GALILEO AND HIS JUDGES. 161 



It is, therefore, generally considered as a critical test 

 of the truth of the system. 



There are two other phenomena, on which however 

 I do not propose to dwell at any length, known as 

 precession and nutation, which it is not easy to 

 explain otherwise than by the modern theory of 

 astronomy and the principle of gravitation ; the latter 

 of these two owed its discovery to Bradley, and the 

 former to Hipparchus, who could not have been aware 

 of its real cause, though he had observed the fact of 

 its occurrence. 



But passing on from these, I may call attention to 

 one most remarkable result of modern scientific 

 research, connected with the stars. In Galileo's day, 

 it was a drawback to the Copernican theory that 

 none of the stars showed the smallest annual 

 parallax ; in popular language, none of them seemed 

 to undergo any change of place, however small, when 

 observed at opposite points of the Earth's orbit, or as 

 the opponents would have said, the Earth's imagined 

 orbit. A displacement of this kind, I need hardly 

 repeat, must not be confounded with that other 

 motion which Bradley observed and explained. This 

 was one of Tycho Brahe's reasons for rejecting the 

 Copernican system, and it was one of the best argu- 

 ments used by the opponents of Galileo. As the 

 enormous distance of the stars from the Earth was, as 

 we have already seen, at that time unknown, the 

 celestial distances generally being under-estimated 

 even by the best astronomers, the argument had an 



