SUCCESSORS OF NEWTON IN ASTRONOMY 



observations of the heavens in the southern hemi- 

 sphere. 



It was while in St. Helena that Halley made his 

 famous observation of the transit of Mercury over the 

 sun's disk, this observation being connected, indi- 

 rectly at least, with his discovery of a method of de- 

 termining the parallax of the planets. By parallax 

 is meant the apparent change in the position of an ob- 

 ject, due really to a change in the position of the ob- 

 server. Thus, if we imagine two astronomers making 

 observations of the sun from opposite sides of the 

 earth at the same time, it is obvious that to these 

 observers the sun will appear to be at two different 

 points in the sky. Half the angle measuring this dif- 

 ference would be known as the sun's parallax. This 

 would depend, then, upon the distance of the earth 

 from the sun and the length of the earth's radius. 

 Since the actual length of this radius has been de- 

 termined, the parallax of any heavenly body enables 

 the astronomer to determine its exact distance. 



The parallaxes can be determined equally well, how- 

 ever, if two observers are separated by exactly known 

 distances, several hundreds or thousands of miles apart. 

 In the case of a transit of Venus across the sun's disk, 

 nple, an observer at New York notes the image 

 of the planet moving across the sun's disk, and notes 

 also the exact time of this observation. In the same 

 manner an observer at London makes similar obser- 

 ns. Knowing the distance between New York 

 and London, and the different time of the passage, it is 

 thus possible to calculate the difference of the paral- 

 laxes of the sun and a planet crossing its disk. The 



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