A HISTORY OF SCIENCE 



ject to precisely the same laws as the other planets. 

 Long familiarity has made these wonderful laws of 

 Kepler seem such a matter of course that it is dif- 

 ficult now to appreciate them at their full value. Yet, 

 as has been already pointed out, it was the knowledge 

 of these marvellously simple relations between the 

 planetary orbits that laid the foundation for the 

 Newtonian law of universal gravitation. Contempo- 

 rary judgment could not, of course, anticipate this 

 culmination of a later generation. What it could 

 understand was that the first law of Kepler attacked 

 one of the most time-honored of metaphysical con- 

 ceptions namely, the Aristotelian idea that the cir- 

 cle is the perfect figure, and hence that the planetary 

 orbits must be circular. Not even Copernicus had 

 doubted the validity of this assumption. That Kep- 

 ler dared dispute so firmly fixed a belief, and one that 

 seemingly had so sound a philosophical basis, evidenced 

 the iconoclastic natiire of his genius. That he did not 

 rest content until he had demonstrated the validity 

 of his revolutionary assumption shows how truly this 

 great theorizer made his hypotheses subservient to the 

 most rigid inductions. 



GALILEO GALILEI 



While Kepler was solving these riddles of planetary 

 motion, there was an even more famous man in Italy 

 whose championship of the Copernican doctrine was 

 destined to give the greatest possible publicity to the 

 new ideas This was Galileo Galilei, one of the most 

 extraordinary scientific observers of any age. Galileo 

 was born at Pisa, on the i8th of February (old style), 



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