SCIENCE OF THE SIXTEENTH CENTURY. 83 



sailors to move in proportion as their vessel moves, and they alone and 

 what is with them seem to be at rest." The reasoning' b'y wiicft Cpper- 

 nicus supports his views is not however always so sound, for he often 

 uses arguments which are as frivolous as those of the Aristoteleans he 

 undertakes to refute. He attributes to the celestial bodies motion only 

 in perfect circles, and accepts the doctrines about " natural " motion and 

 " violent " motion which Aristotle laid down, Copernicus could not 

 therefore discard the epicycles which figure so largely in the Ptole- 

 maian theory; but compared with that system the Copernican was sim- 

 plicity itself. Further, all objections which could be urged against 

 his theory by those who accepted the Aristotelean physics, Copernicus 

 refuted by an appeal to the same principles. Thus, he said, if it be 

 true that the velocity of the earth's motion could cause the dispersion 

 into space of the bodies belonging to it, then must the celestial sphere 

 revolving with an infinitely greater velocity be liable in an infinitely 

 greater degree to this dispersion. 



Copernicus advanced some speculations on gravity which read al- 

 ' most like an anticipation of Newton's grand idea. The followers of 

 Aristotle observing that bodies on the earth's surface tended to move 

 towards its centre, hastily concluded that this point was the centre 

 towards which eveiy body in the universe was attracted. But this, 

 remarks Copernicus, is very doubtful, for gravity being but the tendency 

 of parts to draw together and coalesce in the form of a globe, " it 

 is probable that such a tendency exists in the sun, moon, and other 

 heavenly bodies ; but this does not prevent them from describing 

 their respective orbits. If, then, the earth have other motions, these 

 must be the same as those which other bodies appear to possess." 

 The reasoning of Copernicus on the nature of the motions of the 

 heavenly bodies is however completely Aristotelean. " It is impossible 

 that a single celestial body can move unequally in one orbit ; for that 

 must happen either through the inconstancy of the moving power, 

 whether it be extraneous or belonging to its intimate nature, or through 

 a disparity in the body revolving. But both of these suppositions 

 are repugnant to our understandings." But as the planets plainly do 

 not revolve in circles of which the sun is the centre, Copernicus was 

 obliged to have recourse to the ancient hypothesis of epicycles ; and 

 this was perhaps the best available one until the discovery of the true 

 form of these orbits by Kepler. 



The author of the " Celestial Revolutions" was probably aware that 

 his novel system of the world would encounter much opposition, for 

 he seems to have been anxious to present it in a form as little obnox- 

 ious as possible to the adherents of the established theory. There 

 was indeed some difficulty in the choice of the place where the new 

 views were to be given to the world. Rome was avoided, doubtless 

 for good reasons; and the detestation in which Melanchthon (Schwartz- 

 erde) held astronomical science induced the friends of Copernicus, 



6 2 



