Page:Encyclopædia Britannica, first edition - Volume I, A-B.pdf/584

 484 ASTRO N O M Y. centre falling on the middle thereof, at the middle of the of her’ being full, and the reafon why (he is not eclipfed general eclipfe. The farther the moon changes from at every full, has been Ihewn already. In Plate XLVI. fig. 3. either node, v/ithin 17 degrees of it, the fhorter is the let S be the fun, E the earth, RR the earth’s lhadow, and penumbra’s continuance on the earth, becaufe it goes o- and i? the moon in oppofition to the fun: In this fituaver a lefs portion of the diilc, as is evident by the fi- tion the earth intercepts the fun’s light in its way to the gure. moon ; and when the moon touches the earth’s ftiadow at The nearer that the penumbra’s centre is to the equa- v, ftie begins to be eclipfed on her eaftern limb x, and tor at the mifldie of the general eclipfe, - the longer is the continues eclipfed until her weftern limby leaves the fliaduration of the eclipfe at all thofe places where it is dow at w: At Ihe is in the middle of the ftiadow, central; becaufe, the nearer that any place is to the e- and confequently in the middle of the eclipfe. quator, the greater is the circle it defcribes by the The moon, when totally eclipfed, is not invifible if ftie earth’s motion on its axis: And fo, the place moving be above the horizon and the ftcy be clear; but appears quicker, keeps longer in the penumbra, whofe motion is generally of a dufky colour, like tarniftied copper, which the fame way with that of the place, though falter, as fome have thought to be the moon’s native light. But has been already mentioned. Thus (fee the earth at the true caufe of her being vifible is the fcattered beams D and the penumbra at 12) whilft the point b in the of the fun, bent, into the earth’s ftiadow by going polar circle abed is carried from $ to c by the earth’s through the atmofphere; which, being more or lels diurnal motion, the point d on the tropic of Cancer T denfe near the earth than at confiderable heights above is carried a much greater length from dto D; and there- it, refradts or bends the fun’s rays more inward, the fore, if the penumbra’s centre goes one time over c and nearer they are pafiing by the earth’s furface, than thofe another time over D, the penumbra will be longer in rays which go through higher parts of the atmofphere, palling over the moving place d than it was in palling o- where it is lefs denfe according to its height, until it be ver the moving place b. Confequently, central eclipfes fo thin or rare as to lofe its refradtive power. Let the about the poles are of the Ihortefl duration ; and about circleconcentric to the earth, include the atmofphere whofe refradtive power vanifties at the heights f the equator of the longell. In the middle of fummer, the whole frigid zone, in- and i; fo that the rays JV/nu and Viv go on ftraight cluded by the polar circle abed, is enlightened ; and if it without fuffering the leaft refradlion : But all thofe rays then happens, that the penumbra’s centre goes over the which enter the atmofphere between f and k, and benorth pole, the fun will be eclipfed much the fame num- tween i and /, on oppofite fides of the earth, are gradualber of digits at « as at c; but whilft the penumbra moves ly more bent inward as they go through a greater poreaftward over c, it moves weft ward over *7; becaufe, tion of the atmofphere, until the rays IVk and I7/touchwith refpedt to the penumbra, the motions of a and c ing the earth at m and «, are bent fo much as to meet are contrary : For c moves the fame way with the pe- at q, a little ftiort of the moon ; and therefore the dark numbra towards d, but a moves the contrary way to- ftiadow of the earth is contained in the fpace moqpn, wards b ; and therefore the eclipfe will be of longer du- where none of the fun’s rays can enter : All the reft RR, ration at c than at a. At « the eclipfe begins on the fun’s being mixed by the fcattered rays which are refradted as eaftern limb, but at c on his weftern: At all places lying above, is in fome meafure enlightened by them ; and without the polar circles, the fun’s eclipfes begin on his fome of thofe rays falling on the moon, give her the weftern limb, or near it, and end on of near his eaftern. colour of tarniftied copper, or of iron almoft red hot. At thofe places where the penumbra touches the earth, So that if the earth had no atmofphere, the moon would the eclipfe begins with the rifing fun, on the top of his be as invifible in total eclipfes as ftie is when new. If weftern or uppermoft edge ; and at thofe places where the moon were fo near the earth as to go into its dark the penumbra leaves the earth, the eclipfe ends with the ftiadow, fuppofe about po, fhe would be invifible during fetting fun, on the top of his eaftern edge, which is then her ftay in it; but vifible before and after in the fainter the uppermoft, juft at its difappearing in the horizon. ftiadow RR. If the moon were furrounded by an atraofphere of any When the moon goes through the centre of the earth’s confiderable denfity, it would feem to touch the fun a ftiadow, ftie is diredlly oppofite to the fun : Yet the little before the moon made her appulfe to his edge, moon has been often feen totally eclipfed in the horizon and we 'ftiould fee a little faintnefs on that edge before it when the fun was alfo vifible in the oppofite part of it : were eclipfed by the moon: But as no fuch faintnefs has For, the horizontal refradtion being almoft 34 minutes been obferved, it feems plain, that the moon has no fuch of a degree, and the diameter of the fun and moon being atmofphere as that of the earth. The faint ring of light each at a mean ftate but 32 minutes, the refradtion caufes furrounding the fun in total eclipfes, called by Caffini both luminaries to appear above the horizon when they la ckevelure da foleil, feems to be the atmofphere of the are really below it. fun; becaufe it has been obferved to move equally with When the moon is full at 12 degrees from either of her nodes, ftie juft touches the earth’.s ftiadow, but enthe fun, not with the moon. Having been fo prolix concerning eclipfes of the fhn, ters not into it. In Plate XLVII. fig. 3. let GH be the ewe ihall drop that fubjeA at prefent, and proceed to the cliptic, ef the moon’s orbit where ftie is 12 degrees from do&rine of lunar eclipfes ; which, being more Ample, the node at her full; cd her orbit where fire is 6 degrees from the node, ah her orbit where ftie is full in the may be explained in lefs time. That the moon can never be eclipfed but at the time node, AB the earth’s ftiadow, and M the moon. When