12 EVOLUTION OF THE EARTH 



parts would be condensed and revolve fast while the outer 

 parts were still diffuse and revolved slowly. The mode sug- 

 gested by Laplace for the separation of the rings is also 

 dynamically very unsatisfactory. Moulton has shown that the 

 growth of the planets and the development of rotation in the 

 same direction as their orbital motion could be much better 

 attained from an initial state in which the component particles 

 revolved in the same plane but independently and in highly 

 elliptic orbits about the central nucleus. This is a wide de- 

 parture from the idea of a circular ring revolving as a unit 

 body. 



Still more fundamental objections, emphasized by Cham- 

 berlin and Moulton, are found in certain of the existing 

 dynamical relations of the solar system. It would be expected 

 that in condensation the central mass would continually aban- 

 don matter from its equatorial zone, the inner planets would 

 presumably have possessed the greater masses, and the final 

 sun would now show a high speed of rotation, giving an equa- 

 torial diameter far greater than the polar. Such expectations 

 are contrary to the facts. The sun revolves so slowly on its 

 axis, once in twenty-five days, that it has no measurable equa- 

 torial bulge. In other words, centrifugal force is negligible 

 in the sun. Furthermore, the equatorial plane of the sun, 

 instead of lying precisely in the mean plane of the planets' 

 orbits, is inclined seven degrees to such a mean plane. 



A hypothesis to gain scientific credence must emerge success- 

 ful from the test of observed facts and mathematical theory. 

 The nebular hypothesis has not done so. It is on the defensive 

 and has lost standing during the past generation. Neverthe- 

 less, it would be premature to abandon it entirely. It has the 

 advantage of simplicity in that satellites, planets, and sun are 

 explained as the products of a single process, convergence in a 

 rotating nebula. But nature is often found to be complex in 

 her operations, so that this advantage is of doubtful weight. 



