ADVANCE OF ASTRONOMY. 191 



precession, aberration, refraction, and instrumental 

 errors. And the edition of Bradley's results was 

 only a prelude to fresh catalogues of his own, exe- 

 cuted between 1821 and 1833, and including about 

 62,000 stars. It is hardly necessary to say that 

 Bradley's work was continued through the century 

 by many illustrious astronomers. 



Measuring the Distance of a Star. To the an- 

 cients the stars remained altogether mysterious; 

 they were points of fire set in the concave vault of 

 the firmament and borne by it in daily revolution 

 around the fixed earth. Keppler seems to have been 

 the first to dare to deduce from the Copernican sys- 

 tem the conclusion that the stars are extremely dis- 

 tant suns, so distant that most of them appear un- 

 affected in direction throughout the year ; e. g., when 

 viewed from opposite ends of the earth's orbit. If 

 so distant and yet so clearly visible, they must be 

 sunlike ; i. e., great sources of radiant energy. This 

 conclusion was less hesitatingly accepted by Galilei. 



But while it came to be generally recognised that 

 the stars were unthinkably distant suns, it was not 

 till 1838 that the distance of any star was measured. 

 In that year, Friedrich Wilhelm Bessel (1784- 

 1846), using Fraunhofer's heliometer, or " divided 

 object-glass micrometer," was able to determine the 

 parallax, and thus to deduce the distance of a small 

 star in the constellation of the Swan (61 Cygni). 

 Soon afterwards analogous results were published by 

 Thomas Henderson for a Centauri (1839), and by 

 Struve (1840) for Vega. 



The method of estimating the distance of a star 

 is simple in theory. Suppose that the direction of a 

 star is observed at a certain time with all possible 

 accuracy; suppose that the same star is observed 



