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 45 7 ASTRO N O M Y. the earth goes from G to B. Indeed, bodt rence of longitude. Confequently, if the beginning or itthefewhileappearances be (een of the fecond, third, and ending of a lunar eclipfe be obferved, fuppofe at Lon- fourth fatellite whenmayeclipfed, the earth is between don, to be exaftly at midnight, and in fome other place D and E, or between G and whilft A; but never of the firft at 11 at night, that place is 15 degrees weftward from fatellite, on account of the fmallnefs of its orbit and the meridian of London ; if the fame eclipfe be obferved the bulk of Jupiter, except only when Jupiter is diredtly at 1 in the morning at another place, that place is x 5 de- oppofite to the fun, that is, when the earth is at g; and grees eaftward from the laid meridian. But as it is not eafy to determine the exatt moment even then, ftridtly fpeaking, we cannot fee either the either of the beginning or ending of a lunar eclipfe, be- immerfions or emerfions of any of his fatellites., becaufe body being direCtly between us and his conical (hacaufe the earth’s lhadow, through which the moon paffes, his his fatellites are hid by his body a few moments is faint and ill defined about the edges, we have recourfe dow, to the eclipfes of Jupiter’s fatellites, which difappear fo before they touch his fhadow p and are quite emerged inllantaneoufly as they enter Jupiter’s ihadow, and e- from thence before we can fee them, as it were, juft merge fo fuddenly out of it, that we may fix the pheno- dropping from him. And when the earth is at c, the menon to half a fecond of time. The firfl: or neareft fun, being between it and Jupiter, hides both him and fatellite to Jupiter is the moft advantageous for this pur- his moons from us. pofe, becaufe its motion is quicker than the motion of In this diagram, the orbits of Jupiter’s moons are any of the reft, and therefore its immerfions and emer- drawn in true proportion to his diameter; but, in proportion to the earth’s orbit, they are drawn 81 times fions are more frequent. The Englifti aftronomers have calculated tables for too large. (hewing the times of the eclipfes of Jupiter’s fatellites In whatever month of the year Jupiter is in conjuncto great precifion, for the meridian of Greenwich. Now, tion with the fun, or in oppofition to him, in the next let an obferver, who has thefe tables, with a good tele- year it will be a month later at lead. For whilft the fcope and a well-regulated clock at any other place of earth goes once round the fun, Jupiter defcribes.a twelfth the earth, obferve the beginning or ending of an eclipfe part of his orbit. And therefore, when the earth has of one of Jupiter’s fatelhfes, and note the precife mo- finilhed its annual period, from being in a line with the ment of time that he faw the fatellite either immerge into, fun and Jupiter, it muft go as much forwarder as Jupiter or emerge out of the (hadow, and compare that time has moved in that time, to overtake him again.; juft like with the time (hewn by the tables for Greenwich ; then, the minute-hand of a watch, which muft, from any con15 degrees difference of longitude being allowed for e- junction with the hour-hand, go once round the dial-plate very hour’s difference of time, will give the longitude and fomewhat above a twelfth part more, to overtake of that place from Greenwich, as above; and if there the hour hand again. be any odd minutes of time, for every minute a qua> ter It is found by obfervation, that when the earth is beof a degree, eaft or weft, muft be allowed, as the time tween the fun and Jupiter, as at g, his (atellites are ecof obfervation is later or earlier than the time (hewn by about 8 mffiutes fooner than they fhould be Accor-, the tables. Such eclipfes are very convenient for this lipfed ding to the tables; and when the earth is at B ox C, purpofe at land, becaufe they happen almoft every day; thele eclipfes happen about 8 minutes later than the tables but are of no ufe at fea, becaufe the rolling of the (hip predict them Hence it is undeniably certain, that the hinders all nice telefcopical obfervations. motion of light is not inftantanepus, fince it takes about . To explain this by a figure, in Plate XLII. fig. x. i6i minutes of time to go through a fpace equal to the let J be Jupiter, K, L, M, iVhis four fatellites in their diameter’of the earth’s orbit, which is 162 millions of refpedtive orbits, 1, 2, 3, 4; and let the earth be at f, miles in length ; and confequently the particles of light (fuppofe in November, although that month is no other- fly about 164 thoufand 494 miles every fecond of tiine, wife material than to find the earth readily in this which is above a million of times fwifter than the motion fcheme, where it is (hewn in eight different parts of its of a cannon-bullet. And as light is i&J minutes in.traorbit). Let be a place on1 the- meridian of Green- velling acrofs .the earth’s orbit, it muft be 8^ minutes in wich, and R a place on fome other meridian eaftward coming from the fun to us ; therefore if the fun were from Greenwich. Let a perfon at R obferve the inftan- annihilated, we fhould fee him for 8 J minutes after ; and taneous vaniftiing of the firft fatellite K into Jupiter’s if he were again created, he would be gj minutes old fhadow, fuppofe at three o)clock in the morning; but before we could fee him. by the tables he finds the immerfion of that fatellite to be To illuftrate this progrefltve motion of light, (Plate at midnight.at Greenwich ; he can then immediately de- XLII. fig. 2.), 'let A/Aodi. B be the earth in two diftermine, that as there are three hours difference of time be- ferent parts of its orbit, whofe diftance from each other tween J^and R', and that R is three hours forwarder in is 8 1 millions of miles, equal to the earth’s diftance from reckoning than it muft be 45 degrees of eaft longitude the fun S. It is plain, that if the motion of light were from the meridian of i£. Were this method as practicable inftantaneous, the fatellite 1 would appear to enter into at fea as at land, any failor might almoll: as eafily, and Jupiter’s fhadow FF at the fame moment of time to a with equal certainty, find the longitude as the latitude. fpeCUtor in A, as to another in B. But by many years Whilft the earth is going from C7 to E in its orbit, obfervatipns it has been found, that the immerfion of the only the immerfions of Jupiter’s fatellites into hi^ fha- fatellite ’nto the fhadow is feen 8J minutes fooner when dow are generally feen; and their emerfions put of the earth is at U, than when it is at A. And fo, as Vox-. I. Numb. 19. 3 sZ ' Mr