140 FROM NEBULA TO NEBULA 



corresponding increase in the gyroscopic momentum (i. e. 

 centrifugal motion) and a consequent spreading out of 

 the circle to what extent Nature, herself, tells. 



The whole proceeding may be likened to the action of 

 a boy's peg-top, except that in one case the phenomenon 

 starts from a state of rest and thereafter grows, while in 

 the case of the top the reverse is true. When first 

 thrown, as everybody well knows, the top does not settle 

 at once on one spot, but gyrates at the outset in a widish 

 circle, which gradually narrows until the peg seemingly 

 becomes glued to one spot. Note, if you please, the in- 

 clination of the top in these introductory gyrations. Its 

 axis extended does not remain parallel to itself, but trims 

 out a cone, whose apex lies some distance below the floor, 

 vertically under the central point of these exursions. Our 

 earth is essentially a great top. While rotating on its 

 axis it is likewise, as a member of the sun 's great system, 

 revolving around with him in his orbit, keeping her axis 

 ever pointed toward the Vertex (the point below the floor, 

 of the ecliptic) and trimming out its great cone once every 

 (let us cling to round numbers) 260 centuries. 



This is an appropriate place to emphasize again the 

 fundamental distinction between Newton's explanation 

 of precession and my own. Inasmuch as he had no physi- 

 cal ground for postulating the solar motion (a fact which 

 was not really substantiated until a century and more 

 after his death), he was obliged to treat the solar system 

 as stationary and the earth, too, as stationary, in a cer- 

 tain sense. In his mind the earth's precessional oscillation 

 resembles the top's behavior when, after spinning for a 

 long time on one identical spot, it begins to nod and 

 wobble before finally falling over upon its side. It is 

 true that he did not attribute this nutation to loss of 

 momentum, but to the differential attractions of the sun 

 and moon upon the equatorial ring. Nevertheless, every- 

 one must perceive that the causes Newton assigned were 

 diurnally, monthly, and annually complete; and it was 

 highly inadmissible for him to assume out of hand that 

 the effects could be otherwise than swiftly cyclical to cor- 



