Cygnus (Lat., a swan), a northern constellation, made memorable by containing the first star whose distance from the sun was approximately determined. In 1717 the astronomer Halley, from a comparison of the positions of certain fixed stars with their positions as recorded by Ptolemy from observations made by Hipparchus in 130 B. C, concluded that these stars had a proper motion of their own in space. Observations made by astronomers since that time have established this fact in regard to several stars, among which is the star 61 Cygni. This is a double star of the sixth magnitude, and comparatively inconspicuous; but it is so situated relatively to the other stars of the constellation that a new and refined method of observation and calculation could be brought to bear upon it. In 1837-'8 Bessel of the Konigsberg observatory succeeded in ascertaining the parallax of this star, which he declared to be equal to 0.348", a result since confirmed by other astronomers. The distance of this star from the earth is calculated to be equal to 56,239,807,881,401 miles.

It would require a period of between nine and ten years for light to traverse this interval.