26 A CENTURY'S PROGRESS IN ASTRONOMY. 



motion can be discovered, the proper motions of 

 the stars themselves very minute have to be 

 decomposed into two parts, the real motion of 

 the star, and the apparent motion, resulting 

 from the movement of the Solar System. To 

 any astronomer but Herschel the problem would 

 have been insoluble. Only sixty years had 

 elapsed since Halley had announced the proper 

 motions of the brighter stars which had been 

 previously supposed to be immovable hence 

 the name of "fixed stars." Herschel did not 

 deal with the motions of many stars. Only a 

 few proper motions were known with accuracy 

 when he attacked the problem in 1783. Making 

 use of the proper motions of seven stars, and 

 separating the real from the apparent motion, 

 he found that the Solar System was moving 

 towards a point in the constellation Hercules, 

 the "apex" being marked by the star X Her- 

 culis. The rate of the solar motion, Herschel 

 thought, was "certainly not less than that 

 which the Earth has in her annual orbit." This 

 extraordinary discovery was one of Herschel's 

 greatest works. " Its directness and apparent 

 artlessness," Miss Clerke remarks, " strike us 

 dumb with wonder." In 1805 Herschel again 

 attacked the subject, utilising the proper motions 

 of thirty-six stars. His second inquiry, on the 



