GRAVITATION VERSUS INERTIA 31 



be equally efficacious to explain the so-called axial 

 rotations. They are. 



Students of physics, but unfamiliar with the theory 

 of astronomy, will think I am indulging in playful 

 platitude when I soberly state that planets attract each 

 other by their centers of gravity. On the contrary, I 

 am giving utterance to rank astronomical heresy. More 

 than two centuries ago Newton, who lived in the 

 neolithic age of inductive science, and, for all his won- 

 derful genius, was still human and liable to the errors 

 of his day, expressed the opinion that planets behave as 

 though their entire mass were concentrated at their* 

 centers of fifittrc. This error has been blindly reiter- 

 ated and adhered to for so long that it may be very 

 difficult to eradicate it, notwithstanding its patent 

 absurdity. 



Suppose we were given, for purposes of experiment, 

 a precise miniature of the moon, both as to form and 

 structure, and that we wished first of all to ascertain the 

 position of its center of gravity. Since that planet is 

 considerably heavier than water, our moonlet will not 

 float in that liquid, so let us immerse it in mercury 

 instead. Suppose on doing this we should find that 

 the bottom side of the moonlet, while freely floating 

 in the mercury, exactly corresponded to the face of the 

 real moon as turned toward us. We should then know 

 that it is the bottom of the real moon that is turned our 

 way, and that the gravitation of our earth must have 

 some bearing on the fact. Now conceive this moonlet to 

 be dropped from the top of the Washington Monument, 

 and then successively from greater and greater heights, 

 as far up as the moon itself, will it not in one case as 

 in another, whether dropped from high or low, turn its 

 weightier end toward the earth in falling? If the minia- 

 ture will do so, why do scientists keep declaring that 



