STARS. 



earth, being in the ratio of about 22 to i, when they are 

 both in oppofition, and fuppofiniT them to refled equally 

 in proportion to the whole of the light that falls upon them. 

 But if Jupiter and Saturn, inftead of reflecting the whole of 

 the light that falls upon them, (hould really refleft only a 

 part of it, e. g. a fourth or a fixth, which may be the cafe, 

 their diftances, computed above, muft be increafed in the 

 proportion of 2 or 2i to l, to maice the fun's light no more 

 than equal to their's ; and his parallax would be lefs in 

 the fame proportion. Suppofmg, then, that the fixed liars 

 are of the fame magnitude and brightnefa with the fun, it 

 is no wonder that their parallax (hould hitherto have efcaped 

 obfervation ; fince, if this is the cafe, it could hardly 

 amount to 2", and probably not more than i" in Sinus 

 himfelf, though he had been placed in the pole of the eclip- 

 tic ; and in thofe that appear much lefs luminous, e. g. 

 y Draconis, which is only of the third magnitude, it could 

 hardly be expefted to be fenfible with fuch inftruments as 

 have hitherto been ufed. However, Mr. Michell fuggelU, 

 that it is not imprafticable to conitruft inftruments, capable 

 of dillinguilhing even to the 20th part of a fecor.d, pro- 

 vided the air will admit of that dcgrfee of exaftnefs. This 

 ingenious writer apprehends, that the quantity of light 

 which we receive from Sirius, does not exceed the light 

 which we receive from the leall fixed ftars of the fixth mag- 

 nitude, in a greater proportion than that of 1000 to i, nor 

 in a lefs proportion than that of 400 to i ; and the fmaller 

 ftars of the fecond magnitude feem to be about a mean pro- 

 portional between the other two. Hence, the whole paral- 

 lax of the lead fixed Itars of the fixth magnitude, fuppofing 

 them of the fame fize and native bignefs v/ith the fun, 

 ftiould be from about 2"' to 3"', and their dillance from 

 about eight to twelve million times that of the fun : and 

 the parallax of the fmaller ftars of the fecond magnitude, 

 upon the fame fuppofition, (hould be about 12'", and their 

 dirtance about two million times that of the fun. 



This author farther fuggefts, that, from the apparent 

 fituation of the ftars in the heavens, there is the higheft 

 probability (the odds againft the contrary opinion being 

 many million millions to one) that the ftars are coUefted 

 together in clufters in fome plactc, where they form a kind 

 of fyftem, whilft in others there are either few or none of 

 them, whether this difpofition be owing to their mutual 

 gravitation, or to fome other law or appointment of the 

 Creator. Hence it may be inferred, that fuch double 

 ftars, &.C. as appear to confill of two or more ftars placed 

 very near together, do really confift of ftars placed near 

 together, and under the influence of fome general law: and 

 he proceeds to inquire, whether, if the ftars are collefted 

 into fyftems, the fun does not likewife make one cf fome 

 fyftem, and which are thofe, among the fixed ftars, that 

 belong to the fame fyftem with himfelf. 



Thofe ftars, he apprehends, which are found in clufters, 

 and furrounded by many others at a fmall diltance from 

 them, belong probably to other fyftems, and not to ours. 

 And thofe (iars which are furrounded with nebulae, are 

 probably only very great ftars, which, on account of their 

 luperior magnitude, are fingly vifible ; while the others, 

 which compofe the remaining part of the fame fyftem, are 

 fo fmall as to efcape our fight. And thofe nebulae in 

 which we can difcover either none or only a few ftars, even 

 •with the afliitance of the beft telefcopes, are probably fyf- 

 tems that are ftill more diftant than the reft. 



For other particulars of this inquiry, we muft refer to 

 Phil. Tranf, vol, Ivii. art. 27. p. 234, &c. 



As the diftance of the fixed ftars is beft determined by 

 ilteir parallax, various roetheds have been purfued, hitherto 



without fuccefs, for inveftigating it ; and the refult of the 

 moft accurate oblervations has given us little more than a 

 diftant approximation, from which we may conclude, that 

 the neareli of the fixed ftars cannot be lefs than 40,000 

 diameters of the whole annual orbit of the earth diftant 

 from us. 



Tne method pointed out by Galileo, and firft attempted 

 by Hooke, Flamltead, Mulineux, and Bradley, of taking 

 diftances of Itars from the zenith that pafs very near it, 

 has given us a much jufter idea of the iramenfe diftance of 

 the ftars, and fiirnilhcd us with an approximation to the 

 knowledge of their parallax, that is much nearer the truth 

 than we ever had before. 



Dr. Bradley aflures us (Phil. Tranf. N° 406, or Abr. 

 vol. vi. p. 162.), that, if it had amounted to a fingle fecond, 

 or two at moft, he (hould have perceived it in the great 

 number of obfervations which he made, cfpecially upon 

 y Draconis ; and that it feemed to him very probable, that 

 the annual parallax of this (tar is not fo great as a fingle 

 fecond, and confequer.tly, that it is above four hundred 

 thoufand times farther from ;;s than the fun. 



But Dr. Herfchel, to whofe ingenuity and induftry in 

 exploring the heavens aftronomy is already m.uch indebted, 

 and from whom much more may be expedled, remarks, that 

 the inftrument ufed on this occafion, being the fame with 

 the prefent zenith feftors, can hardly be allowed fufBcienl 

 to (hew an ancfle of one or even two feconds with accuracy : 

 and befides, the (lar on which the obfervations were made, 

 is only a bright ftar of the third or fmall ftar of the fecond 

 magnitude ; and, therefore, its parallax is probably confi- 

 derably lefs than that of a ftar of the firft magnitude : fo 

 that we are not warranted in inferring that the parallax 

 of the ftars in general does not exceed i", whereas thofe 

 of the firft magnitude may have, notwithftanding the refult 

 of Dr. Bradley's obfervations, a parallax of feveral feconds. 



Moreover, the method of zenith diftances is liable to 

 confiderable errors, on account of refraftion, the change of 

 pofition of the earth's axis arifing from nutation, precefTion 

 of the equinoxes, and other caufcs, and the aberration of 

 light. 



Dr. Herfchel has propofed another method, by means of 

 double ftars, which is free from thefe errors, and of fuch a 

 nature, that the annual parallax, even if it (hould not ex- 

 ceed the tenth part of a fecond, may ftill become vifible 

 and be afcertaincd, at leaft to a much greater degree of ap- 

 proximation than it ever has been done. This method, 

 which was firft propofed in an imperfetl manner by Galileo, 

 and has been alio mentioned by other authors, is capable of 

 every improvement which the telefcope and mechanifin of mi- 

 crometers can furnifli. In order to obtain a general idea of 

 it, let O, E, (P/ateXX. JJronomy, fg. 8.) be two oppo- 

 fite points of the annual orbit, taken in the fame plane 

 with two ftars a, b, of unequal magnitudes. Let the angle 

 aO bhe obferved when the earth is at O, and a E 3 be ob- 

 ferved when the earth is at E. From the difference of thefe 

 angles, if there (hould be any, we may calculate the paral- 

 lax of the ftars, according to the theory fubjoined. Thefe 

 two ftars ought to be as near each other as poffible, and alfo 

 to differ as much in magnitude as we can find them. 



Dr. Herfchel's theory of the annual parallax of double 

 ftars, wnth the method of computing from thence what is 

 generally called the parallax of the fixed ftars, or of fingle 

 ftars of the firft magnitude, fuch as are neareft to us, fuppofes, 

 frji, that the ftars, one with another, arc about the fize of 

 the fun : and, Jecondly, that the difference of their apparent 

 magnitudes is owing to their different diftances, fo that the 

 ftar of the fecond, third, or fourth magnitude, is two, three, 



or 



