SECT. XXXVI. DOUBLE STARS. 399 



nearly circular. It is at right angles to the visual ray, and the 

 periodic time is 514 years. The parallax or radius of the earth's 

 orbit as seen from the star is 0"348, while the radius of the star's 

 orbit as seen from the earth is 15"'5 ; hence the radius of the 

 star's orbit is to that of the earth's orbit as 15"'5 to 0"*348, or 

 nearly as 45 to 1. So the orbit described by the two stars of 

 61 Cygni about one another greatly exceeds that which Neptune 

 describes about the sun. Since the mean distance of the 

 stars and their periodic time are given, the sum of the masses 

 of the two stars is computed to be 0*3529, that of the sun being 

 1. Thus our sun is not vastly greater nor vastly less than the 

 stars composing 61 Cygni, which is a small inconspicuous star to 

 the naked eye, not exceeding the 6th magnitude. 



Of all the double stars a Centauri is the most beautiful : it is 

 the brightest star in the southern hemisphere, equal, if not supe- 

 rior, to Arcturus in lustre. The distance between the two stars 

 has been decreasing at the rate of half a second annually since 

 the year 1822, while the angular motion has undergone very little 

 change, which shows that the plane of the orbit passes through 

 the earth like the orbits of 44 Bootes, and IT Serpentarii ; that is to 

 say, the edge of the orbit in these three stellar systems is pre- 

 sented to the earth, so that the revolving star seems to move in a 

 straight line, and to oscillate on each side of its primary. Were 

 this libration owing to parallax, it would be annual from the 

 revolution of the earth about the sun ; but as years elapse before 

 it amounts to a sensible quantity, it can only arise from a real 

 orbital motion seen obliquely. In this case five observations are 

 sufficient for the determination of the orbit, provided they be 

 exact ; but the quantities to be measured are so minute, that it is 

 only by a very long series of observations that accuracy can be 

 attained. In 1834 Captain Jacob determined the periodic time of 

 the revolving star of a Centauri to be 77 years, and the distance 

 between the two to be 17"'5 ; and since the decrease is half a 

 second annually, the distance or radius vector of the revolving star 

 was 12" -5 in the year 1822 ; and as Mr. Henderson had deter- 

 mined the parallax or radius of the earth's orbit as seen from the star 

 to be *913, it follows that the real semi-axis of the revolving star's 

 orbit is 13^ times greater than the semi-axis of the earth's orbit 

 as a minimum. The real dimensions of the ellipse therefore 

 cannot be so small as the orbit of Saturn, and may possibly 



