SKETCH OF NICOLAUS COPERNICUS. 259 



mirable whole which we call the system of Copernicus, and which 

 is really only the correct arrangement of. the planetary system to 

 which we belong." " After long researches/' Copernicus himself 

 said, " I am convinced that if we refer the motions of the other 

 planets to the revolution of the earth, calculation will agree well 

 with observation. ... I do not doubt that mathematicians will 

 be of my opinion, if they will take the pains to make themselves 

 acquainted, not superficially but profoundly, with the demonstra- 

 tion which I shall present in this book." 



He reasoned that " every displacement manifest to our view 

 proceeds either from the object perceived or from the subject 

 which perceives, or from the unequal motions of the two, for an 

 equal and simultaneous motion of the object and the subject could 

 cause no semblance of displacement. The earth is the place 

 whence the movement of the sky is presented to our view. Every 

 motion starting from the earth is reflected in the sky, which will 

 appear to move in the opposite direction. Such is the diurnal 

 revolution, which appears to involve the whole universe except 

 the earth. If now we suppose that the sky has none of this mo- 

 tion, but that the earth turns around itself from west to east (in 

 a contrary direction from the apparent motion of the sky), we 

 shall find that it is really so." Among the chief arguments in 

 support of this view, the astronomer insisted especially on the 

 immensity of the sky as compared with the size of the earth. 

 "The whole mass of the earth," he said, "vanishes before the 

 grandeur of the sky ; the horizon divides the celestial sphere into 

 halves, which could not be if the earth bore any proportion to the 

 extent of the sky, or if its distance from the center of the universe 

 was perceptible. Compared to the sky, the earth is only a point ; 

 it is as a finite quantity compared with an infinite quantity. It 

 is no more admissible to suppose the earth resting in the center 

 of the universe. What! to believe that immensity turns every 

 twenty-four hours around an insignificancy ! " So the inequalities 

 in the movements of the planets their forward and backward 

 movements and stationary positions were referred to two causes : 

 the movement of translation of the earth and the proper motions 

 of the planets ; correctly, as modern astronomers explain them, 

 only Copernicus was not able to give details and exact figures. 



Ptolemy had argued against the idea of these motions of the 

 earth, because if the earth were translated through space it would 

 leave all the loose things on it behind ; and, if it turned on its axis 

 from west to east, it would be impossible for bodies to make any 

 headway to the eastward, for, whatever the rate of their motion, 

 the earth would always reach a given point in that direction first. 

 Hence the former idea was the most ridiculous of all (navriov ytXoio- 

 rara), and the latter altogether ridiculous (ndvv yeXotoTarov). These 



