DOUBLE STARS. 211 



Orion, "\ve have a combination of six probably a system sub- 

 ject to peculiar physical attraction, since the five smaller 

 stars (6-3m: ; 7m.; 8m.; 113m. ; and 12m.) follow the prop- 

 er motion of the principal star, 4 - 7m. No change in their 

 relative positions has yet been observed.* In the ternary 

 combinations of f Librae and Cancri, the periodical move- 

 ment of the two companions has been recognized with great 

 certainty. The latter system consists of three stars of the 

 third magnitude, differing very little in brightness, and the 

 nearer companion appears to have a motion ten times more 

 rapid than the remoter one. 



The number of the double stars, the elements of whose 

 orbits it has been found possible to determine, is at present 

 stated at from fourteen to sixteen. t Of these, Herculis 

 has twice completed its orbit since the epoch of its first dis- 

 covery, and during this period has twice (1802 and 1831) 

 presented the phenomenon of the apparent occultation of one 

 fixed star by another. For the earliest measurements of 

 the orbits of double stars, we are indebted to the industry of 

 Savary ( Ursae Maj.), Encke (70 Ophiuchi), and Sir John 

 Herschel. These have been subsequently followed by Bes- 

 sel, Struve, Madler, Hind, Smyth, and Captain Jacob. Sa- 

 vary's and Encke's methods require four complete observa- 

 tions, taken at sufficient intervals from each other. The 

 shortest periods of revolution are thirty, forty-two, fifty-eight, 

 and seventy-seven years ; consequently, intermediate be- 

 tween the periods of Saturn and Uranus ; the longest that 

 have been determined with any degree of certainty exceed 

 five hundred years, that is to say, are nearly equal to three 

 times the period of Le Terrier's Neptune. The eccentricity 

 of the elliptical orbits of the double stars, according to the 

 investigations hitherto made, is extremely considerable, re- 

 sembling that of comets, increasing from 0-62 (o* Coronas) up 

 to 095 (a Centauri). The least eccentric interior comet 

 that of Faye has an eccentricity of 55, or less than that 

 of the orbits of the two double stars just mentioned. Ac- 

 cording to Madler's and Hind's calculations, 77 Coronas and 

 Castor exhibit much less eccentricity, which in the former is 

 0-29, and in the latter 022 or 0-24. In these double stars the 

 two suns describe ellipses which come very near to those of 



* Madler, Astr., s. 517. Sir John Herschel, Ovtl., p. 568. 



t Compare Madler, Untersuch. uber die Fixitcm-Systeme, th. i., s. 

 225-275 ; th. ii., s. 235-240 ; and his Astr., s. 541 Sir John Herschel, 

 OutL, p. 573. 



