NEBULAR HYPOTHESIS 301 



increased the rate of their rotation, and this caused equatorial 

 bulging, till some of them, following the example of their parent 

 body, shed rings which became satellites. 



In support of this theory many harmonies in the motions of the 

 members of the solar system were cited, and in the early days of 

 the hypothesis, existing nebulae were thought to give it support, 

 for among them, </.s then known, there seemed to be nebulous aggre- 

 gations in various stages of development, from diffuse nebulous 

 masses to forms almost as concentrated as suns; but the best photo- 

 graphs now taken fail to show that any follow the lines of this 

 hypothesis. Grave difficulties arise from the dynamics of the 

 theory, but without some knowledge of celestial mechanics, it is 

 not possible to appreciate the full force of the arguments against 

 it. Some of them may be stated briefly. 



1. In the evolution of a gaseous nebula, it is highly improbable 

 that rings would be formed, for the molecules of gas would separate 

 from the parent nebula one by one. 



2. Even if rings were formed, there are grave difficulties in 

 their development into spheroids as set forth by this hypothesis. 



3. In the intensely hot condition of the assumed ring which 

 was to form the earth and moon, its gravity could hardly have held 

 its gases together. Even now the earth does not appear to hold 

 permanently very light gases, though it holds the heavier ones. 



4. It is probable that the material of a ring, such as the supposed 

 earth-moon ring, would have cooled to solid particles long before 

 it could collect into a spheroid. In this case no secondary ring to 

 form a moon would be developed. 



5. The inner satellite of Mars (Phobos) revolves about that planet 

 three times while the planet rotates once. According to theory, 

 these motions must have corresponded at the time of separation, 

 and since that time the planet should have increased its rotation 

 by cooling. Its period of rotation should therefore be shorter than 

 the period of the satellite's revolution. Explanations have been 

 suggested for this difficulty, but they do not meet the case. The 

 small bodies that make up the inner edge of the inner ring of Saturn 

 also revolve in about half the time that planet rotates. 



6. If the solar system were converted into a gaseous spheroid, 

 with its matter distributed according to the laws of gases, and 

 expanded to Neptune's orbit, and if this nebula had the total 

 momentum (technically, the moment of momentum) of the solar 



