ON ASTRONOMY. 119 



76,000,000 of miles. These results agreed pretty nearly witli those 

 derived from the observations of Richer, Cassini, and Flamsteed, three- 

 quarters of a century earlier. 



In 1760 Mayer, from analytical equations involved m the lunar 

 theory, deduced a value for the sun's par. r= 7". 8. 



But the transit of Venus was now near at hand, which alone could 

 satisfactorily solve this great problem. Dr. Halley was the first to 

 perceive and urge the importance of the passage of Venus over the sun 

 for this purpose. As early as 1716 he called the attention of astronomers 

 to the two transits which were to occur in 1761 and 1769, and most 

 earnestly urged them not to neglect so precious and so rare an oppor- 

 tunity of determining the planetary distances of the solar system. If 

 these opportunities were neglected the like would not again occur for 

 more than a century. 



The transits of Venus invariably occur in June or December — those 

 at the ascending node in June, at the descending, in December. The 

 intervals between them are 8 years and 105^, then 8 and 121^, then 8 

 and lOoA, and so on successively in that order. 



The transit of 1639, December 4, was observed by Horrox and 

 Crabtree, two young men engaged in astronomical studies, in seclu- 

 sion, as Grant says, in one of the northern counties of England. 

 They, by good fortune, (for the time of the transit was not accurately 

 known,) had ''the privilege of witnessing a phenomenon which human 

 eyes had never before beheld, and which no one was destined again to 

 see till more than a hundred years had passed away." 



In anticipation of the transit of June 6, 1761, encouraged by the 

 earnest appeal of Dr. Halley, several European governments and 

 learned academies sent scientific commissions to different parts of 

 the earth to observe it. The English sent Dr. Maskelyne to St. 

 Helena, Mason and Dixon to the Cape of Good Hope. The French 

 Academy sent Pingre to the island of Rodrigues, in the Indian Ocean. 

 The Russians sent Chappe to Tobolsk, in Siberia, and Rumouski to a 

 station near Lake Baikal, on the Mongolian frontier. All the observa- 

 tories in Europe were put in requisition. 



I shall here explain the method by which the sun's parallax is de- 

 termined by the transit of Venus. At different stations on the earth 

 the planet will be seen to traverse the sun's disc in different lines, and 

 will occupy unequal times. As the relative motions of Venus and the 

 sun are accurately known, the duration of the transit will show the 

 particular chord along which it passed. The two chords, as seen by 

 two observers favorably situated, being known, will make known the 

 perpendicular distance between them. This perpendicular distance is 

 then compared witli the diameter of the earth or some known part of 

 that diameter, and at once gives tlie sun's parallax. The peculiar 

 advantage of this method is, that the distance between the two ap- 

 parent chords traversed by the planet is more accurately known by 

 the difference of times than by any direct micrometrical measurement. 

 This process will be easily understood by reference to fig. 13, where 

 A B re{)resents the earth, V Venus, and S the sun. For the most 

 successful result, the stations A and B should be at the extremities of 

 a diameter perpendicular to the phme of the ecliptic. At A Venus 

 will be seen to traverse the chord E n L, at B the chord D m H. 



