396 CATALOGUES OF DOUBLE STARS. SECT. XXXVI. 



eoveries, and with whom the idea of their combination in binary 

 and multiple systems originated ; and that important fact he 

 established by the discovery of a revolving motion in 50 or 60, 

 and by the determination of the revolution of one star about the 

 other of Castor or a Geminorum, the largest and finest double 

 star -in the northern hemisphere. He even assigned the approxi- 

 mate periodic times of this and of several other binary systems. 

 More than 100 stars are now known to be stellar systems. 

 The positions of many hundreds were measured by Sir John 

 Herschel and Sir James South ; and the catalogue of the double 

 stars in the northern hemisphere, which have been micrometrically 

 measured, has been increased to more than 6000 by MM. Bessel, 

 Struve, and British astronomers. 



Extensive catalogues of double stars in the southern hemi- 

 sphere have been published by the astronomers in our colonial 

 establishments. To these Sir John Herschel added 1081 during 

 his residence at the Cape of Good Hope : the angles of posi- 

 tion and distances of the stars from one another he measured, 

 and found that many of them have very rapid orbital motions. 

 The elliptical elements of the orbits and periodic times of fifteen 

 have been determined by the most eminent astronomers with 

 wonderful accuracy, considering the enormous distances and the 

 extreme delicacy and difficulty of the subject. M. Savary has 

 the merit of having first determined the elements of the orbit of 

 a double star from 'observation. The difficulty of doing so is 

 great, because the nearest fixed star is 211,000 times farther 

 from the sun than the earth is, and the orbit itself is only visible 

 with the best telescopes ; consequently a very small error in 

 observation occasions an enormous error in the determination 

 of quantities at that distance. 



In observing the relative position of the stars of a binary 

 system, the distance between them, and also the angle of posi- 

 tion, that is, the angle which the meridian, or a parallel to the 

 equator, makes with the line joining the two stars, are measured. 

 The different values of the angle of position show whether the re- 

 volving star moves from east to west, or the contrary ; whether 

 the motion be uniform or variable, and at what points it is great- 

 est or least. The measures of the distances show whether the 

 two stars approach or recede from one another. From these the 

 form and nature of the orbit are determined. Were observations 



