98 SEVENTEENTH CENTURY. PT. in. 



our moon, which proved that she moved round the sun. You 

 may imagine how delighted he was to find the Copernican 

 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 1618, 

 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, 1818. 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 Jupiter 13 \ times farther off from the sun 

 than Mercury, and he also knew how long each was in going 

 round the sun, and from these two facts he worked the 

 following rule. 



If you take any two planets and cube their distances from 

 the sun and then square the time each takes in going round 



