ASTRONOMY 115 



the sea on the two opposite sides of the earth. This is what 

 causes the tides. Now, if A comes close enough, this differ- 

 ence in the attraction on the two sides of B may tear B to 

 pieces. Of course B would exert the same effect upon A. 

 It can be shown that the final result would be to tear off from 

 B a number of parts, large and small, each of which would 

 revolve around what was left of B in an elliptical path, but 

 that at any one time the several pieces would be distributed 

 around the central piece in two spirals. In speaking about 

 nebulae I said that most of the nebulae in the sky showed 

 just such a spiral form, and the belief of Chamberlain and 

 Moulton is that these spiral nebulae are the result of this 

 process of one star being torn apart by another approaching 

 close to it. The larger ones of these pieces revolving about 

 the central mass would attract the smaller pieces and even- 

 tually would either absorb them and thus grow larger, or else 

 cause them to revolve about the big pieces as satellites. So 

 in time we should have the spiral nebula changing into a sys- 

 tem like our solar system, consisting of a central sun around 

 which revolve several planets, these in turn having satellites 

 about them. 



This theory has not yet been subjected to enough care- 

 ful criticism to bring out its weak points, but it certainly 

 seems to account very well for many of the facts about the 

 planets and their orbits which were very hard to explain on 

 the older theories. It is hard to see, however, how such a 

 process as I have described could explain the great amount 

 of heat which the sun and planets undoubtedly had at one 

 time. I think we should have to assume that the original 

 body was already very hot before it came near the other one. 



Another theory, which has not been worked out very 

 carefully yet, is the one proposed by Arrhenius. He supposes 

 that a spiral nebula is formed by stars and meteorites wan- 

 dering by chance into a nebula originally of irregular form 

 and there growing larger by attracting some of the nebulous 



