THE BORDERLAND OF ASTRONOMY AND GEOLOGY 1 



By Prof. A. S. Eddington, F.R.S. 



The region in which geology and astronomy most conspicuously 

 overlap is in the theories of the origin of our planet. We have, in 

 fact, two main theories — one due originally to an astronomer, La- 

 place, and the other to a geologist, Chamberlin. 



In the last century the evolution of a star seems often to have 

 been regarded as something quite detached from the evolution of 

 the stellar universe. Just as the birth and death of a man is an 

 incident which can occur at any time in the rise and decline of 

 the human race, so it was thought that the birth and extinction of 

 a particular star formed merely a detached incident in the course 

 of progress of the stellar universe — if, indeed, the universe was 

 progressing in any particular direction. Thus it was a natural be- 

 lief that the stars died out and were re-formed by collisions of ex- 

 tinct stars; and that the matter which now forms the sun had 

 undergone many alternations of incandescence and extinction since 

 things first began. But this view is quite at variance with the 

 general tendency of sidereal astronomy in the present century. We 

 have come to recognize that the stellar system is one great organiza- 

 tion, and that the stars which are shining now are more or less 

 coeval with one another. Everyone would admit that Mars and 

 Jupiter were formed as parts of one process of evolution — not nec- 

 essarily at the same moment, but each formed as the process reached 

 the appropriate stage; and similarly we now believe that it was 

 one process of evolution sweeping across the primordial matter 

 which caused it to form itself into stars; and these original stars 

 are the actual stars which we see shining now. No doubt the 

 evolution did not develop at the same rate in all parts of the 

 universe, and there are probably places where stars are still being 

 formed; but you will see that this view is entirely different from 

 the other view that stars were being formed individually by hap- 

 hazard collisions of dark stars, so that each was an independent 

 formation, having no time connection with other stars. 



This view has been forced on us partly by direct evidence of 

 organization among the stars, pointing to a common origin for 

 large groups of stars. We notice scattered groups such as the 

 Hyades, which have almost exactly equal and parallel motions. 



1 A lecture delivered before the Geological Society of London on November 21, 1922. 

 Reprinted by permission from Nature, January 6, 1923. 



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