6 POPULAR SCIENCE MONTHLY. 



class which are near together must be very few in number; indeed, there 

 are probably none among the close double stars whose brightest com- 

 ponent can be seen by the naked eye. 



The time of revolution of the binary systems is so long that there 

 are only about fifty cases in which it has yet been determined with 

 any certainty. Leaving out the 'spectroscopic binaries/ to be hereafter 

 described, the shortest period yet found is eleven years. In only a 

 small minority of cases is the period less than a century. In the large 

 majority either no motion at all has yet been detected, or it is so slow 

 as to indicate that the period must be several centuries, perhaps several 

 thousand years. 



There is a great difficulty in determining the period with precision 

 until the stars have been observed through nearly a revolution, owing 

 to the number of elements, seven in all, that fix the orbit, and the 

 difficulty of making the measures of position angle and distance with 

 precision. It thus happens that many of the orbits of binary systems 

 which have been computed and published have no sound basis. Two 

 cases in point may be mentioned. 



The first magnitude star Castor, or a Geminorum, can be seen 

 to be double with quite a small telescope. The components are in rela- 

 tive motion. Owing to the interesting character of the pair it has 

 been well observed, and a number of orbits have been computed. The 

 periodic times found by the components have a wide range. The fact 

 is, nothing is known of the period except that it is to be measured by 

 centuries, perhaps by thousands of years. 



The history of 61 Cygni, a star ever memorable from being the 

 first of which the parallax was determined, is quite similar. Al- 

 though, since accurate observations have been made on it the com- 

 ponents have moved through an apparent angle of 30°, the observa- 

 tions barely suffice to show a very slight curvature in the path which 

 the two bodies are describing round each other. Whether the period 

 is to be measured by centuries or by thousands of years cannot be de- 

 termined for many years to come. 



In his work on the 'Evolution of the Stellar Systems,' Prof. T. J. J. 

 See has investigated the orbits of forty double stars having the shortest 

 periods. There are twenty-eight periods of less than one hundred years 



In considering the orbits of binary systems we must distinguish 

 between the actual and the apparent orbit. The former is the orbit as 

 it would appear to an observer looking at it from a direction perpen- 

 dicular to its plane. This orbit, like that of a planet or comet mov- 

 ing round the sun, is an ellipse, having the principal star in its focus. 

 The point nearest the latter is called the periastron, or pericenter, and 

 corresponds to the perihelion of a planetary orbit. The point most 

 distant from the principal star is the apocenter. It is opposite the 



