CHAPTER I 



INTRODUCTORY CHAPTER 



SURVEY OF THE PROBLEM 



The Solar System 



1. In 1543 Copernicus published his treatise "De Revolutionibus Orbium 

 Coelestium" in which the apparent motion of the planets was explained by 

 the simple hypothesis that they all described orbits about the Sun at rest. 

 Two thirds of a century later, in the early days of 1610, Galileo first observed 

 the satellites of Jupiter revolving around their primary, and so obtained what 

 amounted almost to direct visual proof of the truth of the Copernican system 

 of astronomy. But in verifying Copernicus' solution of one problem, Galileo 

 had opened up another. For it now became clear that there were at least two 

 systems of almost exactly similar formation in the universe, and a philosophic 

 mind could not but conclude that they had probably originated from similar 

 causes, and would be impelled to conjecture as to what those causes might be. 



In this way the problem of scientific cosmogony had its origin. To the 

 modern astronomer the problem is much richer, wider and more definite, in 

 proportion as the mass of observational . material within his knowledge is 

 greater than that with which Qalileo was acquainted. In the solar system 

 alone, we know that in addition to the eight great planets, there are upwards 

 of 900 minor planets * or asteroids, and all these 908 or more bodies shew the 

 same regularity in their motion. Their orbits are all nearly circular, they are 

 all approximately in one plane, and they are all described in the same direc- 

 tion. If we assume it to be -a priori an even chance that a planet should 

 move either from east to west or from west to east, then the chance against 

 908 planets all moving in the same direction would be 2 907 1 to 1. But if 

 we regard the problem from the point of view of statistical mechanics, and 

 calculate the odds against these orbits being all of small inclination and of 

 small eccentricity, then we arrive at odds in comparison with which the 

 previously calculated odds of 2 907 - 1 to 1 are so small as to look approxi- 

 mately like an even chance. 



* At the end of 1916, numbers had been assigned to 826, and orbits computed for 896. Of 

 the 520 earliest discovered planets, 13 are regarded as lost, having been seen at no opposition 

 since their discovery. 



J. C. 1 



