120 DISTANCES AXD MAGNITUDES 



seconds, and that of Procyon five and seven-tenths. In the same man- 

 ner Calandrelli deduced a parallax of four seconds for Vega ; but these 

 results have all been shown to be incorrect, and it was not till within 

 the last few years that the distance of any star was determined with 

 any degree of certainty. 



Dr. Long appears to have been the first who suggested the method 

 of finding the parallax by micrometrical measurements of the angular 

 distance of two stars in close proximity to each other, or nearly in the 

 same line of vision ; for it is evident that if either star has any sensible 

 parallax whatever, their apparent distance from each other must vary 

 with the earth's motion in its orbit. This suggestion led Sir William 

 Herschel to commence his observations on double stars, in order, if 

 possible, to discover their parallax. He was soon diverted, however, 

 from his original object by the discovery of changes in their relative 

 positions which could not be referred to the orbital motion of the earth. 

 Hence he concluded that the apparent proximity of these stars was not 

 in consequence of their being accidentally situated nearly in the same 

 line of vision ; but that they were really so near as to be within the 

 sphere of each other's attraction, and that such a physical connection 

 existed between them, as caused them mutually to revolve around each 

 other, or rather, each around their common centre of gravity. His sub- 

 sequent observations, together with those of other astronomers, have 

 confirmed this hypothesis. This eminent astronomer continued his ob- 

 servations with characteristic perseverance, until by the aid of his pow- 

 erful telescope he had discovered 500 binary systems, a list and descrip- 

 tion of which he presented to the Royal Society. By the researches of 

 later observers, particularly those of Sir John Herschel, and Professor 

 Struve, this list has been extended to several thousand. 



In November, 1835, Struve commenced his observations on Vega, 

 or Jllplia LyrcB. He selected this star because another smaller one is 

 situated at an angular distance of only forty-two seconds from it. — 

 These are found not to be connected by any common motion \ they 

 must, therefore, be at diflerent distances from the earth, and their angular 

 distance must vary with the earth's annual motion. Having continued 

 his observations for several years with the greatest accuracy and perse- 

 verance, he found the parallax of Mpha Lyrcc to be 0."2613. Hence 

 its distance must be 771400 times greater than that of the sun from the 

 earth, or, 73,383,000,000,000 miles; a distance so great that light, 

 which travels at the rate of 192,000 miles in a second, would not 

 reach us from that star in less than 12 years. So that, were Vega now 

 stricken out of existence, to us it would appear to occupy its place 



