670 



" On the Proper Motion of the Solar System." By Thomas Gal- 

 loway, Esq., A.M., F.R.S. 



The object of this paper is to communicate the results of a calcu- 

 lation for determining the direction of the proper motion of the 

 solar system from the apparent proper motions of stars in the 

 southern hemisphere, deduced mostly from a comparison of the 

 observations made by Lacaille at the Cape, about the middle of the 

 last century, with the recent observations of Mr. Johnson and the 

 late Professor Henderson at St. Helena and the Cape respectively. 

 After adverting to the papers of Sir William Herschel in the Philo- 

 sophical Transactions for 1783 and 1805, and some other investi- 

 gations of the same subject, the author remarks that up to a 

 recent period astronomers seem generally to have entertained the 

 opinion that our knowledge of the proper motions of the stars is 

 not sufficiently advanced to enable us to pronounce positively either 

 on the fact or the direction of the motion of our own system. This 

 opinion was grounded on the discrepancies which present them- 

 selves when it is attempted to explain the observed displacements 

 of individual stars by referring them to the motion of the sun in an 

 opposite direction ; it being always found that whatever direction 

 is assigned to the sun's motion, there are many stars whose proper 

 motions cannot thereby be accounted for. But if the sun be in 

 motion it is very improbable that any star is absolutely at rest ; 

 hence the proper motions deduced from a comparison of catalogues 

 must be regarded as the effect partly of the true proper motions of 

 the stars, and partly of the apparent systematic or parallactic mo- 

 tion caused by the displacement of the point of view ; and as vv^e 

 have no reason for supposing the true proper motion of a star to be 

 more probable in one direction than in another, it may be expected, 

 a priori, that the observed directions will form angles of all different 

 values with the direction of the sun's motion, or any other fixed 

 line. The observed discrepancies are therefore not incompatible 

 with a general drifting of the stars towards a particular region of 

 the heavens ; but in order to deduce the direction of the systematic 

 motion, it becomes necessary to take account of a very considerable 

 number of proper motions, and to represent them by equations, 

 involving the unknown quantities required for determining the 

 direction of the sun's motion, and to solve the equations so as to 

 obtain the most probable values of those quantities. The first person 

 who investigated the subject under this point of view was Professor 

 Argelander of Bonn, in a paper published in the Petersburg Me- 

 moirs for 1837. From the proper motions of 390 stars deduced 

 from a comparison of Bessel's catalogue of Bradley's observations 

 with his own catalogue of stars observed at Abo, Argelander found 

 the direction of the sun's motion, for 1792°5, to be towards the point 

 of the sphere whose right ascension is 259° 47'*6 and declination 

 -|-32° 29'*5. Lundahl, subsequently, from a comparison of the places 

 of 14?7 stars in the catalogues of Bessel and Pond, and not included 

 among those considered by Argelander, found the co-ordinates of 

 the point to be ^=252° 24?'-4, Dec. + U° 26'-l ; and Otto Struve, 



