168 DISCOVERY CH. 



minutes," said Kepler, " we will construct a new theory 

 that will explain the motions of all the planets." 



All previous theories had assumed the existence of 

 uniform circular movement ; and it was only when this 

 doctrine had to be abandoned that Kepler was led to 

 the truths embodied in his three famous laws of planetary 

 motion. In the first of these laws the Platonic principle 

 was disestablished for ever, for it states that the planets 

 do not move in circles but in ellipses, and that the sun 

 is situated at a focus of each ellipse. The second law, 

 announced at the same time, in 1609, defines the rate 

 of movement and shows that a planet moves fastest 

 when nearest the sun and slowest when most distant. 

 The third fundamental truth, which was not discovered 

 until nine years later, is known as the Harmonic law, 

 and gives precisely the relation that exists between 

 periods of revolution and distances from the centre 

 of motion. By means of this law, the distance of a 

 planet from the sun, in comparison with the earth's 

 distance, can be calculated when the period which the 

 planet takes to make a complete revolution round the 

 sun is known. In the work in which he announced this 

 law, the discovery of which had occupied twenty years 

 of his life, Kepler wrote in words of unfeigned delight : 



that for which I joined Tycho Brahe, for which I settled in 

 Prague, for which I have devoted the best part of my life to 

 astronomical contemplations, at length I have brought to light, 

 and recognised its truth beyond my most sanguine expectations 

 . . . the die is cast, the book is written, to be read either now 

 or by posterity, I care not which ; it may well wait a century 

 for a reader, as God has waited six thousand years for an observer. 

 Kepler. 



The meaning of Kepler's three laws of planetary 

 motion remained a mystery until Newton's discovery 

 of the principle or law of universal gravitation. By 



