A NEW ASTRONOMY 197 



the earth is spherical ; third, that the motions of the heavenly bodies 

 are uniform circular motions or compounded of such motions. 

 The slender basis for the first and third of these may be inferred 

 from his statement in regard to certain hypothetical causes of 

 want of uniformity : 



Both of which things the intellect shrinks from with horror, it 

 being unworthy to hold such a view about bodies which are con- 

 stituted in the most perfect order. 



He makes the relative character of the motions involved of 

 fundamental importance. In his own words : 



For all change in position which is seen is due to a motion either 

 of the observer or of the thing looked at, or to changes in the position 

 of both, provided that these are different. For when things are 

 moved equally relatively to the same things, no motion is perceived, 

 as between the object seen and the observer. 



Thus the daily revolution of sun, moon, and stars about a station- 

 ary earth would have the same apparent effect as rotation of the 

 earth in the opposite direction about its own axis, and the ap- 

 parent yearly motion of the sun about the earth is equivalent to 

 an orbital motion of the latter. 



'It is,' he says, 'more probable that the earth turns about its 

 axis than that the planets at their various distances, the comets sweep- 

 ing through space, and the endless multitude of the fixed stars, describe 

 the same regular daily motion about the earth.' 



The apparent irregularities in the motions of the five known 

 planets had been a perpetual stumbling-block to the ancient 

 astronomers, requiring more and more complicated hypotheses for 

 their explanations as accuracy of observations increased. The 

 heliocentric theory of Copernicus, inaccurate as it was in some 

 respects, afforded a simple explanation of the fact that Mercury 

 and Venus seem merely to oscillate east and west of the sun, 

 while Mars, Jupiter, and Saturn recede indefinitely from it, ex- 

 hibiting also periodic reversals of the direction of their motion. 



