514 Dr. Bohert S. Ball [Feb. 11, 



WEEKLY EVENING MEETING, 



Friday, February 11, 1881. 



William Bowman, Esq. F.K.S. Vice-President, in the Chair. 



Robert S. Ball, Esq. LL.D. F.E.S. 



ASTRONOMER ROYAL OF IRELAND. 



The Distances of the Stars. 



Every one who is acquainted with the rudiments of astronomy knows 

 that the sun with its attendant planets is merely an island group in 

 the vast realms of space. 



An island the size of this room in the middle of the Atlantic 

 would be over a thousand miles from the coasts of Europe and 

 America on either side. Yet that island would not be more remotely 

 apart from the surrounding shores than is our solar system from the 

 bodies which surround it in space. To determine the distance from 

 this solar island to the stars which surround it, is the problem for our 

 consideration to-night. 



Becent Besearches on 61 Cygni, 



It is now almost exactly forty years (February 12, 1841) since the 

 gold medal of the Eoyal Astronomical Society was awarded to Bessel 

 for his discovery of the annual parallax of 61 Cygni. On that 

 occasion Sir John Herschel delivered an address, in which he glanced 

 at the labours of Struve and Henderson as well as Bessel. The 

 discovery of the distances of the stars was alluded to as " the greatest 

 and most glorious triumph which practical astronomy has ever 

 witnessed." From this date the history of our accurate knowledge 

 of the subject may be said to commence. Each succeeding race of 

 astronomers takes occasion to investigate the parallax of 61 Cygni 

 anew, with the view of confirming or of correcting the results arrived 

 at by Bessel. 



[The parallactic ellipse which the stars appear to describe, having 

 been briefly explained, the method of deducing the distances of the 

 stars was pointed out.] 



The attention of Bessel was directed to 61 Cygni by its proper 

 motion of five seconds per annum. When Bessel was at his laboui'S in 

 1838, the pair of stars forming the double were in the position 

 indicated on Fig. 1. When 0. Struve undertook his labours in 1853 

 the pair of stars forming 61 Cygni had moved considerably, as is 

 shown on the figure. Finally, when the star was observed at 

 Dunsink in 1878, it had made another advance in the same direction 

 as before. In forty years this object had moved over an arc of the 

 heavens upwards of three minutes in length. 



