428 MASSES OF THE STARS. [SECT, xxxvn. 



and a Crucis. It comprises the bright stars of the constel- 

 lations Orion, Canis Major, the Cross, Centaur, Lupus, and 

 Scorpio. The axis of the zone is inclined at an angle of 20 

 to the medial line, or circle, passing through the centre of the 

 Milky Way. 



M. Aglander has calculated that the apparent magnitude 

 of the stars depends upon their distance. Supposing them 

 to be all of the same size, the smallest visible in Sir William 

 Herschel's 20-feet reflecting telescope, namely, those of the 

 seventeenth magnitude, would be 228 times farther off than 

 those of the first magnitude ; and Mr. Peters, of Polkora, 

 from the annual parallax of thirty-five, seven of which are 

 now very accurately determined, has ascertained the dis- 

 tance of the nearest of them to be such, that light, flying at 

 the rate of 95 millions of miles in a second, would take 

 15 years and a half to come from them to the earth ; and 

 that the smallest seen in the 20-feet reflector might be ex- 

 tinguished for 3541 years before we should be aware of it. 



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 accu- 

 racy. In some cases, however, the binary 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 round 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 



