Solar and Planetary Evolution. 



59 



of matter separates off and is left suspended around the con- 

 tracting nucleus. Other rings form in the same way, until 

 a series of them surrounds the central mass which has con- 

 tracted into a sphere. But these rings are not everywhere 

 of equal thickness, and the tendency of their constituent 

 particles to gravitate toward the thickest part causes them 

 to separate at the thinnest point, and finally to form oblate 

 spheroids of nebulous matter, which ultimately condense 

 into spheres. 



Fig. 4. The planet Saturn, showing rings, and size as compared with the earth. 



Let us illustrate this by supposing that we have a grind- 

 stone, on which, while rotating, we pour a film of water. If 

 we revolve it fast enough the water will fly off, but we may 

 revolve it just fast enough to cause the centrifugal and cen- 

 tripetal forces to balance. Then if we could make the stone 

 contract in size, the layer of water would be left suspended 

 in the air as a ring surrounding the revolving grindstone. 

 In the case of the revolving nebula, the contraction of the 

 inner sphere of nebulous matter assists in the separation of 



