264 COSMOS. 



laxes. If, for instance, the plane of the orbit which the secon- 

 dary star describes around the central body is not at right angles 

 to the line of vision from the earth to the double star, but 

 coincides nearly with this line of vision itself, then the secon- 

 dary star in its orbit will likewise appear to describe nearly a 

 straight line, and the points in that portion of its orbit which 

 is turned towards the earth will all be nearer to the observer 

 than the corresponding points of the second half, which is 

 turned away from the earth. Such a division into two halves 

 produces not a real but an apparent unequal velocity, with 

 which the satellite in its orbit recedes from, or approaches, the 

 observer. If the semi-diameter of this orbit were so great 

 that light would require several days or weeks to traverse 

 it, then the time of the half revolution through its more 

 remote side will prove to be longer than the time in the 

 side turned towards the observer. The sum of the two un- 

 equal times will always be equal to the true periodic time ; 

 for the inequalities caused by the velocity of light reciprocally 

 destroy each other. From these relations of duration, it is 

 possible, according to Savary's ingenious method of changing 

 days and parts of Jays into a standard of length, (on the as- 

 sumption that light traverses 14356 millions of geographical 

 miles in twenty- four hours), to arrive at the absolute mag- 

 nitude of a semi-diameter of the earth's orbit; and the distance 

 of the central body and its parallax may be then deduced 

 from a simple determination of the angle under which the 

 radius appears to the observer. 2 * 



In the same way that the determination of the parallaxes 1 

 instructs us as to the distances of a small number of the fixed 

 stars, and as to the place which is to be assigned to them in 

 the regions of space, so the knowledge of the measure and 



38 Savary, in the Connaissance des Temps pour 1830, pp. 5G 

 -69, and pp. 163-171; and Struve, ibid. p. clxiv. 



