388 DISTANCES OF FIXED STARS. SECT. XXXVI. 



of parallax, an arc of 1" is assumed as the parallactic unit. Now 

 radius is to the sine of 1" as 206,265 is to 1 ; hence, a Centauri 

 is 206,265 times more distant from the sun than the sun is 

 from the earth. Light flying at the rate of 192,000 miles in a 

 second must take 3 years and 83 days to come to us from that 

 star. 



One or two tenths of a second becomes a very great error when 

 the maximum amount of parallax is only 1", and on that account, 

 with the exception of a Centauri, it has been found impracticable 

 to determine the annual changes in -the apparent motions of 

 single stars affected by precession, nutation, aberration, and the 

 variations of temperature of the instruments used in observing. 

 However, as two stars in juxtaposition are equally affected by all 

 of these, the difference in their motions is independent of them. 

 Of two. stars apparently in close approximation, one may be far 

 behind the other in space. They may seem near to one another 

 when viewed from the earth in one part of its orbit, but may 

 separate widely when seen from the earth in another position, just 

 as two terrestrial objects appear to be one when viewed in the 

 same straight line, but separate as the observer changes his posi- 

 tion. In this case the stars would not have real, but only apparent 

 motion. One of them would seem to oscillate annually to and 

 fro in a straight line on each side of the other, a motion that 

 could not be mistaken for that of a binary system where one star 

 describes an ellipse about the other ; or if the edge of the orbit 

 be turned towards the earth, where the oscillations require years 

 for their accomplishment. The only circumstances that can 

 affect the stars unequally, and which must be eliminated, are the 

 proper motion of the stars in space, and specific aberration, a very 

 minute quantity arising from peculiarities in the star's light. 

 This method of finding the distances of the fixed stars was pro- 

 posed by Galileo and attempted by Dr. Long without success. 

 Sir William Herschel afterwards applied it to some of the binary 

 groups ; and although he did not find the thing he sought for, it 

 led to the discovery of the orbital motions of the double stars. 



M. Struve was the first to apply this method, and that in a 

 very difficult case. He perceived that a very small star is close 

 to a Lyrse, and by a series of most accurate differential measure- 

 ments from 1835 to 1838 he found that a Lyrge has a parallax of 

 0"'261, which was afterwards corroborated by the observations of 



