GRAVITATION VERSUS INERTIA 33 



are ballasted just that way. The longitudinal libration 

 of the moon is a variation whose cause will be discussed 

 in the special chapter on that body. 



The third class, Pseudo, includes the sun, Jupiter, 

 Saturn, Uranus, and Neptune. These do not rotate on 

 their axes at all, but only seem to do so from the fact 

 that they REVOLVE around the centers of their respective 

 systems in such restricted orbits as to create the illu- 

 sion of rotation. As previously explained, the sun is 

 the center of a balancing system, and has his part to 

 perform in this exercise as well as the planets circulat- 

 ing about him. Each instant some one point within him 

 is the absolute pivot of the system, but this pivot is as 

 elusive as a drop of quicksilver, and as fickle as the 

 bubble of a spirit-level in a nervous hand. Neverthe- 

 less, the sun keeps circulating around it just as obedi- 

 ently as does his humblest asteroid. 



Now the reason the sun does not seem to us to shift 

 appreciably out of position on this account is that the 

 planets largely counterbalance each other, so that the 

 adjustments left to him are so minute as effectually to 

 elude our detection. 



At first glance, it may seem a small matter 

 whether we say that the sun "rotates'' or "revolves," 

 but the distinction is not only important but funda- 

 mental. Thus: 



1. Were his movement really rotational, it would 

 be arbitrary and anti-gravitational in character, and 

 would demand the specification by science of a strange, 

 remote, and long-spent cause. 



2. The persistence of sun-spot cavities in a gaseous 

 globe like the sun ? not merely for days- but for many 

 months together, is incompatible with the rotational 

 theory. 



