| XXX | (468) | XXX |
463 ASTRO N O M Y. earth at that time. But this difficulty vaniffies when fourth fatellite d is always progreffive, making neither we confider, that a common impulfe on any fyftem of loops nor angles in the heavens; but comes to its next bodies affefts not their relative motions; but that they conjunction at e between the numeral figures 16 and 17, will continue to attraft, impel,-or circulate round one an- or in 16 days 18 hours. In order to have a tolerably other, in the fame manner as if there was no fuch im- good figure of the paths of thefe fatellites, take the pulfe. The moon is fo near "the earth, and both of them following method. fo far from the-fun, that the attractive power of the fun It appears by the fcheme, that the three firft fatellites may he confidered as equal on both ; and therefore, the come almoft into the fame line or pofition every feventh moon will continue to circulate round the earth in the day ; the firft being only a little behind with the fecond, fame manner as if the fun did not attradl them at all; a -d the fecond behind with the third. But the period of like bodies in the cabbin of a (hip, which move round, the fourth fatellite is fo incommenfurate to the periods of or impel one another, in the fame manner when the (hip the other three, that it cannot be gueffed at by the diais under fail, as when it is at reft, becaufe they are all gram when it would fall again into a line of conjunCHon equally affeCted by the common motion of the {hip. If with them, between Jupiter and the fun. And no wonby any other caufe, fuch as the near approach of a comet, der ; for fuppofing them all to have been once in conthe moon’s diftance from the earth fhould happen to be junction, it will require 3,087,043,493,260 years to fo much increafed, that the difference of their gravita- bring them in conjunction again. ting forces towards the fun fliould exceed that of the In Plate XLIV. fig. 1. we have the proportions of the moon towards the earth ; in that cafe, the moon, when orbits of Saturn’s five fatellites, and of Jupiter’s four, in conjunction, would abandon the earth, and be either to one another, to our moon’s orbit, and to the difk of drawn into the fun, or comet, or circulate round a- the fun. S is the fun ; M rn the moon’s orbit, (the bout it. earth fuppofed to be at E ;), J Jupiter ; 1234 the The curves which Jupiter’s fatellites deferibe, are all orbits of his four moons or fatellites ; Sat Saturn ; and of different forts from the path deferibed by our moon, x 2 3 4 5 the orbits of his five moons. Hence it apalthough thefe fatejlites go round Jupiter, as the moon pears, that the fun would much more than fill the whole goes round the earth. In Plate XLIII. fig. 3. let orbit of the moon ; for the fun’s diameter is 763,000 j4BCDE, &c. be as much of Jupiter’s orbit as he de- miles, and the diameter of the moon’s orbit ^only feribes in 18 days from A to T; and the curves abed 480,000. In proportion to all thefe orbits of the fatelwill be the path's of his four moons going round him in lites, the radius of Saturn’s annual orbit would be 21-J yards, of Jupiter’s orbit n-J, and of'the earth’s 2^* his progreffive motion. Now let us fuppofe all thefe moons to fet out from a taking them in round numbers. conjunction with the fun, as feen from Jupiter at A; then his firft or neareft moon will be at a, his fecond at Chap. XIV. The Phenomena of the Harveflb, his third at c, and his fourth at d. At the end of 24 terreftrial hours after this conjunction, Jupiter has moon explained by a common Globe: The moved to Bt his firft moon or fatellite has deferibed the years in which the Harvejl-moons are leajl curve ai, his fecond the curve by his third ci, and his and mojl beneficial from 1751, to 18(^1. fourth di. The next day, when Jupiter is at C, his firft fatellite has deferibed the curve «2, from its con- The long Duration of Moon-light at the junction, his fecond the curve hi, his third the curve a, Poles in Winter and his fourth the curve ^2, and fo on. The numeral figures under the capital letters (hew Jupiter’s place in his It is generally believed that the moon rifes about 48 path every day for 18 days, accounted from A to T minutes later every day than on the preceding; but this and the like figures fet to the paths of his fatellites, fhew is true only v/ith regard to places on the equator. In wherq they are at the like times. The firft fatellite, al- places-of confiderable latitude there is a remarkable difmoft under C, is ftationary at ft- as feen from the fun ; ference, efpecially in the harveft time ; with which farand retrograde from + to 2 : at 2 it appears ftationary mers were better acquainted than aftronomers till of again, and thence it moves forward until it has paft 3, late ; and gratefully aferibed the early rifing of the full and is twice ftationary, and once retrograde, between 3 moon at that time of the year to the goodnefs of God, and 4. The path of ihis fatellite interfeCts itfelf every in ordering it fo on purpofe to give them an immediate fupply of moon-light after fun-fet for their greater con42t in the 2 3 houfs, 5 7 9 10making 12 14fuch16 loops 18, aaslittle afterdiagram every con-at veniency in reaping the fruits of the earth. And indeed. junction. The fecond fatellite b, moving flower, barely In this inftance of the harveft-,moon, as in many others crofles its path every 3 days 13 hours ; as at 4 7 11 difcoverable by aftronomy, the wifdom and beneficence 14 18, making only five loops and as many conjunctions of the Deity is confpicuous, who really ordered the in-the time that the firft makes ten. The third fatellite courfe of the moon fo, as to beftow more or lefs light c moving ftill flower, and having deferibed the curve on all parts of the earth as their feveral circumftances £ 1 2 3 4 5 6 7, comes to an angle at 7 in con- and feafons render it more or lefs ferviceable. About junction with the fun at the end of 7 days 4 hours ; and the equator, where there is no variety of feafons, and fo gaps on to deferibe fuch another curve 789101! the weather changes feldom, and at ftated times, moon1.2 13 14, and is at 14 in its next conjunction. The light is not neceffary for gathering in the produce of the ground ;
