ORIGIN OF THE WORLD 



explanation was put forward that might conceivably; 

 account for Phoebe's eccentricity. In particular Pro- 

 fessor Pickering himself explained it picturesquely 

 by assuming that Saturn had originally rotated in the 

 reverse direction, but that subsequent to the detach- 

 ment of the ninth satellite the planet had turned com- 

 pletely over owing to what is called the precessional 

 effect of the sun's gravitational pull on its bulging 

 equator, which acted as a sort of tidal brake. 



You can illustrate the effect very clearly if you will 

 experiment with a kind of top or gyroscope that 

 whirls in so-called gimbal rings. This top will spin 

 for a long time without changing its direction of axis 

 if not interfered with. But if you touch your ringer to 

 its rim (tidal brake) you will cause it to twist to one 

 side, and as you continue the pressure you will see the 

 top turn completely over and remain there (the ob- 

 struction being removed) spinning in the opposite di- 

 rection to that which it had at first. 



This interesting theory of the overturning of the 

 planets applies of course to other members of the sys- 

 tem, including the earth. It finds partial support in 

 the vertical revolution of the satellites of Uranus. 

 But another complication was introduced when Pro- 

 fessor Perrine in 1905 made photographic discovery 

 of two new moons in the system of Jupiter (the sixth 

 and seventh) which seemed to be revolving in almost 

 the same orbit, but one of them going forward, the 

 other (No. VII) in retrograde direction. 



Even the turning upside down of planets cannot ac- 

 count for two moons revolving in opposite direction 

 at about the same distance from their primary. 



13 



