170 



On certain Drifting Motions of the Stars. 



[Jan. 20, 



Looking, therefore, on the stars as severally in motion, with velocities ex- 

 ceeding the sun's on the average, it cannot hut he looked upon as highly 

 significant that in any large region of the heavens there should be a com- 

 munity of motion such as I have described. We seem compelled to look 

 upon the stars which exhibit such community of motion as forming a 

 distinct system, the members of which are associated indeed with the ga- 

 lactic system, but are much more intimately related to each other. 



In other parts of the heavens, however, there are instances of a star- 

 drift opposed to the direction clue to the solar motion. A remarkable 

 instance may be recognized among the seven bright stars of Ursa Major. 

 Of these, the stars (3, y, c, e, and £ are all drifting in the same direction, 

 and almost exactly at the same rate, towards the " apex of the solar mo- 

 tion," that is, the point from which all the motions due to the sun's trans- 

 lation in space should be directed. If these five stars, indeed, form a system 

 (and I can see no other reasonable explanation of so singular a community 

 of motion), the mind is lost in contemplating the immensity of the periods 

 which the revolutions of the components of the system must occupy. 

 Madler had already assigned to the revolution of Alcor around Mizar 

 (£ Ursse) a period of more than 7000 years. But if these stars, which 

 appear so clear to the naked eye, have a period of such length, what must 

 he the cyclic periods of stars which cover a range of several degrees upon 

 the heavens ? 



In like manner the stars a, ft, and y of Arietis appear to form a single 

 system, though the^motion of a is not absolutely coincident either in mag- 

 nitude or direction with that of /3 and y, which are moving on absolutely 

 parallel lines with equal velocit)^. 



There are many other interesting cases of the same kind. I hope soon 

 to be able to lay before the Society a pair of maps in which all the well- 

 recognized proper motions in both hemispheres are exhibited on the stereo- 

 graphic projection. In the same maps also the effects due to the solar 

 motion are exhibited by means of great circles through the apex of the 

 solar motion, and small circles or parallels having that apex for a pole. 



It appears to me that the star-drift I have described serves to explain 

 several phenomena which had hitherto been thought very perplexing. In 

 the first place, it accounts for the small effect which the correction due to 

 the solar motion has been found to have in diminishing the sums of the 

 squares of the stellar proper motions. Again, it explains the fact that 

 many double stars which have a common proper motion appear to have 

 no motion of revolution around each other ; for clearly two members of a 

 drifting-system might appear to form a close double, and yet be in reality 

 far apart and travelling not around each other, but more closely around 

 the centre of gravity of the much larger system they form part of. 



I may add that, while mapping the proper motions of the stars, I 

 have been led to notice that the rich cluster around ^ Persei falls almost 

 exactly on the intersection of the Milky Way with the great circle which 



