CH. xix. if ALLEY'S METHOD. 155 



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

 ever observed. 



1691. Halley suggests that the Sun's Distance may 

 be measured by the Transit of Venus in 1761.- 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 measuring 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 

 1716, 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 mention two facts which astronomers 

 knew already. The proportion of the distances of the 

 planets was ascertained, as you will remember, by Kepler 

 (see p. 98). Therefore it was known that Venus is (in 

 round numbers) 2-*- times as far from the sun as she is from 

 the earth. It was also known by the apparent size of the 

 sun that the sun's distance is about 108 times his diameter, or, 

 in other words, if you could measure the number of miles 

 across the face of the sun and multiply that number by 108, 

 it would give you the sun's distance from the earth. 



