1909] on Recent Results of Astronomical Research. 565 



Pickering, of course, had only the case of Phoebe to account for, 

 but he suggested an explanation which is known as the theory of 

 planetary inversion, and which now receives strong confirmation from 

 the discovery of an outer retrograde satellite attending Jupiter. 



On this theory it is supposed that each planet, when it separated 

 from the original nebula, or whatever may have been the material 

 from wdiich the solar system was evolved, had originally a retrograde 

 rotation, i.e. a rotation from east to west, or in the opposite direction 

 to the motion round the sun. The motions of Phoebe, and we may 

 now add that of J YIII, are relics of this original retrograde motion, 

 Jupiter and Saturn having now turned right over so as to spin in the 

 forward direction, and all their inner satellites having turned over 

 along with the planets, or else only having detached themselves after 

 the parent-planets had turned over. Similarly the Earth and Mars 

 must have turned over, for they are now rotating forwards. In the 

 case of the two outermost planets of our system, Uranus and Nep- 

 tune, although we do not know their direction of rotation, the 

 motions of their satellites lend some support to the idea of an 

 original retrograde direction. Still, until the discovery of J VIII, 

 the evidence for the hypothesis rested very largely on Phoebe alone. 



How, then, has it come about that the traces of this original 

 rotation, believed once to have been universal, are so few ? The 

 cause suggested is tidal influence. Ever since Sir George Darwin 

 published his classical researches on the Tides, which showed what a 

 remarkable influence tidal effects have exerted in the formation especi- 

 ally of the Earth-Moon system, their importance in all evolutionary 

 processes has become more and more recognised. It is possible that 

 there may be a tendency to press the hypothesis too much nowadays : 

 the theory is to this extent speculative, that we have very little 

 direct evidence as to the magnitude of the tidal forces, or how many 

 million years are required to produce the effects that are assigned to 

 them. But the effects, though certainly small, are steadily cumu- 

 lative, so that, granted sufficient time, and there seems no reason why 

 time should be denied, these theoretical results must be brought about. 



The results of the action of tidal forces are too complicated for 

 us to do more than touch generally on. The tides raised by the sun 

 and each satellite have each to be considered, and then the attractions 

 of the sun and satellites on their own and each other's tides. The 

 viscosity of the planet is also an important factor. So many par- 

 ticular cases arise that it seems necessary to consider every planet and 

 satellite separately. The mathematics have been worked out in 

 considerable detail by Mr. Stratton, who concludes that tidal action 

 appears to be adequate to explain the characteristics of the various 

 systems of satellites attending on the planets. The mode in which 

 tidal action reverses the direction of the rotation is by tilting over the 

 axis. If you turn a planet over so that what was formerly the north 

 end becomes the south end, this is equivalent to reversing the direction 



