66 GALILEO AND HIS JUDGES. 



greatly as the stars vary in brightness, they present 

 no appreciable diameter at all to the eye ; not even 

 those classed as being of the first magnitude. 



Another and more weighty objection to Copernicus 

 is, however, urged by the mouth of Simplicio, and it 

 is this if the Earth really makes an annual revolu- 

 tion round the Sun, why do not the fixed stars, 

 viewed as they must be at different seasons of the 

 year from points so widely distant, change their 

 apparent positions in the heavens ? We have just 

 seen that the true distance of the Sun was not known 

 at that time ; if it had been known, and if the men 

 of that age had been aware that the diameter of the 

 Earth's orbit was about 184,000,000 miles in length, 

 the objection would have been still more forcible. 

 But the modern answer to it is conclusive : the stars, 

 or rather a certain number of them, do actually under- 

 go a small displacement in their apparent position every 

 year, or in the technical language of astronomy, they 

 have an annual parallax, a fact which not merely 

 disposes of the objection, but actually confirms the 

 truth of the Copernican theory. 



Galileo's reply (by the mouth of Salviati) is to the 

 effect that the followers of Ptolemy admit that it 

 takes 36,000 years to effect a complete revolution 

 of the starry sphere ; then, judging from the planets, 

 the length of time required for the orbit is in propor- 

 tion to the distance, and we suppose the distance of the 

 starry sphere to be, on such assumption, 10,800 semi- 

 diameters of the Earth's orbit (or Sun's orbit, as 



