252. By looking at the Moon with an ordinary telescope we perceive that her surface is diversified with long tracts of prodigious high mountains and deep cavities. Some of her mountains, by comparing their height with her diameter (which is 2180 miles) are found to be three times higher than the highest hills on our Earth. This ruggedness of the Moon’s surface is of great use to us, by reflecting the Sun’s light to all sides: for if the Moon were smooth and polished like a looking-glass, or covered with water, she could never distribute the Sun’s light all round; only in some positions she would shew us his image, no bigger than a point, but with such a lustre as would be hurtful to our eyes. [Sidenote: Why no hills appear on her edge.] 253. The Moon’s surface being so uneven, many have wondered why her edge appears not jagged, as well as the curve bounding the light and dark places. But if we consider, that what we call the edge of the Moon’s Disc is not a single line set round with mountains, in which case it would appear irregularly indented, but a large Zone having many mountains lying behind one another from the observer’s eye, we shall find that the mountains in some rows will be opposite to the vales in others; and so fill up the inequalities as to make her appear quite round: just as when one looks at an orange, although it’s roughness be very discernible on the side next the eye, especially if the Sun or a Candle shines obliquely on that side, yet the line terminating the visible part still appears smooth and even. [Illustration: Plate VII. _J. Ferguson delin._ _J. Mynde Sculp._] [Sidenote: The Moon has no twilight. Fig. I.] 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 As the Sun can only enlighten that half of the Earth which is at any moment turned towards him, and being withdrawn from the opposite half leaves it in darkness; so he likewise doth to the Moon: only with this difference, that the Earth being surrounded by an Atmosphere, and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.]