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 441 ASTRO N O M Y. taining the length of their year, though their method fide next her is feen ; the other half being hid below the and ours mufl 'differ. For we can know the length of horizon of all places on that circle. To her the earth our year by the return of our equinoxes; but the lunafeems to be the largeft body in the univerfe, for it aprians, having always equal day and night, mufl have pears thirteen times as large as Ihe does to us. The moon has no atmofphere of any vifible denfity recourfe to another method; and we may fuppofe, they furrounding her; for if Ihe had, we could never fee her meafure their year by obferving when either of the poles edge fo well defined as it appears ; but there would be a of our earth begins to be enlightened, and the other to fort of a mifi: or hazinefs around her, which would make difappear, which is always at our equinoxes, they being the ftars look fainter, when they are feen through it. conveniently fituated for obferving great tracks of land But obfervation proves, that the flats which difappear about our earth’s poles, which are entirely unknown to behind the moon retain their full lullre until they feem us. Hence we may conclude, that the year is of the to touch her very edge, and then they vanifli m a mo- fame abfolute length both to the earth and moon, though ment. The faint light which has been feen all around very different as to the number of days; we having 3654 the moon in total eclipfes of the fun, has been obferved, natural days, and the lunarians omy 12-r?; every day during the time of darknefs, to have its centre coincident and night in the mqon-being as long as 29-!-on the earth. with the centre of the fun; and was therefore much The moon’s inhabitants on the fide next the earth may more likely to arife from the atmofphere of the fun than as eafily find the longitude of their places as we can find from that of the moon ; for if it had been owing to the the latitude of ours. For the earth keeping conflantly, latter, its centre would have gone along with the moon’s. or very nearly fo, over one meridian of the moon, the If there were feas in the moon, Ihe could have no eafl or well diflances of places from that meridian are as clouds, rains, nor florms, as we have; becaufe fhe has eafily found as we can find our diflance from the equator no fuch atmofphere to fupport the vapours which occafion by the altitude of our celeflial poles. them. And every one knows, that when the moon is The planet Mars is next in order, being the firft above our horizon in the night-time, fhe is vifible, un- above the earth’s orbit. His diflance from the fun is lefs the clouds of our atmofphere hide her from our computed to be 125 millions of miles; and by travelling view, and all parts of her appear conflantly with the at the rate of 47 thoufand miles every hour, as in the fame clear, ferene, and calm afpedt. But thofe dark circle cf, he goes round the fun in 636 of our days and parts of the moon, which were formerly thought,to be 23 hours; which is the length of his year, and contains feas, are now found to i>e only va/t deep cavities, and 667-|- of his days, every day and night together being 40 places which refledt not the fun’s light fo flrongly as o- minutes longer than with us. His diameter is 4444 thers, having many caverns and pits, whofe fhadows fall miles, and by his diurnal rotation the inhabitants about within theip, and are always dark on the fides next the his equator are carried 556 miles every hour. His quanfun, which detnonflrates their being hollow; and molt tity of vght and heat is equal but to one half of ours ; of thefe pits have little knobs like hillocks Handing with- and the fun appears but half as big to him as to us. in them, and calling fhadows alfo; which caufe thefe This planet being but a fifth part fo big as the earth, thefe places to appear darker than others which have if any moon’ attends him, fhe mufl be very fmall, and fewer or lefs remarkable caverns. All thefe appear- has not yet been difcovered by our befl telefcopes. He ances fhew, that there are no feas in the moon; for if is of a fiery red colour, and by his appulfes to fome of there were any, their furfaces would appear fmooth and the fixed ftars feems to be encompaffed by a very grofs atmofphere. He appears fometimes gibbous, but never even, like thofe on the earth. There being no atmofphere about the moon, the hea- horned; which both fhews that his orbit includes the vens in the day-time have the appearance of night to a earth’s within it, and that he fhines not by his own light. lunarian who turns his back toward the fun; and when To Mars, our earth and moon appear like two moons, he does, the flars appear as bright to him as they do in a bigger and a lefs, changing places with one another, the night to us. For it is entirely owing to our atmo- and appearing fometimes horned, fometimes half or three fphere that the heavens are bright about us in the day. quarters illuminated, but never full, nor at mofl above As the earth turns round its axis, the feveral conti- one quarter of a degree from each other, although they nents, feas, and iflands appear to the moon’s inhabitants are 240 thoufand miles afunder. like fo many fpots of different forms and brightnefs, mo- Our earth appears almofl as big to Mars as Venus ving over its furface, but much fainter at fome times than does to us, and at Mars it is never feen above 48 degrees others, as our clouds cover them or leave them. By thefe from the fun; fometimes it appears to pafs over the difk fpots, the Idnarians can determine the time of the earth’s of the fun, and fo do Mercury and Venus; but Mercury diurnal motion, juft as we do the motion of the fun; and can never be feen from Mars by fuch eyes as ours, unafperhaps they meafure their time by the motion of the fifled by proper inflruments; and Venus will be as feldom feen as we fee Mercury. Jupiter and Saturn are as viearth’s fpots, for they cannot have a truer dial. The moon’s axis is fo nearly perpendicular to the ec- fible. to Mars as to us. His axis is perpendicular to the liptic, that the fun never removes fenfibly from her equa- ecliptic, and his orbit is 2 degrees inclined to it. tor ; and the obliquity of her orbit, being only 5-j- de- Jupiter, the largefl of all the planets, is flill higher grees, which is next to nothing as feen from the fun, in the fyflem, being about 426 millions of miles from the cannot caufe the fun to decline fenfibly from her equator. fun; and going at the rate of 25 thoufand miles every Yet her inhabitants are not.deflitute of means for afcer- hour in his orbit, as in the circle 2/, finifhes his annual 5T period Vol. I. No. 19. 3