| XXX | (453) | XXX |
ASTRONOMY. 453 another as the cubes of their diameters ; whence, if the found by the following analogy, their periods round fun be 81 millions of miles from the earth, to appear as him being afcertained by obfervation. As the fquare of big as' the moon, whofe diftance does not exceed 240 the earfh’s period round the fun is to the cube of its dithoufand miles, he muft, in folid bulk, be 42 millions ftance from him, fo is the fquare of the period of any other planet to the cube of its diftance, in fuch parts or 87? thoufand times as big as the moon. The horizontal parallaxes are beft obferved at the e- meafures as the earth’s diftance was taken. This proquator. 1. Becaufe the heat is fo nearly equal every portion gives us the relative mean diftances of the planets day, that the refraciions are almoft conftantly the dame. from the fun to the greateft degree of exaftnefs; and are as follow, having been deduced from their pe2. Becaufe the paralladtic angle is greater there, as at they A (the diftance from thence to the earth’s axis being riodical times, according to the law juft mentioned, greater) than upon any parallel of latitude, as a or b. which was difcOvered by Kepler, and demonftrated by The earth’s diftance from the fun being determined, Sir Ifaac Newton. the diftances of all the other planets from him are eafily Periodical Revolution to the fame fixed Star in Days, and decimal Parts of a Day. Saturn, Of Mercury, Venus, The Earth, Mars, Jupiter, 224.6176 365.2564 686.9785 4332.514 10759.275 Relative mean dijlances from the fun. 38710 72333 ■ 100000 152369 520096 954006 From thefe numbers we deduce, that if the fun's 'horizontal parallax be 10", the real snean difances of the planets from the fun in Englifh miles are, 31,742,200 S9'3J3»06o 82,000,000 124,942,580 426,478,720 782,284,920 But if the fun's parallax be 11", their difances are no more than 29,032,500 54,238,570 75,000,000 114,276,750 390,034,560 715,504,500 Errors in difance, ariftng from the mifake of in the fun's parallax. 2,709,700 5,074,490 7,000,000 10,665,830 36,444,220 66,780,420 But, from the tranfit of Venus, A. D. 1761, the fun's parallax appears to be only 8" 4s $■ and according to that, their real difance in miles are 36,668,373 68,518,044 94,725,840 144,588,575 492,665,307 903,690,197 Thefe numbers (hew, that although we have the rela- throughout the year; which, by no means, difproves tive diftances of the planets from'the fun to the greateft the earth s annual motion, but plainly proves the diftance nicety, yet the beft obferyers could not afcertain their of the ftars to be exceeding great. true diftances until the above tranfit appeared, which The fmall apparent motion of the ftars, difeovered by we muft confefs was embarrafled with feveral difficulties. that great aftronomer, he.found to be no ways owing to But the late tranfit of Venus over the fun, on the third their annual parallax (for it came out contrary thereto) of June, was much better fuited to this great pro- but to the aberration of their light, which can refult blem. from no known caufe befides that of the earth’s annual The earth’s axis produced to the ftars, being carried motion ; and as it agrees fo exadly therewith, it proves, parallel to itfelf during the earth’s annual revolution, de- beyond difpute, that the earth has fuch a motio-n: For feribes a circle in the fphere of the fixed ftars equal to this aberration completes all its various phenomena evethe orbit of the, earth. But this orbit, though very ry year ; and proves that the velocity of ftar-light is fuch large, would feem no bigger than a point if it were as carries it through a fpace equal to the fun’s diftance viewed from the ftars; and confequently, the circle de- from us in 8 minutes 13 feconds of time. Hence, the feribed in the fphere of the ftars, by the axis of the velocity of light is 10 thoufand 21O times as great as earth produced, if viewed from the earth,' muft appear the earth’s velocity in its orbit; which velocity (from but as a point ; that is. its diameter appears too little to what we know already of the earth’s diftance from the be meafured by obfervation : For Dr Bradley has allured fun) may be afferted to be at leaft between 57 and 58 us, that if it had amounted to a fingle fecond, or two at thoufand miles every hour : And fuppofrng it to be moft, he ffiould have perceived it in the great number of 58000, this number, multiplied by the above 10210, ebfervations h.e has made, efpecially upon y dragonis ; gives 592 million 180 thoufand miles for the hourly and that it feemed to him very probable that the annual motion of light; which laft number, divided by 3600, parallax of this ftar is not fo great as a fingle fecond; and the number of feconds in an hour, ffiews that light flies confequently, that it is above 400 thoufand times far- at the rate of more than a hundred and fixty-four ther from us than the fun. Hence, the celeftial poles thoufand miles every fecond of time, or fwing of a comfeem to continue in the fame points of the heavens mon clock pendulum, Vol. I. No. 19. 3 Chap, SY
