THE STORY OF TRANSITS 431 



Combining these, we see that transit years are 1631, 1639, 

 1761, 1769, 1874, 1882, 2004, 2012, 2117, 2125. 



These transits have come in pairs for the last two thousand 

 years, and will continue to do so for about a thousand years 

 more. At the end of that time they will occur singly and 

 Venus will cross the central part of the solar disc. 



The table shows us that if we failed to see the transit of 1 882, 

 there is very little likelihood of ever witnessing one of the most 

 interesting sights in the whole of naked-eye astronomy. 



The year 1631, with which we have begun the table, was 

 the first in which astronomers began to look for the new appear- 

 ance of Venus as a black spot against the brilliant background 

 of the sun. Two years previously Kepler had predicted the 

 phenomenon for the early part of December, and Gassendi 

 searched carefully for it on the 4th, 5th, 6th, and 7th ; but he 

 failed to witness it, and we now know that the transit actually 

 took place during the night of the 6th and 7th. The first to be 

 observed was, therefore, that of 1639, and, as far as we know, 

 this was seen by two people only. A Lancashire clergyman 

 named Horrocks had been making some calculations in con- 

 nection with the event, and these pointed to its occurrence on 

 December 4. This was a Sunday and the young vicar had to 

 arrange his day as best he could so that he could watch the skies 

 in the intervals between the services in his church. After an 

 anxious day his labours were rewarded just before sunset, when 

 the clouds cleared away and revealed the planet on the brilliant 

 solar disc. Horrocks had previously informed his friend Crab- 

 tree of his prediction, and he, too, saw the transit take place. 



Transits of Mercury are far more frequent than those of 

 its sister world, for Mercury is so much nearer the sun that 

 there is much less likelihood of it passing over or beneath the 

 solar disc. 



Precisely the same general ideas apply to transits of Mercury 

 as apply to those of Venus. Let us take another hoop of wire 

 and try and represent the path of this smaller planet on the 

 clock face we used just now. We shall find that the line of nodes 

 will be the one that joins the 22 and the 52 minute marks, the 

 nodes themselves being at a distance, from the centre, of about 

 one-third of the radius. The earth will be at the extremities 

 of this new line of nodes on May 7 and November 9, so that, 

 when transits of Mercury occur at all, they must take place on 

 or about one or other of these dates. The ascending node is 

 the November one and is considerably nearer the sun than is 

 the May node, since the orbit of Mercury departs from the 

 circular form to quite an appreciable extent. We should, 



