CHAPTERS ON THE STARS. 7 



pericenter and corresponds to the aphelion of a planetary orhit. The 

 law of motion is here the same as in the case of a body of the solar 

 system; the radius vector, joining the two bodies, sweeps over equal areas 

 in equal times. The apparent orbit is the orbit as it appears to us. It 

 differs from the actual orbit because we see it from a more or less 

 oblique direction. In some cases the plane of the orbit passes near our 

 system. Then to us the orbit will appear as a straight line and the 

 small star will seem to swing from one side of the large one to the other 

 like a pendulum, though the actual orbit may differ little from a circle. 

 In some cases there may be two pericenters and two apocenters to the 

 apparent orbit. This will be the case when a nearly circular orbit is 

 seen at a considerable obliquity. 



It is a remarkable and interesting fact that the law of areas holds 

 good in the apparent as in the actual orbit. This is because all parts 

 of the planeof the orbit are seen at the same angle, so that the obliquity 

 of vision diminishes all the equal areas in the same proportion and thus 

 leaves them equal. 



The two most interesting binary systems are those of Sirius and 

 Procyon. In the case of each the existence and orbit of the com- 

 panion were inferred from the motions of the principal star before the 

 companion had been seen. Before the middle of the century it was 

 found that Sirius did not move with the uniform proper motion which 

 characterizes the stars in general; and the inequality of its motion was 

 attributed to the attraction of an unseen satellite. Later Auwers, from 

 an exhaustive investigation of all the observations of the star, placed 

 the inequality beyond doubt and determined the elements of the orbit 

 of the otherwise unknown satellite. Before his final work was pub- 

 lished the satellite was discovered by Alvan G. Clark, of Cambridgeport, 

 Mass., son and successor of the first and greatest American maker of 

 telescopes. Additional interest was imparted to the discovery by the 

 fact that it was made in testing a newly constructed telescope, the 

 largest refractor that had been made up to that time. The discoverer 

 was, at the time, unaware of the work of Peters and Auwers demon- 

 strating the existence of the satellite. The latter was, however, in the 

 direction predicted by Auwers, and a few years of observation showed 

 that it was moving in fairly close accordance with the prediction. 



The orbit as seen from the earth is very eccentric, the greatest dis- 

 tance of the satellite from the star being about ten seconds, the least 

 less than three seconds. Owing to the brilliant light of Sirius the satel- 

 lite is quite invisible, even in the most powerful telescopes, when near- 

 est its primary. This was the case in the years 1890-92 and will again 

 be the case about 1940, when another revolution will be completed. 



The history of Procyon is remarkably similar. An inequality of its 

 motion was suspected, but not proved, by Peters. Auwers showed from 



