LAPLACE. 135 



which flow from this moYement. Two of these have more especially 

 attracted attention. 



Bj reason of the precession of the equinoxes, it is not always the same 

 groups of stars, the same constellations, which are perceived in the 

 heavens at the same season of the year. In the lapse of ages the con- 

 stellations of winter will become those of summer,, and reciprocally. 



By reason of the precession of the equinoxes, the pole does not always 

 occupy the same place in the starry vault. The moderately bright star 

 which is very justly named in the present day the pole star, was far 

 removed from the pole in the time of Hipparchus ; in the course of a 

 few centuries it will again appear removed from it. The designation 

 of pole-star has been and will be applied to stars very distant from 

 each other. 



When the inquirer, in attempting to explain natural phenomena, has 

 the misfortune to enter ui^ou a wrong path, each precise observation 

 throws him into new comiilications. Seven spheres of crystal did not 

 suffice for reijresentiug the phenomena as soon as the illustrious astron- 

 omer of Khodes discovered precession. An eighth 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 preces- 

 sion. 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 position by a small 

 quantity ; in a word, instead of sui^posing the circumpolar stars to ad- 

 vance in a certiiiu way toward the pole, he makes the pole advance 

 toward the stars. This hypothesis divested the mechanism of the 

 universe of the greatest complicat»ion which the love of theorizing had 

 introduced into it. A new Alphonse would have then wanted a pre- 

 text 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 

 iust seen, introduced a striking improvement into the mechanism of the 

 heavens, it still remained to discover the motive force which, by alter- 

 ing the position of the terrestrial axis during each successive year, 

 would cause it to describe an entire circle, of nearl}" 50° in diameter, in 

 a period of about 2G,U00 years. 



Newton conjectured tbat 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 i)rocess. Now this great man had introduced into 

 philosophy the severe and just rule : Consider as certain only what has 



