192 SPECIAL RESULTS IN THE URANOLOGICAL PORTION 



double stars, the aberration of light may be used for deter- 

 mining the parallax. If the plane of the orbit, which the 

 secondary star describes round the central body, is not per- 

 pendicular to the line of sight from the earth to the double 

 star, but, on the contrary, nearly coincides with it, then the 

 course of the secondary star will appear to be in a right 

 line, and the points on the half of its orbit which is turned 

 towards the earth will all be nearer the observer, than the 

 corresponding points of the other half Which is turned from 

 the earth. Such a division into two halves produces, not 

 a really, but to the observer an apparently, unequal velocity 

 according as the smaller star is approaching or receding 

 from him. If, then, the semi-diameter of the orbit is so 

 large that light requires several days or weeks to traverse it, 

 then the time of the semi-revolution on the farther side will 

 be greater than on the side turned towards the observer. 

 The sum of the two unequal numbers which express the 

 duration of the two semi-revolutions, is still equal to the 

 true period of entire revolution, since the inequalities occa- 

 sioned by the cause referred to mutually destroy each other. 

 In Savary's ingenious method, by converting days and parts 

 of days into a standard of length (light traverses 14356 

 millions of geographical miles in 24 hours), it is possible 

 to deduce from these ratios of duration the absolute magni- 

 tude of the semi-diameter of the orbit, and by the simple 

 determination of the angle under which the semi-diameter 

 presents itself to the observer, the distance of the central 

 body and its parallax. ( 317 ) 



As the determination of the parallaxes informs us con- 

 cerning the distances of a small number of fixed stars anil 



