362 A Short History of Astronomy [CH. xil 



at a distance of rather more than three miles. In other 

 words, the distance of the star is about 400,000 times the 

 distance of the sun, which is itself about 93,000,000 miles. 

 A mile is evidently a very small unit by which to measure 

 such a vast distance ; and the practice of expressing such 

 distances by means of the time required by light to perform 

 the journey is often convenient. Travelling at the rate of 

 186,000 miles per second ( 283), light takes rather more 

 than six years to reach us from 6 1 Cygni. 



279. Bessel's solution of the great problem which had 

 baffled astronomers ever since the time of Coppernicus was 

 immediately followed by two others. Early in 1839 Thomas 

 Henderson (1798-1844) announced a parallax of nearly i" 

 for the bright star a Centauri which he had observed at the 

 Cape, and in the following year Friedrich Georg Wilhelm 

 Struve (1793-1864) obtained from observations made at 

 Pulkowa a parallax of j* for Vega ; later work has reduced 

 these numbers to f " and T V respectively. 



A number of other parallax determinations have subse- 

 quently been made. An interesting variation in method was 

 made by the late Professor Charles Pritchard ( 1808- 1893) 

 of Oxford by photographing the star to be examined and its 

 companions, and subsequently measuring the distances on 

 the photograph, instead of measuring the angular distances 

 directly with a micrometer. 



At the present time some 50 stars have been ascertained 

 with some reasonable degree of probability to have measur- 

 able, if rather uncertain, parallaxes ; a Centauri still holds 

 its own as the nearest star, the light-journey from it being 

 about four years. A considerable number of other stars 

 have been examined with negative or highly uncertain 

 results, indicating that their parallaxes are too small to be 

 measured with our present means, and that their distances 

 are correspondingly great. 



280. A number of star catalogues and star maps too 

 numerous to mention separately have been constructed 

 during this century, marking steady progress in our know- 

 ledge of the position of the stars, and providing fresh 

 materials for ascertaining, by comparison of the state of 

 the sky at different epochs, such quantities as the proper 

 motions of the stars and the amount of precession. Among 



