154 A CENTURY'S PROGRESS IN ASTRONOMY. 



there was an annual displacement which could 

 only be attributed to parallax. In order to have 

 no mistake, he made another year's observa- 

 tions, which confirmed the results he arrived at 

 previously, and all doubt was removed by a 

 third series. The resulting parallax was 0*3483", 

 corresponding to a distance of 600,000 times 

 the Earth's distance from the Sun. This was 

 confirmed some years later by C. A. F. Peters 

 at Pulkowa, and still later by Otto Struve, who 

 estimated the distance at forty billions of miles. 

 Meanwhile, F. G. W. Struve, working at Pul- 

 kowa, found a parallax of 0*2613" for Vega, 

 but this was afterwards found to be consider- 

 ably in error. Accordingly, Struve does not 

 rank with Bessel as a successful measurer of star- 

 distance. But independently of Bessel, another 

 accurate measure had been made by Thomas 

 Henderson, the great Scottish astronomer. 



Born in Dundee in 1798, Thomas Henderson 

 was the youngest of five children of a hard- 

 working tradesman. After education in his 

 native town he went to Edinburgh, where he 

 worked for years as an advocate's clerk, pur- 

 suing studies in astronomy as a recreation from 

 his boyhood. In 1831 he had become so well 

 known, that he received the appointment of 

 Astronomer -Royal at the new observatory at 



