60 SCIENCE PROGRESS 



and five hours. The lunar orbit was practically in the plane of 

 the equator ; the initial eccentricity is uncertain, as it depends 

 altogether on the actual variation of the viscosity with the 

 time. 



The question that next arises is, what was the condition 

 just before this ? The natural suggestion is that the two bodies 

 formed one mass. The cause of the separation is, however, 

 open to some doubt. It has been thought that the rapidity 

 of the rotation would be enough to cause instability, in which 

 case the original body might break up into two parts. Moulton, 

 on the other hand, has shown that the actual rotation could not 

 be so rapid as to make the system unstable. It is more likely 

 that Darwin's original suggestion is correct, namely, that at the 

 epoch considered the period of rotation was nearly double the 

 period of one of the free vibrations of the mass ; consequently 

 the amplitude of the semidiurnal tide would be enormous, and 

 might easily lead to fission in a system not possessing much 

 strength. 



The Prevalence of Direct Motion in the Solar System. — On all 

 of the theories of the origin of the solar system that have here 

 been described it is necessary that all the planets should revolve 

 in the same direction. On the Planetesimal Theory this would 

 be the direction of the motion of the perturbing body relative 

 to the sun at the time of the initial disruption. In addition 

 to this, however, all the planets except probably Uranus 

 and Neptune have a direct rotation, and all the satellites except 

 those of these two planets and the outer ones of Jupiter and 

 Saturn have a direct revolution. The fact that three satellites 

 revolve in the opposite direction to the rotation of their primaries 

 is in flagrant contradiction to the original form of the Nebular 

 Hypothesis. It was, however, suggested by Darwin that all 

 the planets might have originally had a retrograde rotation, 

 and that the friction of the solar tides has since reversed the 

 rotation of all except the two outermost. Jupiter and Saturn 

 would then be supposed to have produced their outer satellites 

 before the reversal took place, and the others afterwards. An 

 objection to this theory has been raised by Moulton, who 

 points out that the secular retardation of the rotation of Saturn 

 due to solar tides is only about yrtnny of that of the earth, so 

 that there probably was not time for this to occur. On the other 

 hand, this retardation is proportional to the seventh power of 



