2 7 8] 



The Parallax of b\ Cygni 



indication of nearness, and selected a star (61 Cygni) 

 which was barely visible to the naked eye but was re- 

 markable for its large proper motion (about 5" per annum) ; 

 evidently if a star is moving at an assigned rate (in miles 

 per hour) through space, the nearer to the observer it is the 

 more rapid does its motion appear to be, so that apparent 

 rapidity of motion, like brightness, is a 

 probable but by no means infallible 

 indication of nearness. A modification 

 of Galilei's differential method (chap- 

 ter vi., 129, and chapter XIL, 263) 

 being adopted, the angular distance 

 of 6 1 Cygni from two neighbouring 

 stars, the faintness and immovability 

 of which suggested their great distance 

 in space, was measured at frequent 

 intervals during a year. From the 

 changes in these distances <r a, a- b 

 (in fig. 85), the size of the small ellipse 

 described by o- could be calculated. 

 The result, announced at the end of 

 1838, was that the star had an annual 

 parallax of about " (chapter VIIL, 

 161), i.e. that the star was at such 

 distance that the greatest angular dis- 

 tance of the earth from the sun viewed 

 from the star (the angle s a- E in fig. 86, 

 where s is the sun and E the earth) 

 was this insignificant angle.* The 

 result was confirmed, with slight altera- 

 tions, by a fresh investigation of 

 Bessel's in 1839-40, but later work FIG. 86. The parallax 

 seems to shew that the parallax is a of61 c >'S ni - 

 little less than ".t With this latter 

 estimate, the apparent size of the earth's path round the 

 sun as seen from the star is the same as that of a halfpenny 



* The figure has to be enormously exaggerated, the angle s <r E as 

 shewn there being about 10, and therefore about 100,000 times too 

 great. 



f Sir R. S. Ball and the late Professor Pritchard ( 279) have 

 obtained respectively -47" and '43" ; the mean of these, '45", may be 

 provisionally accepted as not very far from the truth. 



