THE STELLAR UNIVERSE. 



Let the line s s' represent the direction of the motion of the 

 system, and let s and s' represent its positions at any two epochs. 

 At s, the stars ABC would be separated by intervals measured 



by the angles A s u, and B s c, while at s' they would appear 

 separated by the lesser angles A s' B, and B s' c. Seen from s', 

 the stars ABC would seem to be closer together than they were 

 when seen from s. For like reasons, the stars a It c, towards 

 which the system is here supposed to move, would seem to be 

 closer together when seen from s, than when seen from s'. Thus, 

 in the quarter of the heavens towards which the system is moving, 

 the stars might be expected to separate gradually, while in the 

 opposite quarter they would become more condensed. In all the 

 intermediate parts of the heavens they would be affected by a 

 motion contrary to that of the solar system. Such, in general, 

 would be the effects of a progressive motion of our system. 



66. Although no general effect of this kind has been manifested 

 in any conspicuous manner among the fixed stars, many of these 

 objects have been found, in long periods of time, to have shifted 

 their position in a very sensible degree. Thus, for example, the 

 three stars, Sirius, Arcturus, and Aldebaran, have undergone, 

 since the time of Hipparchus (130 B.C), a change of position 

 southwards, amounting to considerably more than half a degree. 

 The double star 61 Cygni has, in half a century, moved through 

 nearly 4*3', the two stars composing it being carried along in 

 parallel lines with a common velocity. The stars e Indi and /* 

 Cassiopeia} move at the rate of 7 '74" and 3-74" annually. 



Yarious attempts have been made to render these and other 

 like changes of apparent position of the fixed stars compatible 

 with some assumed motion of the sun. Sir W. Herschel, in 

 1783, reasoning upon the proper motions which had then been 

 observed, arrived at the conclusion, that such appearances might 

 be explained by supposing that the sun has a motion directed to a 

 point near the star A Herculis. About the same time, Prevost 

 came to a like conclusion, assigning, however, the direction of the 

 supposed motion to a point differing by 27 from that indicated by 

 Sir W. Herschel. 



Since that epoch, the proper motions of the stars have been 

 8 



