344 -A Short History of Astronomy [CH. xii. 



should not be so also, their motion not having been rapid 

 enough to be clearly noticeable during the quarter of a 

 century or so over which Herschel's observations extended ; 

 and this probability entirely destroyed the utility of double 

 stars for the particular purpose for which Herschel had 

 originally sought them. For if a double star is binary, 

 then the two members are approximately at the same 

 distance from the earth and therefore equally affected by 

 the earth's motion, whereas for the purpose of finding the 

 parallax it is essential that one should be much more 

 remote than the other. But the discovery which he had 

 made appeared to him far more interesting than that which 

 he had attempted but failed to make ; in his own picturesque 

 language, he had, like Saul, gone out to seek his father's 

 asses and had found a kingdom. 



265. It had been known since Halley's time (chapter x., 

 203) that certain stars had proper motions on the celestial 

 sphere, relative to the general body of stars. The conviction, 

 that had been gradually strengthening among astronomers, 

 that the sun is only one of the fixed stars, suggested the 

 possibility that the sun, like other stars, might have a 

 motion in space. Thomas Wright, Lambert, and others 

 had speculated on the subject, and Tobias Mayer (chapter x., 

 225-6) had shewn how to look for such a motion. 



If a single star appears to move, then by the principle of 

 relative motion (chapter iv., 77) this may be explained 

 equally well by a motion of the star or by a motion of the 

 observer, or by a combination of the two ; and since in this 

 problem the internal motions of the solar system may be 

 ignored, this motion of the observer may be identified with 

 that of the sun. When the proper motions of several stars 

 are observed, a motion of the sun only is in general inade- 

 quate to explain them, but they may be regarded as due 

 either solely to the motions in space of the stars or to 

 combinations of these with some motion of the sun. If 

 now the stars be regarded as motionless and the sun be 

 moving towards a particular point on the celestial sphere, 

 then by an obvious effect of perspective the stars near 

 that point will appear to recede from it and one another 

 on the celestial sphere, while those in the opposite region 

 will approach one another, the magnitude of these changes 



