STARS. 



^^rious means. For if our own folar fyftem be conceived 

 to change it? place with refpeA to abfolute fpace, tliis 

 might, in procefs of time, occafion an apparent change in 

 the asngular diftanccs of the fixed ftars ; and in fuch a cafe, 

 the places of the nearefl (tars being more affefted than of 

 thofe that arc very remote, their relative pofition might 

 feem to alter, though the ftars themfelves were really im- 

 moveable : and vice •verfii, we may furmife, from the ob- 

 ferved motion of ihu ilars, that our fun, with all his planets 

 and comets, may have a motion towards fome particular part 

 of the heavens, on account of a greater quantity of matter 

 coUefted in a number of flars and their iiirrouuding planets 

 there fituated, which may perhaps occafion a gravitation of 

 our whole fular fyltem towards it. If this lurmife ftiould 

 have any foundation, as Dr. Herfchel obferves, ubi/uprn, 

 p. 103, it will fhew itfelf in a ferics of fome years ; as from 

 that motion will arife another kind of hitherto unknown 

 parallax (fuggefted by Mr. Michell, Phil. Tranf. vol. Ivii. 

 p. 252.1, the inveltigation of which may account for fome 

 part of the motions already obferved in fome of the princi- 

 pal itars ; and for the purpofe of determining the direftion 

 and quantity ot fuch a motion, accurate obfervations of the 

 didance of ftars that are near enough to be meafured with 

 a micrometer, and a very high power of telefcopes, may be 

 of confiderable ufe, as they will undoubtedly give us the 

 relative places of thofe ftars to a much greater degree of 

 accuracy than they can be had by tranfit inltruments or fee- 

 lers, and thereby much fooner enable us to difcover any ap- 

 parent change in their fituation, occafioned by this new kind 

 of fyftematical parallax, jf we may fo exprefs the change 

 arifing from the motion of the whole folar lyftem. For an 

 account of Dr. Herlchel's theory of the annual parallax of 

 double ftars, w'ith 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 ncarell to us, 

 fee the preceding part of this article, under Dtftance of the 

 Star.s. 



On the other hand, if our fyftem be at reft, and any of 

 the ftars really in motion, this might likewife vary their ap- 

 parent pofitions ; and the more fo, the nearer they arc to us, 

 or the fwifter their motions are, or the more proper the di- 

 reftion of the motion is to be rendered perceptible by us. 

 Since then the relative places of the ftars may be changed 

 from fuch a variety of caufes, confidering the amazing dif- 

 tance at which it is certain fome of them are placed, it may 

 require the obfervations of many ages to determine the laws 

 of the apparent changes even of a llngle ilar ; much more 

 difficult, therefore, muft it be to iettle the laws relating to 

 all the moft remarkable ftars. 



When the caufes which afleft the places of all the ftars in 

 general are known, fuch as the precellion, aberration, and 

 nutation ; it may be of fingular ufe to examine nicely the 

 relative fituations of particular liars, and elpecially of thofe 

 of the greateft luftre, which, it may be prelumed, lie nearell 

 to us, and may therefore be lubjetl to more linfible changes, 

 either from their own motion, or from that of our fyftem. 

 And if at the lame time that the brighter ftars are compared 

 with each other, we likewiic determine the relative pofitions 

 of fome of the imallcft that appear near them, whole places 

 can be afcertained with fufiicient exactncfs, we may perhaps 

 be able to judge to what caufe the change, if any be ob- 

 fervable, is owing. The uncertainty tiiat we are at preleiit 

 under, with refpcCl to the degree of accuracy with which 

 former aftronomers could oblerve, makes us unable to deter- 

 mine feveral thing 'clat !ng fo this fubjedt ; but the im- 

 provements which have of late )-ars been mauc in the me- 

 thods of taking the pi -"e!! of f|ie heavenly bodies are fo 

 great, that a few year, n.ay hereatter be fuliicient to fettle 



Vol. XXXIII. 



fome pomts, which cannot now be fettled, by comparing' 

 even the earlieft obfervations with thofe of the prefent age. 



The firft pcrfon who pointed out the method of invc-fti- 

 gating the parallax of the ftars, by taking thedit'.ai.cea from 

 the zenith of thofe that pafs very near it, was Galileo. The 

 execution of this method was alfo attempted by Hnke, 

 Flamftead, Molyneux, and Bradley. See Dtftance of the 

 Staus, fiipra. 



Dr. Hooke communicated feveral obfervations on the ap- 

 parent motions of the fixed ftars ; and as this was a matter 

 of great importance in aftronomy, feveral of the learned 

 were defiroui of verifying and confirming his obfervations. 

 An inftrument was accordingly contrived by Mr. George 

 Graham, and executed with lurprifing exaftnefs. 



With this inftrument the liar -/, in the conftellation Draco, 

 vvas frequently obferved by Mellrs, Molyneux, Bradley, and 

 Graham, in the years 172J, 1726; and the obfervations were 

 afterwards repeated by Dr. Bradley with an inftrument con- 

 trived by the fame ingenious perfon, Mr. Graham, and In 

 exaft, that it might be depended on to half a fecond. The 

 refult of thefe obfervations was, that the ftar did not always 

 appear in the fame place, but that its diftance from the 

 zenith varied ; and that the difference of the apparent place* 

 of Themis, the third fatcUite of Jupiter, amounted to 21 

 or 22 feconds. Similar obfervations were made on other 

 ftars, and a like apparent motion was found in them, pro- 

 portional to the latitude of the ftar. This motion was by 

 no means fuch as was to have been expected, as the cfTcd of 

 a parallax ; and it was fome time before any way could be 

 found of accounting for this new phenomenon. At length 

 Dr. Bradley refolved all its variety, in a fatisfaftory manner, 

 by tlie motion of light and the motion of the earth com- 

 pounded together. See Light, and Phil. Tranf. N° 406. 

 p. 364, or Abr. vol. vi. p. 149, &c. 



Our excellent aftronomer. Dr. Bradley, had no fooner 

 difcovered tlie caufe, and fettled the laws of the aberration 

 of the fixed ftars, than his attention was again excited by 

 another new phenomenon, w-. ax\ annual change of declina- 

 tion in fome of the fixed ftars, which appeared to be feiifibly 

 greater than a preceffion of the equinoctial points of 50" m 

 a year, the mean quantity now ulually allowed by aftrono- 

 mers, would have occafioned. 



This apparent change of declination was obferved in the 

 ftars near the cquinodtial colure, and there appearing at the 

 fame time an efl'eft of a quite contrary nature in fome ftars 

 near the lolftitial colure, which feemed to alter their declina- 

 tion lefs than a precefiion of 50" required. Dr. Bradley was 

 thereby convinced, that all the phenomena in tiie different 

 ftars could not be accounted for merely by fuppofing that 

 he had allumed a wrong quantity for the precefiion of the 

 equinodtial points. He had alfo, after many trials, fufficient 

 reafon to conclude, that thefe fecond unexpected deviations 

 of the ftars were not owing to any imperfection of his in- 

 ftruments. At length, from repeated obfervations, he b'gan 

 to guefs at the real caufe of thefe phenomena. 



It appeared from the doctor's obfervations, during his 

 refideiicc at Wanftcad, from the year 1727 to 1732, that 

 fome of the ftars near the foMitial colure had changed their 

 declinations y" or 10" lefs tiiai. a prccciriou of 50 would 

 have produced ; and, at the fume tiine, that o;>,iMii near the 

 cquinodtial colure had altered theirs about ihe unit quantity 

 more than a like preceffion Wviuld have occafioned ; the 

 north pole of the equator feeniini; to have appro..eaed the 

 flara which come to the meridian witi the fun about the 

 vernal equinox and the winter folftite ; 41. J to luve recn'.ed 

 from thofe which come to the meiidian with '''c fun about 

 t.he autun iial equinox and the lutrmer folltice. 



From the confid'-'aljoii of tln.ic clrcumftaIlC^.^, and the 

 { £ luuution 



