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.]