SECT. VII. TRANSIT OF VENUS. 53 



sun like a black spot. If we could imagine that the sun and 

 Venus had no parallax, the line described by the planet on his 

 disc, and the duration of the transit, would be the same to all 

 the inhabitants of the earth. But, as the semi-diameter of the 

 earth has a sensible magnitude when viewed from the centre of 

 the sun, the line described by the planet in its passage over 

 his disc appears to be nearer to his centre, or farther from it, 

 according to the position of the observer ; so that the duration of 

 the transit varies with the different points of the earth's surface 

 at which it is observed (N. 133). This difference of time, being 

 entirely the effect of parallax, furnishes the means of computing 

 it from the known motions of the earth and Venus, by the same 

 method as for the eclipses of the gum. In fact, the ratio of the 

 distances of Venus and the sun from the earth at the time of 

 the transit is known from the theory of their elliptical motion. 

 Consequently the ratio of the parallaxes of these two bodies, 

 being inversely as their distances, is given ; and as the transit 

 gives the difference of the parallaxes, that of the sun is obtained. 

 In 1769 the parallax of the sun was determined by observations 

 of a transit of VenUs made at Wardhus in Lapland, and at 

 Tahiti in the South Sea. The latter observation was the object 

 of Cook's first voyage. The transit lasted about six hours at 

 Tahiti, and the difference in duration at these two stations was 

 eight minutes ; whence the sun's horizontal parallax was found 

 to be 8"*72. But by other considerations it has been reduced by 

 Professor Encke to 8"*5776 ; from which the mean distance of 

 the sun appears to be about ninety-five millions of miles. This 

 is confirmed by an inequality in the motion of the moon, which 

 depends upon the parallax of the sun, and whicli, when compared 

 with observation, gives 8"'6 for the sun's parallax. The transits 

 of Venus in 1874 and 1882 will be unfavourable for ascertaining 

 the accuracy of the solar parallax, and no other transit of that 

 planet will take place till the twenty-first century ; but in the 

 mean time recourse may be had to the oppositions of Mars. 



The parallax of Venus is determined by her transits ; that of 

 Mars by direct observation, and it is found to be nearly double 

 that of the sun, when the planet is in opposition. The distance 

 of these two planets from the earth is therefore known in ter- 

 restrial radii, consequently their mean distances from the sun 

 may be computed ; and as the ratios of the distances of the 

 planets from the sun are known by Kepler's law, of the squares 



