SOO Astronomy. [Lecture 19. 



Though the sun's distance, therefore, is so 

 great that the earth's diameter is only a point in 

 comparison, and his parallax, for the reasons 

 already assigned, could not be determined with 

 accuracy, the case is very different when Venus 

 is perceptibly between the earth and the sun, for 

 her distance is between three and four times less 

 than that of the sun. If, therefore, when Venus 

 in her transit enters upon the sun's disc, she is 

 observed by two different spectators on different 

 parts of the earth's surface, she will appear to 

 each of them at the same instant on different 

 parts of the sun. Dr. Halley, therefore, re- 

 commended that some scientific men should be 

 sent to different parts of the world, where the 

 transit could be observed with accuracy ; that 

 the precise times of her entrance and egress from 

 the face of the sun should be carefully noted by 

 each ; and from these observations, compared 

 with the time which she would occupy in passing 

 over the sun's surface, as seen (by supposition) 

 from the earth's centre, he demonstrated that 

 not only the parallax of Venus but that of the 

 sun might be found. 



I shall not trouble you with the detail of this 

 problem. It is founded on the principles al- 

 ready explained in treating of the moon's hori- 

 zontal parallax, and is explained at large in 

 different treatises on Astronomy *. Let it suffice 



* To those who wish to enter more deeply into the sub- 



