to _s_ the
Sun sets, and when it comes to _w_ the twilight ends. But as the Moon
turns round her Axis, which is only once a month, the moment that any
point of her surface comes to _r_ (see the Moon at _G_) the Sun rises
there without any previous warning by twilight; and when the same point
comes to _s_ the Sun sets, and that point goes into darkness as black as
at midnight.
[Sidenote: The Moon’s Phases.]
255. The Moon being an opaque spherical body, (for her hills take off no
more from her roundness than the inequalities on the surface of an
orange takes off from its roundness) we can only see that part of the
enlightened half of her which is towards the Earth. And therefore, when
the Moon is at _A_, in conjunction with the Sun _S_, her dark half is
towards the Earth, and she disappears as at _a_, there being no light on
that half to render it visible. When she comes to her first Octant at
_B_, or has gone an eighth part of her orbit from her Conjunction, a
quarter of her enlightened side is towards the Earth, and she appears
horned as at _b_. When she has gone a quarter of her orbit from between
the Earth and Sun to _C_, she shews us one half of her enlightened side
as at _c_, and we say, she is a quarter old. At _D_ she is in her second
Octant, and by shewing us more of her enlightened side she appears
gibbous as at _d_. At _E_ her whole enlightened side is towards the
Earth, and therefore she appears round as at _e_, when we say, it is
Full Moon. In her third Octant at _F_, part of her dark side being
towards the Earth, she again appears gibbous, and is on the decrease, as
at _f_. At _G_ we see just one half of her enlightened side, and she
appears half decreased, or in her third Quarter, as at _g_. At _H_ we
only see a quarter of her enlightened side, being in her fourth Octant,
where she appears horned as at _h_. And at _A_, having compleated her
course from the Sun to the Sun again, she disappears; and we say, it is
New Moon. Thus in going from _A_ to _E_ the Moon seems continually to
increase; and in going from _E_ to _A_, to decrease in the same
proportion; having like Phases at equal distances from _A_ or _E_, but
as seen from the Sun _S_, she is always Full.
[Sidenote: The Moon’s Disc not always quite round when full.]
256. The Moon appears not perfectly round when she is Full in the
highest or lowest part of her Orbit, because we have not a direct view
of her enlightened side at that time. When Full in the highest part of
her orbit, a small deficiency appears on her lower edge; and the
contrary when Full in the lowest part of her Orbit.
[Sidenote: The Phases of the Earth and Moon contrary.]
257. ’Tis plain by the Figure, that when the Moon changes to the Earth,
the Earth appears Full to the Moon; and _vice versâ_. For when the Moon
is at _A_, _New_ to the Earth, the whole enlightened side of the Earth
is towards the Moon: and when the Moon is at _E_, _Full_ to the Earth,
it’s dark side is towards her. Hence a _New Moon_ answers to a _Full
Earth_, and a _Full Moon_ to a _New Earth_. The _Quarters_ are also
reversed to each other.
[Sidenote: An agreeable Phenomenon.]
258. Between the third Quarter and Change, the Moon is frequently
visible in the forenoon, even when the Sun shines; and then she affords
us an opportunity of seeing a very agreeable appearance, wherever we
find a globular stone above the level of the eye, as suppose on the top
of a gate. For, if the Sun shines on the stone, and we place ourselves
so as the upper part of the stone may just seem to touch the point of
the Moon’s lowermost horn, we shall then see the enlightened part of the
stone exactly of the same shape with the Moon; horned as she is, and
inclining the same way to the Horizon. The reason is plain; for the Sun
enlightens the stone the same way as he does the Moon: and both being
Globes, when we put ourselves into the above situation, the Moon and
stone have the same position to our eyes; and therefore we must see as
much of the illuminated part of the one as of the other.
[Sidenote: The nonagesimal Degree, what.]
259. The position of the Moon’s Cusps, or a right line touching the
points of her horns, is very differently inclined to the Horizon at
different hours of the same days of her age. Sometimes she stands, as it
were, upright on her lower horn, and then such a line is perpendicular
to the Horizon: when this, happens, she is in what the Astronomers call
_the Nonagesimal Degree_; which is the highest point of the Ecliptic
above the Horizon at that time, and is 90 degrees from both sides of the
Horizon where it is then cut by the Ecliptic. But this never happens
when the Moon is on the Meridian, except when she is at the very
beginning of Cancer or Capricorn.
[Sidenote: How the inclination of the Ecliptic may be found by the
position of the Moon horns.