A MOVEMENT AND ITS CAUSE. 117 



parchus, was occupied by the sun in spring, has no longer 

 any value as a commemorative sign; it gives place nowa- 

 days to the constellation of the Fishes, and corresponds tc 

 the constellation of Taurus, or the Bull ; the constellation of 

 Taurus to Gemini, or the Twins; the constellation of the 

 Twins to Cancer, and so on. But little more than a month, 

 then (a month of 2000 years !), of the great year (a year of 

 25,000 years !) has elapsed since the epoch of Hipparchus. It 

 is to astronomy especially that, with a slight variation, we may 

 apply the aphorism of Hippocrates " Brevis vita, ars longa " 

 (Life is short, and art long). 



The precession of the equinoxes explains why the pole of 

 our starry vault does not occupy invariably the same point of 

 the firmament, and why the constellations which we now see 

 shining during the nights of a given season change their 

 places as time glides by. 



But what is the cause of this movement ? 



Before this question, as before a sovereign tribunal, appear 

 the two opposite doctrines which have been enunciated on 

 the value of the earth and the sun in the world's system. 

 According to the doctrine at once the oldest and most in- 

 tolerant, the earth occupies immovably the centre of the 

 world ; the sun and the planets are only its satellites ; they, 

 like the moon, revolve around the earth ; finally, all the starry 

 sphere, the whole celestial vault, rotates upon its own axis in 

 four-and-twenty hours. We have been speaking as if this 



