392 PHILOSOPHICAL TRANSACTIONS. [aNNO 1773. 



tween n Bootis and Arcturus was 5° 22' 30" .0 by each of the observations, which 

 difference should have been, supposing neither of the stars to have any proper 

 motion, 5° 'll' 11*7 in May 1768. But it was found to be 28".3 less; by so 

 much therefore had Arcturus moved westward in 24 years. 



On the 24th of May, and 8th of June, 1746, the difference in right ascension 

 between the same stars was 5° 22' ^6'''.2, by taking a mean of the two observa- 

 tions; that difference should have been 5° 22' p"..") in 1768. But it was observed 

 = 5° 21' 43'''.4. Arcturus therefore in 22 years has moved 26 '.1 to the west. 



Lastly, on the l6th of April, and 27th and 28th of May, 1747, the differ- 

 ence in right ascension between »] Bootis and Arcturus, by taking a mean of the 

 3 observations, was 5° 22' 1b".0. By computation the variation in the difference 

 of right ascension was l6".0, by which the ascensional difference should have 

 been diminished, and = 5° 22' g'.O, But by observation it was found = 5° 2l' 

 43".4; Arcturus therefore by this last observation appears to have gone 1b".Q, 

 westward. 



By the observations therefore at Shirburn castle, 

 Arcturus appears to have gone westward, as in the 

 annexed table; in the last column of which are 

 contained the quantities resulting from the obser- 

 vations of each year, reduced to 78 years. 



The mean of all the observations, when reduced to an interval of 78 years, 

 is 1' 35".14, which differs only r''.17 from the mean of the other comparisons. 



As then the proper motions of Arcturus westward in right ascension = 1' 

 33* 974, and 2' 36".81 in declination southward, seem well established, the real 

 motion of Arcturus is inclined in an angle of 30° 56' to the west of the meridian 

 or horary circle, and to be in that direction 3' 2".8] in 78 years, or at the rate 

 of 2".343 in a year. As this direction of its motion is nearly perpendicular to 

 the plane of the ecliptic, the latitude of Arcturus must diminish yearly almost in 

 the same proportion; and its longitude will alter less than that of other stars, 

 though not so considerably as its right ascension. The proper motion of Arc- 

 turus then, in right ascension westward, being l".205, and in declination 2".005, 

 its annual precession in right ascension is 41 ".108, and in declination 19".]33; 

 and the true right ascension of Arcturus, on Jan. 1, 1773, is 211° 19'47".4, 

 and declination north 20° 22' 23".3. 



As none of the other principal stars have been found to have a motion so con- 

 siderable as this, though many of the stars of the first magnitude, as for in- 

 stance, Sirius, Procyon, x Aquilae, * Orionis, as also (3 Aquilae of inferior mag- 

 nitude, do really vary their positions, and perhaps all of the first order will here- 

 after be found to have a proper motion, we may fairly conclude, that Arcturus 

 is the nearest star to our system, visible in this hemisphere. If therefore the 



