STARS. 



Dr. Bradley's obfervations of the ftary Draconis, then all the 

 phenomena of the feveral Itars which he obferved will be very 

 nearly folved by his hypothefis. But for the particulars of 

 his folutioii, and the application of his theory to the prac- 

 tice of adronomy, we mud refer to the excellent author 

 himfelf, our intention being only to give the hittory of the 

 invention. 



The correftions arifing from the aberration of light, and 

 from the nutation of the earth's axis, mult not be neglefted 

 in aftronomical obfervations ; fince fuch negledis might 

 produce errors of near a minute in the polar dillances of 

 fomc Itars. 



As to the allowance to be made for the aberration of 

 light, Dr. Bradley afl'ures us, that having again examined 

 thofe of his own obfervations, which were moft proper to 

 determine the tranfverfe axis of the ellipfis which each (tar 

 feems to defcribe, he found it to be nearelt to 40" ; and 

 this is the number he makes ufe of in his computations relat- 

 ing to the nutation. 



Dr. Bradley fays, in general, that experience ha's taught 

 him, that the obfervations of fuch Itars as lie nearelt the 

 zenith, generally agree bell with one another, and are there- 

 fore the fittelt to prove the truth of any hypothefis. Phil. 

 Tranf. No. 4S5. vol. xlv. p. i, &c. 



Monfieur d'Alembert has publifhed a treatife, entitled 

 «' Recherches fur la Prcceflion dcs Equinoxes et fur la Nu- 

 tation de la Terre dans le Sylteme Newtonien," 4to. Paris, 

 1749. The calculations of this learned gentleman agree, in 

 general, with Dr. Bradley's obfervations. But Monfieur 

 d'Alembert finds, that the pole of the equator delcribes an 

 ellipfn in the heavens, the ratio of whofe axes is as 4 to 3 ; 

 whereas, according to Di'. Bradley, the curve delcribed is 

 either a circle or an ellipfis, the ratio of whole axes is as 18 

 to 16. 



From this digrefllon we fliall return to the proper motion 

 of the itars and folar lyllem. This motion was firll ob- 

 ferved, as we have already iuggeltcd, by Dr. Halley, and 

 afterward'! by Le Monnier and Caffini. Tobias Mayer had 

 the merit of giving the firll explanation of this proper mo- 

 tion ; he compared the places of about eigiity itars, as de- 

 termined by Roemer, with his own obfervations, and he 

 found that the greater number of them had a proper motion. 

 He was apprized that this change of place in the pofition of 

 the ftars might be explained by a progrelTive motion of the 

 fun towards one quarter of the heavens ; but the refult of his 

 obfervations notaccording with this hypothefis, he concluded, 

 that many centuries might elapfe before the true caufe of 

 this motion is explained. The pofiibility of a folar motion 

 was fuggedcd, upon theoretical principles, by the late Dr. 

 Wilfon of Glalgow ; and M. de la Laiide deduced the fame 

 opinion from the rotatory motion ot the fun ; liut another 

 kind of argument has completely confiruK-d thefe con- 

 jeftures. 



If the fun has a m.otion in abfolute fpace, directed to- 

 wards any quarter of the heavens, it is obvious tliat the liars 

 in that quarter mull appear to recede from eacli other, while 

 thofe in the oppofite region feem gradually approaching. 

 The proper motion of the Itars, therefore, in thofe oppofite 

 regions, as afcertained by a comparifon of ancient with mo- 

 dern obfervations, ought to correlpond with this hypothefis. 



Dr. Herfchel has examined this fubjcdl with his ufual 

 fuccefs, and he has certainly difcovcred the direttion in 

 which our fyitem is gradually advancing. He found that 

 the apparent proper motions of about forty-four llarb out of 

 fifty-fix are very nearly in the dire£tion which flioidd rcfult 

 from a motion of the fun towards the conllellation Hercules, 

 or, more accurately, to a point in the heavens, whole right 



afcenfion is 250' 52' 30", and whofe north polar dillancc u 

 40'' 22'. 



Dr. Herfchel, in ellabliihing this motion of the (tarry and 

 folar fyftems, begins with principks drawn from the theory 

 of attraftion, which evidently oppofe every idea of abiolute 

 reil in any one of the liars, when once it is known that fome 

 of them are in motion ; for the change that mud arife by 

 fuch motion, in the value of a power which afts invcrfely a> 

 the fquares of the dillances, mud be felt in all the neighbour- 

 ing liars ; and if thefe be influenced by the motion of the 

 former, they will again affedl thofe that are next to them, 

 and fo on till all are in motion. Now as we know that 

 feveral ftars, in divers parts of the heavens, do aflually 

 change their place, it will follow, that the motion of our 

 folar fydem is not a mere hypothefis ; and this confidcration 

 will be confirmed by the reafons that induce us to fuppofe, 

 that mod of thofe very ftars which have been obferved to 

 move, to be fuch as are nearelt to us ; and, therefore, their 

 influence on our fituation would of itfelf be a powerful argu- 

 ment in favour of the proper motion of the fun, if it had 

 been originally at red. Moreover, it feems very natural, 

 that the changes which are obferved to take place among 

 the ftars ; fuch as the increafe of magnitude in many, and 

 the gradual difappearance of others ; the periods of fome not 

 befwre obferved, and the dillances of many that are adtually 

 changing, while many more are fnfpcftcd to have a confider- 

 able motion ; afford a itrong lufpicion that moll probably 

 every itar in the heavens is more or lefs in motion. Allow- 

 ing the proper motion of the ftars, it mud f lUow, that our 

 fydem, with all its planets and comets, tint is, the folar 

 fydem, is not lefs liable to fuch a general aijitation as is 

 found to obtain among all the other celedial bodies. Adro- 

 nomcrs have already obferved what they call a proper mo» 

 tion in feveral of the fixed liars, and the fame may be fup- 

 pofed of them all. We ought, therefore, to relolvc that 

 which is common to all the dars, which arc found to have 

 what has been called a proper motion, into a fin.ile real mo- 

 tion of the folar fydem, as far as that will anfwer the known 

 facts, and only to attribute to the proper motion of each 

 particular dar the deviations from the general law the itars 

 feem to follow in thofe movements. 



By Dr. Maflielyne's account of the proper motion of 

 fome principal liars, we find that Sirius, Caitor, Procyon, 

 Pollux, Regulus, Ardturus, and a. Aquilae, appear to have 

 refpeclivcly the following proper motions in right alcenfion ; 

 -o".63;— o".28;-o".8o; - o".93 ; -o".4i ; - i".40; 

 and -|- o".57 ; and two of them, Sirius and Artlurus, in de- 

 clination, W2. i".20 and 2".oi, both fouthward. 



Since the apparent motions of thefe feven liars may be ac- 

 counted for, either by fuppofing them to move jud as they 

 appear todo, orelfe by fuppofing the lun alone to have a mo- 

 tion in a direction fuch as Dr. Herfchel aifigns to it, we are no 

 more authorifed to fuppofe the fun at red than we fliould be 

 to deny the diurnal motion of the earth, except in this refptcl, 

 that the proofs of the latter are very numero;is, whereas the 

 former reds only on a few though capital telliinonies. Our 

 author finds from other confiderations, founded on a t.ible by 

 M. de la Lande of the proper motion of twelve Itars, both 

 in right afcenfion and declination, for fifty years, (Phil. Tranf. 

 vol.lxxiii. p. 270, S:c.), that this motion of the fun, or 

 folar fydem, is direfted towards the liar \ Herculis ; and he 

 mentions fome very llriking circumilanccs in the quanlilics 

 of the real proper motions of the ftars that deferve notice, 

 I'ird, Arfturus and Siriun being the larged of the liars, and 

 therefore probably the nearelt, ought to have the moll ap- 

 p;irent motion, both in right afcenfion and declination, which 

 \i agreeable to obfervation. Next, in regard to the ri^ht 

 ^ E 2 afcealiou 



