THE AGE OF THE EARTH FROM THE POINT OF VIEW 



OF ASTRONOMY. 



By ERNEST W. BROWN. 

 {Read April 22, 1922.') 



Astronomical evolution is considered under three heads : First, 

 that method of observation in which it is assumed that all stages in 

 the process are visible in the sky and so can be traced step by step. 

 Second, physical theory, based on well-known laws such as those of 

 gravitation, heat, etc. Third, pure speculation. When we attempt 

 to apply these methods to the solar system, we find a complete absence 

 of any observational evidence from the first point of view, because 

 we have no stellar systems sufficiently near for us to detect planets if 

 such exist. Thus evolution in the solar system is mainly a mixture 

 of physical theory and speculation. 



All theories of evolution use the idea of contraction under gravi- 

 tation, which in general causes a gain of heat and of angular velocity. 

 The chief differences between the theories consist in the forms of 

 matter which are assumed to come into existence under the operation 

 of the process of contraction. Laplace imagined that a planetary 

 nebula contracted and in the course of the process left behind rings 

 of matter which later condensed into planets. Roche showed that 

 under certain conditions matter will be thrown off along the equator. 

 G. H. Darwin and Poincare developed the processes of fission from 

 which it was hoped that planetary bodies might be shown to have 

 developed through successive divisions of the central body. Later 

 workers at the theory, and particularly Jeans, have proved that this 

 hypothesis is very improbable for planetary evolution on account of 

 the fact that in this process of division the masses should be of the 

 same order of magnitude and not, as in the case of the planets, of 

 very different orders of magnitude. It has, however, been applied 

 with considerable success to the evolution of close double stars. 

 Finally there are the tidal hypotheses in which the matter is supposed 



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