Page:A short history of astronomy(1898).djvu/433

§ 278] indication of nearness, and selected a star (61 Cygni) which was barely visible to the naked eye but was remarkable for its large proper motion (about 5" per annum); evidently if a star is moving at an assigned rate (in miles per hour) through space, the nearer to the observer it is the more rapid does its motion appear to be, so that apparent rapidity of motion,

like brightness, is a probable but by no means infallible indication of nearness. A modification of Galilei's differential method (chapter, § 129, and chapter , § 263) being adopted, the angular distance of 61 Cygni from two neighbouring stars, the faintness and immovability of which suggested their great distance in space, was measured at frequent intervals during a year. From the changes in these distances σ a, σ b (in fig. 85), the size of the small ellipse described by σ could be calculated. The result, announced at the end of 1838, was that the star had an annual parallax of about $1⁄3$" (chapter, § 161), i.e. that the star was at such distance that the greatest angular distance of the earth from the sun viewed from the star (the angle σ  in fig. 86, where  is the sun and  the earth) was this insignificant angle. The result was confirmed, with slight alterations, by a fresh investigation of Bessel's in 1839-40, but later work seems to shew that the parallax is a little less than $1⁄2$". With this latter estimate, the apparent size of the earth's path round the sun as seen from the star is the same as that of a halfpenny