CH. XIX. THE TRANSIT OF VENUS FIRST SEEN. 157 



the telescope as a round black spot upon the sun's face. 

 With Mercury this happens at intervals of from seven to 

 thirteen years ; but with Venus it is much more rare, for 

 though two transits generally come together with an in- 

 terval of only 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 Gasscndi took 

 advantage of this prediction, and managed to observe Mer- 

 cury passing across the face of the sun on November 7, 163 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 tliis too was seen. 

 You will remember 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 



