II 



GRAVITATION VERSUS INERTIA AS THE 

 MOTIVE POWER OF PLANETS 



Probably the first system of celestial motions mer- 

 iting the epithet "scientific-" was that devised by 

 Claudius Ptolemy about the middle of the second cen- 

 tury of onr era. According to him, the earth was a fixed 

 point and the absolute center of the universe around 

 which the sun, moon, planets, and stars, all revolved, 

 at various distances, every twenty-four hours. So far 

 the theory may be called the ImprcxxioiiiKtic, inasmuch 

 as it relied wholly upon the concerted evidence of the 

 three senses sight, hearing, and feeling which of 

 themselves lead to no other conclusion. 



Having once assumed that the heavenly bodies re- 

 volve daily around the earth, it was only natural to 

 infer that they do so in circles. But the ancients went 

 further, and gave what seemed to them an unanswer- 

 able ground for this inference, namely, that it is meta- 

 physicnllv abhorrent to suppose that the Creator would 

 choose any but the perfect curve (every point of the 

 circumference of a circle being equi-distant from the 

 center) for the heavenly bodies to pursue. 



However, systematic observation showed vagaries 

 of movement on the part of the planets that appeared 



15 



