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ASTRONOMY, ETC., OF SEVENTEENTH CENT. 215 



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to be visible to European observers ; and fot this purporip {ie repaired j ' 

 to the island of St. Helena, where he remained! f^r a year. An obser- 

 vation was made by Halley during his residence at/Sj. Helena of ,a 

 transit of the planet Mercury over the sun's disc, which suggested to 

 him the idea of employing the transits of Mercury and Venus /Is jthe 

 means of determining the parallax of the sun, and consequently the 

 distance between the sun and the earth. We shall briefly explain the 

 method of determining the sun's distance which Halley was the first 

 to propose, and this explanation will have the more interest from the 

 fact of the recent transit of Venus having been observed in various 

 parts of the world with great care, in order to realize with all the re- 

 finements of modern appliances the suggestions of the illustrious as- 

 tronomer of the seventeenth century. Parallax signifies that apparent 

 change in the position of an object which is owing to a real change 

 in the position of the observer. Two observers, who at the same 

 instant view the sun or the moon from different places- on the earth's 

 surface, refer the luminary to different positions in the sky, just as 

 two persons in a room would see a lamp on the table intercepting 

 from view a different part of the opposite wall. The angle which 

 measures this difference in the apparent positions of an object is called 

 by astronomers its parallax, and the positions to which it is referred 

 are usually assumed to be the centre of the earth and a point on its 

 surface. The parallax in this sense would therefore be the same as 

 the angle which the earth's radius would subtend when viewed from 

 the sun ; and in like manner the parallax of Venus would be the angle 

 subtended by the earth's radius at that planet. The radius of the earth 

 being known by the actual measurement of the length of a degree on 

 its surface, the knowledge of the parallax of any heavenly body would 

 give us its distance. Now, when the planet Venus comes betwixt us 

 and the sun, she has of course a greater parallax, and an observer at 

 one part of the earth will see the dark body of the planet crossing over 

 the sun at a part of his disc different from that to which an observer 

 at another part will refer the planet's path. Thus, the path for one 

 observer rnay appear to cut off but a small segment of the sun's disc, 

 as at c D, Fig. 101, while to another a parallel track, traversing perhaps 

 a very large part of the disc, will be presented. 

 In each case the observers notice very accurately 

 the time occupied in the transit, and from this the 

 parallax can be determined within the possibility 

 of a very small error. Of course the difference 

 of the times of passage of the planet is due to 

 the parallax of the planet and the sun jointly;, 

 but as the relative distances of the sun and the FlG - I01 - 



planet are known by Kepler's law, and hence the 

 ratio of their parallaxes, the actual parallax in each case is easily de- 

 ducible from the observation which gives their difference. It was in 



