PLANETARY EVOLUTION 



were habitable, his rings would prove a formi- 

 dable nuisance. Mr. Proctor has shown that, 

 in latitudes corresponding to that of New York 

 and Naples, they cause total eclipses of the 

 sun, which last seven terrestrial years at a time. 

 But the problem which natural theology thus 

 fails to solve is completely solved bv a verv 

 simple mechanical consideration. Since the de- 

 tachment of a moon-forming ring from a con- 

 tracting planet depends on the excess of cen- 

 trifugal force over gravity at its equator, it is 

 evident that rings will be detached in greatest 

 numbers from those planets in which the cen- 

 trifugal force bears the highest ratio to gravita- 

 tion. Such planets will have the greatest number 

 of moons. And such, in fact, is the case. Of 

 the four inner planets, which rotate slowly, and 

 in which the centrifugal force is therefore small, 

 only the earth is known to have a satellite. 1 

 But Jupiter, whose centrifugal force is twenty 

 times greater than that of any of the inner plan- 

 ets, has four satellites. Uranus, with still greater 

 centrifugal force, has at least four, and proba- 

 bly six or eight moons. And finally Saturn, in 

 which the centrifugal force is one sixth of grav- 



1 It is not improbable that Venus may have a satellite also. 

 Several astronomers have declared that they have seen such a 

 satellite ; but as their testimony seems difficult to reconcile 

 with that of other astronomers, equally competent as observers, 

 the question must remain an open one for the present. 

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