MIRACLES OF SCIENCE 



corrections that will give the size of the angle sub- 

 tended by the earth's radius as viewed from the sun, 

 which is the parallax desired. Such is, in point of fact, 

 the method by which the distance of the moon is de- 

 termined. But in attempting to apply the method to 

 measurement of the parallax of the sun, complications 

 arise that have proved quite insuperable. The direct 

 observation of the sun presents obvious difficulties 

 because of the intense brilliancy and heat of its sur- 

 face, and it is found almost impossible to focus on its 

 precise centre with needful accuracy. The actual 

 angle to be measured is now known (thanks to 

 other methods) to be not far from 8.8 seconds of arc; 

 and this, as Professor F. R. Moulton points out, 

 makes the apparent shift in the sun's position as 

 viewed from opposite sides of the earth closely 

 equivalent to the shift of an object a mile distant as 

 viewed first with one eye and then with the other. 

 The angle is too small to be measured under the 

 given conditions. So it is necessary to find another 

 way of estimating the sun's parallax. 



It was suggested by the celebrated Halley, dis- 

 coverer of the comet that bears his name, that a 

 means of measuring the sun's parallax might be 

 afforded by observation of a transit of Venus; that 

 is to say of the passage of Venus across the sun's 

 face on one of those rare occasions when the course 

 of our sister planet carries her directly between the 

 earth and the sun. Owing to the slightly varying 

 planes of the planetary orbits, a transit of Venus is 

 a relatively infrequent occurrence. When a transit 

 does occur, however, it is repeated after an interval 



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