CHAPTERS ON THE STARS. 655 



as great. A parallax of 0"20 implies a distance of more than a million 

 times that of our unit of measure. 



The first conclusive result as to the extreme minuteness of the 

 parallax of the brighter stars was reached by Struve, at Dorpat, about 

 1830. In the high latitude of Dorpat the right ascension of a star 

 can be determined with great precision, not only at the moment of its 

 transit over the meridian, but also at transit over the meridian below 

 the pole, which occurs twelve hours later. He, therefore, selected a 

 large group of stars which could be observed twice daily in this way 

 at certain times of the year, and made continuous observations on them 

 through the year. It was not possible, by this method, to certainly 

 detect the parallax of any one star. What was aimed at was to deter- 

 mine the limit of the average parallax of all the stars thus observed. 

 The conclusion reached was that this limit could not exceed one-tenth 

 of a second and that the average distance of the group could not, 

 therefore, be much less than two million times the distance of the sun; 

 if, perchance, some stars were nearer than this, others were more distant. 



By a singular coincidence, success in detecting stellar parallax was 

 reached by three independent investigators almost at the same time, 

 observing three different stars. 



To Bessel is commonly assigned the credit of having first actually 

 determined the parallax of a star with such certainty as to place the 

 result beyond question. The star having the most rapid proper motion 

 on the celestial sphere, so far as known to Bessel, was 61 Cygni, which 

 is, however, only of the fifth magnitude. This rapid motion indicated 

 that it was probably among the stars nearest to us, much nearer, in 

 fact, than the faint stars by which it is surrounded. 



After several futile attempts, he undertook a series of measurements 

 with a heliometer, the best in his power to make, in August, 1837, and 

 continued them until October, 1838. The object was to determine, 

 night after night, the position of 61 Cygni, relative to certain small 

 stars in its neighborhood. Then he and his assistant, Sluter, made a 

 second series, which was continued until 1840. All these observations 

 showed conclusively that the star had a parallax of about 0".35. 



While Bessel was making these observations, Struve, at Dorpat, 

 made a similar attempt upon Alpha Lyrae. This star, in the high north- 

 ern latitude of Dorpat, could be accurately observed throughout almost 

 the entire year. It is one of the brightest stars near the Pole and has a 

 sensible proper motion. There was, therefore, reason to believe it 

 among the nearest of the stars. The observations of Struve extended 

 from 1835 to August, 1838, and were, therefore, almost simultaneous 

 with the observations made by Bessel on 61 Cygni. He concluded that 

 the parallax of Alpha Lyrae was about one-fourth of a second. Subse- 



