THE COMING TRANSIT OF VENUS. 19 



the sun could be seen, the case would be different, for 

 they would serve as index points, by means of which to 

 estimate the sun's displacement. But although stars 

 not lying near the sun's place on the heavens can be 

 seen by day with powerful telescopes, those close around 

 him are quite invisible. This method failing, the 

 astronomer has to look for other means of solving the 

 problem. The planet Venus, which comes at times 

 much nearer to the earth than the sun is, and in fact 

 nearer than any celestial body except the moon, na- 

 turally claims attention as a suitable object for the 

 astronomer's purpose. For it is to be remembered that 

 the proportions of the solar system have long been 

 accurately determined ; so that as soon as the distance 

 of any one planet is ascertained, the scale of the whole 

 solar system becomes also known. 



Venus, however, when at her nearest, is lost in the 

 sun's light, and, though discernible in powerful tele- 

 scopes, is quite unsuitably placed for the delicate obser- 

 vations which would alone avail to determine her 

 distance. 



This brings us at once to the recognition of the 

 importance of a transit of Venus. When Venus passes 

 between the sun and earth, in such a way as not to 

 cross the sun's face, that is, when she passes above 

 or below the long and almost linear portion of space 

 lying actually between the earth and sun, she cannot 

 be well observed ; but when, in making the passage, 

 she comes so close to the line joining the earth and sion 

 as actually to be seen on the sun's face, she can be 



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