1 8 EVOLUTION OF THE EARTH 



side of the basin. The matter shot out toward the passing star 

 would be attracted sideways after it as the star receded into 

 space. On falling back toward the sun it would consequently 

 pass to one side and elliptical orbits of the separate particles 

 would become established. The material shot out in the 

 reverse direction, from the opposite side of the sun, would 

 meet much the same conditions except that the sidewise pull 

 of the passing star would be less on it than on the sun. It is 

 seen that the lateral or deflecting force acting on both arms of 

 the nebula would be due to the difference between the pull of 

 the passing star on each arm and on the central body. The 

 initial spiral arms do not then represent the path along which 

 the material was shot out, but mark the rotation around the 

 central body or sun, both of the axis of expulsion during the 

 passing of the star and of the matter after it is expelled, as 

 shown in Figure 2. 



<i-^^-* ^ 



Fig. 2. — Origin of a spiral nebula according to Chamberlin and Moulton. 



The spiral nebula would be developed in a plane. That 

 plane is established by the hyperbolic orbit of the passing star 

 with the sun at the focus of the orbit. The new system would 

 thus show in its nature features imposed by both its parents. 



From such a nebulous fiery birth Moulton especially has 

 shown how, in accordance with the laws of celestial mechanics, 

 a planetary system could result. 



The matter which has converged into the planets would be 

 that residue of the solar tidal disruption which did not pass 

 beyond gravitative control and did not fall back into the body 



