60 TRANSIT OF VENUS. [SECT. vii. 



finding the parallax of an object as near as the moon, it will 

 not answer for the sun, which is so remote that the smallest 

 error in observation would lead to a false result. But that 

 difficulty is obviated by the transits of Venus. When that 

 planet is in her nodes (N. 130), or within 1^ of them, that is, 

 in, or nearly in, the plane of the ecliptic, she is occasionally 

 seen to pass over the 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 ob- 

 served (N. 131). 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 sun. In fact, the ratio of 

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

 of the transit are 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 deter- 

 mined by observations of a transit of Venus made at Wardhus 

 in Lapland, and at Otaheite in the South Sea. The latter 

 observation was the object of Cook's first voyage. The tran- 

 sit lasted about six hours at Otaheite, 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 ap- 

 pears to be about ninety-five millions of miles. This is con- 

 firmed by an inequality in the motion of the moon, which 



