372 DISTANCE OF BINARY SYSTEMS. SECT. XXXVIL 



quently it is at a greater distance than 61 Cygni : that 

 of a Centauri amounts to a second of space, so that it is 

 nearer the earth than any star that is known : whereas 

 Mr. Airy has found that the parallax of a Lyra? is al- 

 together inappreciable ; and as this is generally the case 

 with the fixed stars, we may conclude that their dis- 

 tances are beyond the hope of mensuration. 



All the ordinary methods fail when the distances are 

 so enormous. An angle even of two or three seconds, 

 viewed in the focus of our largest telescopes, does not 

 equal the thickness of a spider's thread, which makes it 

 impossible to measure such minute quantities with any 

 degree of accuracy. In some cases, however, the bi- 

 nary systems of stars furnish a method of estimating an 

 angle of even the tenth of a second, which is thirty 

 times more accurate than by any other means. From 

 them the actual distances of some of the more remote 

 stars will ultimately be known. 



Suppose that one star revolves about another in an 

 orbit which is so obliquely seen from the earth as to 

 look like an ellipse in a horizontal position, then it is 

 clear that one half of the orbit will be nearer to us than 

 the other half. Now, in consequence of the time which 

 light takes to travel, we always see the satellite star in 

 a place which it has already left. Hence when that 

 star sets out from the point of its orbit which is nearest 

 to us, its light will take more and more time to come to 

 us in proportion as the star moves round to the most 

 distant point in its orbit. On that account the star will 

 appear to us to take more time in moving through that 

 half of its orbit than it really does. Exactly the con- 

 trary takes place in the other half: for the light will 

 take less and less time to arrive at the earth in propor- 

 tion as the star approaches nearer to us, and therefore 

 it will seem to move through this half of its orbit in less 

 time than it really does. This circumstance furnishes 

 the means of finding the absolute breadth of the orbit in 

 miles, and from that the true distance of the star from 

 the earth. For, since the greatest and least distances 

 of the satellite star from the earth differ by the breadth 

 of its orbit, the time which the star takes to move from 

 the nearest to the remotest point of its orbit is greater than 



