154 SEVENTEENTH CENTURY. FT. IIL 



eight years between them, yet after this there is a gap of 

 more than a hundred years before another transit occurs. 



After Kepler had finished the famous Rudolphine Tables 

 he was able to use them to calculate when these transits 

 would take place ; and he showed that both Mercury and 

 Venus would cross the sun's disc on certain days in the 

 year 1631. A French philosopher named Gassendi. took 

 advantage of this prediction, and managed to observe Mer- 

 cury passing across the face of the sun on November 7, 1 63 1. 

 He was the first who ever observed a transit. With Venus 

 he was not so fortunate, for the transit of that planet took 

 place when it was night at Paris, and so Gassendi had no 

 chance of observing it 



It was not long, however, before this too was seen. It 

 must be remembered that two transits of Venus occur close 

 together with only eight years between them. Now Kepler 

 had imagined that in 1639 Venus would pass a little to the 

 south of the sun, and so no transit would take place. A 

 young Englishman, however, named Jeremiah Horrocks, 

 only twenty years of age, after going carefully over Kepler's 

 tables, felt convinced that there would be a transit, and he 

 even calculated within a few minutes the time when Venus 

 would enter upon the sun's face. Full of enthusiasm at the 

 chance of seeing this grand sight, he wrote to a friend at a 

 distance, begging him also to watch through the telescope at 

 three o'clock on the afternoon of December 4, 1639. His 

 expectations were not disappointed, for at fifteen minutes 

 past three on that day the planet began to creep over the 

 face of the sun. For twenty minutes Horrocks watched it, 

 and then the sun set and he could see no more. He had 

 been able to notice, however, that Venus was much smaller 

 in comparison with the sun than had been formerly supposed. 

 Horrocks and his friend Crabtree were the only people in 



