Page:Encyclopædia Britannica, first edition - Volume I, A-B.pdf/523

 ASTRONOMY. 419 hours 48 minutes 52-5- feconds, as feen at that place; made both in Britain and abroad; and finds it to have which was 2 minutes 3-J feconds lefs than as feen at Her- been 8//.52 on the day of the tranfit when the fun was very nearly at his greateft diftance from the earth ; and nofand in Sweden. At Madrals, the-Reverend Mr Hirft obferved the total confequentl-y 8".6j when the fun is at his mean diftance ingrefs to be at 7 hours 47 minutes 55 feconds apparent from the earth. time in the morning, and the beginning of egrefs at 1 The log. fine (or tangent) of 8".63 is 5.6219140, hour 39 minutes 38 feconds part noon.—The duration which being fubtradted from the radius 10.0000000, between thefe two internal contafts was 5 hours 51 mi- leaves remaining the logarithm 4.3780860, whofe number is 23882.84; which is the number of femidiameters nutes 43 feconds. Profeifor Mathenci at Bologna obferved the beginning of the earth that the fun is diftant from it.—And this of egrefs to be at 9 hours 4 0minutes 38 feconds. 0 laft number, 23882.84, being multiplied by 3985, the At Calcutta, (latitude 22 30' north, nearly 92 eaft number of Englifh miles contained in the earth’s femidilongitude from London), Mr William Magee obferved ameter, gives 95,173,117 miles from the earth’s mean the total ingrefs to be at 8 hours 20 minutes 58 feconds diftance from the fun.—But becaufe it is impoflible, from in the morning, and the beginning of egrefs to be at 2 the niceft obfervations of the fun’s parallax, to be fure hours 11 minutes 34 feconds in the afternoon.; the du- of his true diftance from the earth within 100 miles, we ration between the two internal contacts 5 hours 50 mi- lhall at prefent, for the fake of round numbers, ftate the nutes 36 feconds. earth’s mean diftance frem the fun at 95,173,000 Englifh At the Cape of Good Hope, (1 hour 13 minutes 35 miles. feconds eaft from Greenwich), Mr Mafon obferved the And then, from the numbers and analogies in § 11. & 14. beginning of egrefs to be at 9 hours 39 minutes 50 fe- of Mr Short’s differtation, we find the mean diftances of conds in the morning. all the reft of the planets from the fun, in miles, to be as All thefe times are colleifted from the obfervers ac- follows.—Mercury’s diftance, 36,841,468 ; Venus’s dicounts, printed in the Philofophical Tranfadtions for the ftance, 68,891,486 ; Mars’s diftance, 145,014,148; Juyears 1762 and 1763, in which there are feveral other piter’s diftance, 494.990,976; and Saturn’s diftance, other accounts that are not tranfcribed.—The inftants of 907,956,130. Venus’s total exit from the fun are likewife mentioned, The femidiameter of the earth’s annual orbit being ebut they are here left out, as not of any ule for finding qual to the earth’s mean dfftance from the fun, viz, the fun’s parallax. 95,173,000 miles, the whole diameter thereof is - Whoever compares thefe times of the internal contadls, 190,346,ooq miles.- And fince the circumference of a as given in by different obfervers, will find fuch differ- circle is to its diameter as 355 is to 113, the circumfeences among them, even thofe which were taken upon rence of the earth’s orbit is 597,989,646 miles. the fame fpot, as will fhew, that the inftant of either And, as the earth defcribes’this orbit in 365 days 6 contaft could not be fo accurately perceived by the ob- hours (or in 8766 hours) it is plain that it travels at fervers as Dr'Halley thought it could ; which probably the rate of 68,216.9 miles every hour, and confequentarifes from the difference of peoples eyes, and the dif- ly 1136.9 miles every minute; fo that its velocity in its ferent magnifying powers of thofe telefcopes through orbit is at leaft 142 times as great as the velocity of a which the contadls were feen.—If all the obferVers had cannon-ball, fuppofing the ball to move through 8 miles made ufe of equal magnifying powers, there can be no in a minute, which it is found to do very nearly: And at doubt but that the times would have more nearly coinci- this rate it would take 22 years 228 days for a cannonded; fince it is plain, that fuppofing all their eyes to be ball to go from the earth to the fun. equally quick and good, they whofe telefcopes magnified Oh the 3d of June, in the year 1769, Venus agairr moft would perceive the point of internal contadi fooheft, parted over the fun’s diflc, in fuch a manner, as to afford a‘ and of the total exit lateft. much eafier and better method of inveftigating the fun’s Mr Short, in a paper publifhed in the Philofophical parallax than her tranfit in the year 17611 But as few Tranfadtions, Vol. LII. Part II. has taken an incredible of the obfervations upon this tranfit have as yet been deal of pains in deducing the quantity of the fun’s pa- made public, we can only give the following, made by rallax, from the beft of thofe obfervations which were different obfervers at London. Thread of Regular light comMagcircumfei pleated, External’ rences in the internalor Telefcopesmade nifying contact. contaift. ‘ contaft. ufe of. power. h. ' " h. ' " j h. ' " N. Mafkelyne, 7 to 58 31 7 29 23 2 feet refteftor, 140 M. Hitchins, 7 10 54 77 28 28. 5.7 6 f. refleftor, 90 W. Hirft, 7 11 11 28 47 77 29 refleftor, 55 J. Horfley, 7 10 44 7 28 15 7 29 1828 1 o2 f.f. achromatic, 50 S. Dunn, 7 10 37 7 29 287 7 2929 2048 3t f. achromatic, 140 P. Dollond, 7 11 19 3t f. achromatic, 150 E„ Nairne, 7 n 3° 120. 17 29 20 2 f. refleftor, When