SECT. XXXVI. APPARENT MAGNITUDE OF STARS, 389 



M. Peters ; hence a Lyras is 789,600 times more distant from the 

 sun than the earth is. 



It was natural to suppose 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 splendour. This is 

 inferred from the comparative velocity of their proper motions ; 

 all the stars have a general motion of translation, which tends 

 ultimately to mix those of the different constellations ; but none 

 that we know of moves so rapidly as 61 Cygni, and on that 

 account it was reckoned to be nearer to us than any other, 

 for an object seems to move more quickly the nearer it is. Now 

 M. Bessel saw that two minute and probably very remote stars 

 are very near 61 Cygni, their directions from that star being at 

 right angles to one another ; so that, during the revolution of the 

 earth, one of these distances was a maximum and the other a 

 minimum alternately every three months. This alternation, 

 although it indicated a parallax or difference of parallaxes of 

 only 0"-348, was maintained with such perfect regularity every 

 three months, that it leaves not a doubt of its accuracy, which 

 was afterwards confirmed by the observations of M. Peters at 

 Polkova. It follows from that small parallax that 61 Cygni 

 must be 592,700 times farther from the earth than the sun is 

 a distance that light would not pass over in less than nine years 

 and three months. 



Mr. Henderson found the parallax of Sirius, the brightest of 

 all the stars, to be only 0"'230 ; it is consequently more distant 

 than 61 Cygni, though the latter is but of the 6th magnitude. 



M. Argelander has calculated that the apparent magnitude of 

 the stars depends upon their distance. Supposing them all to 

 be of the same size, the smallest visible in Sir William Herschel's 

 20 feet reflecting telescope, namely those of the 17th magnitude, 

 would be 228 times farther off than those of the first magnitude ; 

 and M. Peters of Polkova from the annual parallax of thirty-five, 

 seven of which are now very accurately determined, has ascer- 

 tained the distance of the nearest of them to be such, that light 

 flying at the rate of 95 millions of miles in a second would take 

 15 years and a half to come from them to the earth, and that a 

 star of the 17th magnitude might be extinguished for 3541 years 

 before we should be aware of it. (N. 231.) 



