124 



A Short History of Astronomy CH. i v , 92 



far off that any motion due to this cause was too small 

 to .be noticed. If, for example, the earth moves in six 

 months from E to E', the change in direction of a star at 

 s' is the angle E' s' E, which is less than that of a nearer 

 star at s ; and by supposing the star s' sufficiently remote, 

 the angle E' s' E can be made as small as may be required. 

 For instance, if the distance of the star were 300 times 

 the distance E E', i.e. 600 times as far from the earth as 



FIG. 50. Stellar parallax. 



the sun is, the angle E s' E' would be less than 1 2', 

 a quantity which the instruments of the time were barely 

 capable of detecting.* But more accurate observations 

 of the fixed stars might be expected to throw further light 

 on this problem. 



* It may be noticed that the differential method of parallax 

 (chapter vi., 129), by which such a quantity as 12' could have 

 been noticed, was put out of court by the general supposition, shared 

 by Coppernicus, that the stars were all at the same distance from 

 us. 



