398 CEBITS AND PERIODIC TIMES. SECT. XXXVI. 



present century has enabled Sir John to determine the form and 

 position of the elliptical orbit of the revolving star with extraor- 

 dinary truth by the preceding method. According to his calcu- 

 lation, it came to its perihelion on the 18th of August of the year 

 1834. Its previous velocity was so great that the revolving star 

 described an angle of 68 in one year. By the laws of elliptical 

 motion its angular velocity must diminish till it arrives at its 

 aphelion. The accuracy with which the motions of the binary 

 systems are measured, and the skill employed in the deduction of 

 the elliptical elements, are now so great, that the periodic time of 

 y Virginia, determined by Sir John Herschel and Admiral Smyth 

 from their respective observatories, combined with those of Sir 

 William Herschel, only differ by two years, Sir John having 

 obtained a period of 182 years, Admiral Smyth that of 180. By 

 the aid of more numerous observations Mr. Fletcher has found 

 that the true period is 184*53 years, and that the revolving star 

 passed its perihelion in 1837. It is by such successive steps that 

 astronomy is brought to perfection (N. 232). 



Some of the double stars have very long periods, such as a 

 Coronse, where the revolving star takes 737 years nearly to accom- 

 plish a circuit. Others again have very short periods, as 77 Corona?, 

 Cancri, and Ursaj Majoris, whose periodic times are 42*500, 

 58*91, and 58*26 years respectively : therefore each of these has 

 performed more than one entire revolution since their motions 

 were observed. Herculis, whose periodic time is only about 30f 

 years, has accomplished two complete circuits, the lesser star 

 having been eclipsed by the greater each time. The first of these 

 two truly wonderful events, of one sun eclipsing another sun, was 

 seen by Sir William Herschel in 1782. 



The orbits and periodic times of so many of these binary systems 

 having been determined proves beyond a doubt that sun revolves 

 about sun in the starry firmament by the same law of gravitation 

 that makes the earth and planets revolve about the sun (N. 232). 



Since the parallax of 61 Cygni and that of a Centauri have 

 been determined, Sir John Herschel has made the following ap- 

 proximation to the dimensions of their orbits and masses. The 

 distance between the two stars of 61 Cygni, that is the radius 

 vector of the revolving star, has hardly varied from 15"'5 ever 

 since the earliest observations ; while in that time the star has 

 moved through 50 ; it is evident therefore that the orbit must be 



