150 ON THE ORIGIN OF THE PLANETARY SYSTEM. 



that double stars moved about each other in elliptical 

 paths, and that therefore the same law of gravitation 

 must hold for them as for our planetary system. The 

 distance of some of them could be calculated. The 

 nearest of them, a, in the constellation of the Centaur, 

 is 270,000 times further from the sun than the 

 earth. Light, which has a velocity of 186,000 miles 

 a second, which traverses the distance from the sun to 

 the earth in eight minutes, would take four years to 

 travel from a Centauri to us. The more delicate 

 methods of modern astronomy have made it possible 

 to determine distances which light would take thirty- 

 five years to traverre ; as, for instance, the Pole Star ; 

 but the law of gravitation is seen to hold, ruling the 

 motion of the double stars, at distances in the heavens, 

 which all the means we possess have hitherto utterly 

 failed to measure. 



The knowledge of the law of gravitation has here 

 also led to the discovery of new bodies, as in the 

 case of Neptune. Peters of Altona found, confirming 

 therein a conjecture of Bessel, that Sirius, the most 

 brilliant of the fixed stars, moves in an elliptical path 

 about an invisible centre. This must have been due 

 to an unseen companion, and when the excellent and 

 powerful telescope of the University of Cambridge, in 

 the United States, had been set up, this was discovered. 

 It is not quite dark, but its light is so feeble that it 



