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 465 A S T R C 1 N O M Y. doth to the moon; only with this differ6480 years hence; and in double of that time, or 12,960 foence,he likewife that the earth being furrounded by an atjnofphere, -years, it will be direfted towards the ftar or point C; which will then be the north pole of the heavens, al- and the moon having none, we have twilight after the though it is at prefent Si degrees fouth of the zenith of fun fets ;■ but the lunar inhabitants have an immediate London L. The prefent pofition of the equator E0^_, tranfition from the brighted fun-fhine to the blacked will then be changed into eOq; the tropic of Cancer darknefs. For, (Plate XLIII. fig. 3.) let tkrsiv be TSBZ, into Vt25 ; and the tropic of Capricorn VTrs, the earth, and ABCDEFGH the moon in eight different into trsZ; as is evident by the figure. And the fun, parts of her orbit. As the earth turns round its axis in the fame part of the heavens where he is now over the from wed to ead, when any place comes to t the twilight earthly tropic of Capricorn, and makes the fhorteft days begins there, and when it revolves from thence to r the and longed nights in the northern hemifphere, will then the fun S rifes ; when the place comes to s the fun fets, be over the earthly tropic of Cancer, and make the days and when it comes to ou the twilight ends. But as the longed and nights fhorted. So that it will require moon turns round her axis, which is only once a-month, 12,960 years yet more, or 25,920 from the prefent the moment that any point of her furface comes, to r time, to bring the north pole N quite round, fo as to be (fee the moon at GJ the fun rifes there without any predirected towards that point of the heavens which is ver- vious warning by twilight; and when the fame point tical to it at prefent. And then, and not till then, the comes to / the fiut fets, and that point goes into darknefs fame dars which at prefent defcribe the equator, tropiqp, as black as at midnight. and polar circles, be. by the earth’s diurnal motion, The moon being an opaque fpherical body, (for her hills take off no more from her roundnefs than the inewill defcribe them over again. qualities on the fufface of an orange takes off from its roundnefs), we can only fee that part of the enlightened Chap. XIII. The moorts furface mountain- half of her which is towards the eatth. And therefore, ous : Her phafes deferibed . Her path and when the moon is at A, in coojundtion with the fun S, dark half is towards the earth, and (lie difappears, the paths of Jupiter’ moons delineated: her there being no light on that half to render it viThe proportions of the diameters of their or- asfible.at a,.When comes to her fird odtant at B, or has bits, and thofe of Saturn’s moons, to each gone an eighthihe ipart of her orbit from her conjunflion, a other, and to-ihe diameter of the Sun. quarter of her enlightened fide is towards the earth, and file appears horned, as at b. When {he has gone a quarBy looking at the moon with an ordinary telefcope, ter of her orbit from between the earth and fun to C, we perceive that her furface is diverfified with long tracts {he {hews us one half of her enlightened fide, as at c, of prodigious high mountains and deep cavities. Some and we fay, fixe is a quarter old. At D in her fecond of her mountains, by comparing their height with her odtant, and by {hewing us more of her enlightened fide diameter (which is 2180 miles) are found to be three {he appears gibbous, as at d. At E her whole enlighttimes higher than the highed hills on our earth. This ened fide is towards the earth, and therefore Ihe apruggednefs of the moon’s furface is of great ufe to us, pears round, as at e, when we fay, it is full. moon. by refle&ing the fun’s light to all fides; for if the In her third odtant at F, part of her dark fide being tomoon .were fmooth and polifhed like a looking-glafs, or wards the earth, fire again appears gibbous, and is on covered with water, (he could never didribute the fun’s the decreafe, as at f. At G we fee juft one half of hef light all round ; only in fome pofitions the would fhew us enlightened fide, and {he appears half decreafed, or in her his image no bigger than a point, but with fuch a ludre third quarter, as at g. At H we only fee a quarter of as would be hurtful to our eyes. her enlightened fide, being in her fourth odtant, where The moon’s furface being fo uneven, many have won- {he appears horned, as at h. And at A, having corndered why her edge appears not jagged, as well as the pleated her courfe from the fun to Hie fun again, Ihe difcurve bounding the light and dark places. But if we appears, and we fiy, it is new moon. Thus, in going confider, that what we call the edge of the moon’s dilk from A to E, the moon feems continually to increafe ; is not a {ingle line fet round with mountains, in which and in going from .ff to A, to decreafe in the fame procafe it would appear irregularly indented, but a large portion ; having like phafes at equal diftances from A or zone having many mountains lying behind one another E, but as feen from the fun S, fhe is always full. from the obferver’s eye, we ftiall find that the mountains The moon appears not perfedtly round when ftie is full in fome rows will be oppofite to the vales in others, and in the higheft or loweft part of her orbit, becaufe we fo fill up the inequalities as to make her appear quite have not a full view of her enlightened fide at that time. round; jud as when one looks at an orange, although When full in the higheft part of her orbit, a fmall defiits roughnefs be very difcernible on the fide next the eye, ciency appears on her lower edge ; and the contrary when efpecially if the fun or a candle ftiines obliquely on that full in the loweft part of her orbit. fide, yet the line terminating the vilible part dill appears It is plain by the figure, that when the moon changes fmooth and even. to the earth, the earth appears full to the moon; and As the fun can only enlighten that half of the earth vice verfa. For when the moon is at A, new to the which *s at any moment turned towards him, and being earth, the wdiole enlightened fide of the earth is towards withdrawn from the oppofite half, leaves it in darknefs ; the moon.; and w hen the moon is at E, full to the earth, Vox. '!. No. 20. 3 6 B its