25?] fJerschfl's Astronomical Programme 331 



Coppernicus and his successors had found that the apparent 

 motions on the celestial sphere of the members of the solar 

 system could only be satisfactorily explained by taking 

 into account their actual motions in space, so that the 

 solar system came to be effectively regarded as consisting 

 of bodies at different distances from the earth and separated 

 from one another by so many miles. But with the fixed 

 stars the case was quite different : for, with the unimportant 

 exception of the proper motions of a few stars (chapter x., 

 203), all their known apparent motions were explicable as 

 the result of the motion of the earth ; and the relative or actual 

 distances of the stars scarcely entered into consideration. 

 Although the belief in a real celestial sphere to which the 

 stars were attached scarcely survived the onslaughts of 

 Tycho Brahe and Galilei, and any astronomer of note 

 in the latter part of the lyth or in the i8th century would, 

 if asked, hive unhesitatingly declared the stars to be at 

 different distances from the earth, this was in effect a 

 mere pious opinion which had no appreciable effect on 

 astronomical work. 



The geometrical conception of the stars as represented 

 by points on a celestial sphere was in fact sufficient for 

 ordinary astronomical purposes, and the attention of great 

 observing astronomers such as Flamsteed, Bradley, and 

 Lacaille was directed almost entirely towards ascertaining 

 the positions of these points with the utmost accuracy or 

 towards observing the motions of the solar system. More- 

 over the group of problems which Newton's work suggested 

 naturally concentrated the attention of eighteenth-century 

 astronomers on the solar system, though even from this 

 point of view the construction of star catalogues had con- 

 siderable value as providing reference points which could 

 be used for fixing the positions of the members of the solar 

 system. 



Almost the only exception to this general tendency 

 consisted in the attempts hitherto unsuccessful to find 

 the parallaxes and hence the distances of some of the 

 fixed stars, a problem which, though originally suggested 

 by the Coppernican controversy, had been recognised as 

 possessing great intrinsic interes*. 



Herschel therefore struck out an entirely new path when 



