CYGNUS ("the swan"), in astronomy, a constellation of the northern hemisphere fabled by the Greeks to be the swan in the form of which Zeus seduced Leda. In this constellation 13 Cygni is a fine coloured double star, consisting of a yellow star, 3 mag., and a blue star, 51 mag. A wide double star 61 Cygni was, on account of its large proper motion of 5" per annum, singled out by Bessel for the first successful attempt to measure stellar par allax. The parallax is about 0.31".