loo SEVENTEENTH CENTURY. pt. hi. 



second law, therefore, was that planets describe equal areas 

 about their centre in equal times. 



Not many months after Kepler published these two 

 laws, he heard of Galileo's discoveries with his telescope — 

 that Jupiter had four satellites, and that Venus was proved 

 to move round the sun by having phases like our moon. 

 You may imagine how delighted he was to find the Co- 

 pernican theory made so much more certain, and to see 

 that the telescope was opening the way for so many new 

 discoveries. * Such a fit of wonder,' he said, ' seized me 

 at this report, and I was thrown into such agitation, that 

 between the joy of the friend who told me, my imagination, 

 and the laughter of both, confounded as we were by such 

 a novelty, we were hardly capable, he of speaking or I of 

 listening.' 



For many years after this Kepler was beset with troubles. 

 The Emperor, being at war with his brother Matthias, had no 

 money to spare for salaries. Kepler was thus harassed by 

 poverty ; his favourite son died of the small-pox, which the 

 troops had brought into the city, and his wife died of grief 

 not long afterwards. It was not till the year 1 6 1 8, after he had 

 re-married and had been rescued from his poverty by the 

 new Emperor Matthias, that the unfortunate astronomer had 

 energy and leisure to turn again to his favourite planets. 



Kepler's Third Law, 1618.— It was in that year that he 

 worked out with immense labour his third and most famous 

 law — by which he showed how much longer the planets were 

 going round the sun, according as they were farther off 

 from it. This is difficult to understand, but we must try to 

 form some idea of it. He did not know in figures how far 

 each planet was from the sun, but he knew the proportion of 

 their distances, as for example, that Mars is 4 times and 



