314 . LAPLACE. 
sphere was then wanted to account for a movement in 
which all the stars participated at the same time. 
Copernicus having deprived the earth of its alleged 
immobility, gave a very simple explanation of the most 
minute circumstances of precession. He supposed that 
the axis of rotation does not remain exactly parallel to 
itself; that in the course of each complete revolution of 
the earth around the sun, the axis deviates from its posi- 
tion by a small quantity ; in a word, instead of supposing 
the circumpolar stars to advance in a certain way towards 
the pole, he makes the pole advance towards the stars. 
This hypothesis divested the mechanism of the universe 
of the greatest complication which the love of theorizing 
had introduced into it. A new Alphonse would have 
then wanted a pretext to address to his astronomical 
synod the profound remark, so erroneously interpreted, 
which history ascribes to the king of Castile. 
If the conception of Copernicus improved by Kepler 
had, as we have just seen, introduced a striking im- 
provement into the mechanism of the heavens, it still 
remained to discover the motive force which, by altering 
the position of the terrestrial axis during each successive 
year, would cause it to describe an entire circle of nearly 
50° in diameter, in a period of about 26,000 years. 
Newton conjectured that this force arose from the 
action of the sun and moon upon the redundant matter 
accumulated in the equatorial regions of the earth: thus - 
he made the precession of the equinoxes depend upon 
the spheroidal figure of the earth; he declared that 
upon a round planet no precession would exist. 
All this was quite true, but Newton did not succeed in 
establishing it by a mathematical process. Now this 
great man had introduced into philosophy the severe and 
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