284 A Short History of Astronomy [Cu. X. 



which rendered practicable an entirely different method 

 of finding the longitude (chapter vi., 127). 



227. The astronomers of the i8th century had two 

 opportunities of utilising a transit of Venus for the deter- 

 mination of the distance of the sun, as recommended by 

 Halley ( 202). 



A passage or transit of Venus across the sun's disc is 

 a phenomenon of the same nature as an eclipse of the 

 sun by the moon, with the important difference that the 

 apparent magnitude of the 'planet is too small to cause any 

 serious diminution in the sun's light, and it merely appears 

 as a small black dot on the bright surface of the sun. 



If the path of Venus lay in the ecliptic, then at-every 

 inferior conjunction, occurring once in 584 days, she would 

 necessarily pass between the sun and earth and would 

 appear to transit. As, however, the paths of Venus and the 

 earth are inclined to one another, at inferior conjunction 

 Venus is usually far enough " above " or " below " the 

 ecliptic for no transit to occur. With the present position 

 of the two paths which planetary perturbations are only 

 very gradually changing transits of Venus occur in pairs 

 eight years apart, while between the latter of one pair and 

 the earlier of the next pair elapse alternately intervals of 

 105! and of 12 1 1 years. Thus transits have taken place in 

 December 1631 and 1639, June 1761 and 1769, December 

 1874 and 1882, and will occur again in 2004 and 2012, 

 2117 and 2125, and so on. 



The method of getting the distance of the sun from a 

 transit of Venus may be said not to differ essentially from 

 that based on observations of Mars (chapter vin., 161). 



The observer's object in both cases is to obtain the 

 difference in direction of the planet as seen from different 

 places on the earth. Venus, however, when at all near 

 the earth, is usually too near the sun in the sky to be 

 capable of minutely exact observation, but when a transit 

 occurs the sun's disc serves as it were as a dial-plate on 

 which the position of the planet can be noted. Moreover 

 the measurement of minute angles, an art not yet carried 

 to very great perfection in the i8th century, can be avoided 

 by time-observations, as the difference in the times at 

 which Venus enters (or leaves) the sun's disc as seen at 



