158 SEVENTEENTH CENTURY. pt. ill. 



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 

 the whole world who saw this transit of Venus, the first one 

 ever observed. 



Halley suggests that the Sun's distance may be 

 measured by the Transit of Venus, 1691.— This was all 

 that was known about transits when Halley went to St. 

 Helena in 1676 to study the stars of the southern hemi- 

 sphere. Here he also observed a transit of Mercury, and 

 after watching the small black spot travelling across the 

 face of the sun, and noting the time it took in going from 

 one side to the other, the idea occurred to him that it 

 would be possible to learn the distance of the sun by mea- 

 suring the path of a planet across its face. As Mercury, 

 however, is very far from us, and near to the sun, it would 

 not answer the purpose so well as Venus, which is much 

 nearer the earth. 



Halley knew that another transit of Venus would take 

 place in 1761, and as he could not hope to live till then, he 

 read a paper to the Royal Society in 1691, and another in 

 1 7 16, beseeching the astronomers who should live after him 

 not to let such an opportunity pass, and describing the way 

 in which the observations should be made. It is this 

 method which we must now try to understand as far as it is 

 possible without mathematics. 



First of all I must tell you two facts which astronomers 

 knew already. The proportion of the distances of the 

 planets was ascertained, as you will remember, by Kepler 

 (see p. 100). Therefore it was known that Venus is (in round 



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