SKCT. XXXVII. DISTANCE OF BINARY SYSTEMS. 371 



without success. Sir William Herschel afterward ap- 

 plied it to some of the binary groups ; and though he 

 did not find the thing he sought for, it led to the dis- 

 covery of the orbitual motions of the double stars. 



Though the absolute distance of most of the stars is 

 still a desideratum, a limit has been found under which, 

 probably, none of them come. It was natural to sup- 

 pose that in general the large stars are nearer to the 

 earth than the small ones ; but there is now reason to 

 believe that some stars, though by no means brilliant, 

 are nearer to us than others which shine with greater 

 splendor. This is inferred from the comparative ve- 

 locity of their motions. All the stars have a general 

 motion of translation, which tends ultimately to mix the 

 stars of the different constellations, but none that we 

 know of moves so rapidly as 61 Cygni; and on that 

 account it is reckoned to be nearer to us than any 

 other, for an object seems to move more quickly the 

 nearer we are to it. This circumstance induced MM. 

 Arago and Mathieu to endeavor to determine its an- 

 nual parallax, that is, to ascertain what magnitude the di- 

 ameter of the earth's orbit would have as seen from the 

 star, and from that to compute its distance from the 

 earth (N. 223). This has been accomplished with more 

 accuracy by M. Bessel, who has found by observation, 

 that the diameter of the earth's orbit of 190 millions of 

 miles would be seen from the star under an angle of 

 only one-third of a second, whence 61 Cygni must be 

 592,200 times farther from the earth than the sun is, 

 a distance which light, flying at the rate of 190,000 

 miles in a second, would not pass over in less than 

 nine years and three months. 



The apparent motion of five seconds annually which 

 this star has, seems to us to be extremely small, but at that 

 distance an angle of one second corresponds to twenty- 

 four millions of millions of miles ; consequently the an- 

 nual motion of 61 Cygni is one hundred and twenty 

 millions of millions of miles, and yet, as M. Arago ob- 

 serves, we call it a fixed star ! 



From the observations of Professor Henderson it ap- 

 pears that Sirius, the brightest star in the heavens, has 

 a parallax of less than the third of a second ; conse- 



