140 Science Religion and Reality 



He still pursued the main object of his life, the foundation of 

 an astronomy in which demonstrable causes should replace arbitrary 

 hypothesis. The next subject that Kepler set himself to investigate 

 was the relation of the distances of the planets to their time of 

 revolution round the central sun. It was clear that the time of 

 revolution was not proportional to the distance. For that the outer 

 planets were too slow. Why wa? this ? " Either," he said, " the 

 moving intelligence of the planets is weakest in these that are 

 furthest, or there is one moving intelligence in the sun that forces 

 all j-ound but most the nearest, languishing and weakening in the 

 most distant by attenuation of its virtue by remoteness." 



As the sixteenth century turned into the seventeenth century 

 Kepler received a great incentive to work by joining Tycho Brahe 

 as assistant. By the death of Tycho in 1601 Kepler became 

 effectively his literary legatee. The next nine years saw him 

 largely occupied with the papers of Tycho and with work on optics, 

 in the course of which he developed an approximation to the law 

 of the refraction of light. In 1 609 was issued his greatest work, 

 the " New Astronomy, with Commentaries on the Motions of 

 Mars." It is full of important discoveries and suggestions. 

 Among them we may enumerate the following : 



{a) Important truths relating to gravity are enunciated, e.g., 

 that the earth attracts a stone rather than the stone seeks the earth, 

 and that two bodies near each other will always attract each other 

 if adequately beyond influence of a third body. 



{b) A theory of the tides is developed in relation to attraction 

 by the moon. 



{c) An attempt to explain planetary revolutions results in a 

 theory of vortices not unlike that elaborated later by Descartes. 



Above and beyond all, the work sets forth the cardinal principles 

 of modern astronomy, the so-called first two planetary laws of 

 Kepler by which 



(i) Planets move round the sun not in circles but in ellipses. 



(ii) Planets move not uniformly but in such a way as to sweep 

 out equal areas about their centres in equal times. 



It was another nine years before Kepler enunciated his third 

 law to the effect that 



(iii) The squares of the period of revolution round the sun are 

 proportional to the cubes of their distance (161 8). 



The Aristotelian physics and cosmology now lay derelict and 



