QC0 CAUS £ OF THE CHANGES Of REJ-ATIVE 



Obfcrvations Every argument, to examine the caufe of this motion, which 



refpefting the ^^** heiin uled with £ Bootis, in the paragraphs from (a) to (i), 



changes otrela- will completely apply to this flar; from this we may conclude, 



fiars extremely" ^^^^ themoft natural way of accounting for the obferved changes, 



near each otlier. is to admit the two ftars to form a binary fyftem. In this cafe 



we calculate, with contiderable probability, that the periodical 



time of a revolution of the fmall ftar round ^ Serpentis, muft 



be abotit 375 years. 



y Virginis. 

 This double ftar, which has long been known to aftronomers,* 

 has undergone a vifible change fince the year 1780, when I 

 firft began my obfcrvations of it. The 21 ft of November, 

 1781, I meafured the poiition of the two ftars, which was 

 40° 44' fouth following. The ftars are fo nearly equal, that I 

 have but lately afcertained the following one to be rather larger 

 than its companion; the poiition, therefore, ought now to be 

 called north preceding. By a mean of three meafures, that were 

 taken on the 15 th of April, 1803, the angle was SO^* 20' np. 



The diftance, as far as eftimations by the diameter can deter- 

 mine, when the ftars are fo far afunder as thefe are, remains 

 without alteration. May 21, 1781, they were 2| diameters 

 afunder; and, by eftimations lately made, with the fame in - 

 ilrument and power as were ufed 21 years ago, the ftars are 

 ftill at the fame diftance of 2| diameters. 



A very fmall proper motion in declination, of 0''',02 towards 

 the fouth, has been afligned to this double ftarjf but the quan- 

 tity is hardly fufficient for us to rely much upon the accuracy 

 of the determination. I (hall therefore rather confider y Virginis 

 as one of the ftars of which we have no proper motion afcer* 

 tained; and the arguments to which I fliall refer, will confe- 

 quently be thofe which have been given with e Bootis. 



The change of the angle of pofition, in the time of 21 years 

 and 145 days, amounts to 10^ 24' ; from which we obtain the 

 annual motion of 29 ',16. The obferved and calculated angles, 

 with their differences, on which it will not be necelfary to 

 make any remarks, are in the following Table. 



* Memoires deVAcademie des Sciences, Ann. 1720. 

 t Connoiffance des Temps y Annee VI. page 213. 



Times 



