The angle subtended at a star by a standard distance of one astronomical unit, the radius of the Earth's orbit. It is usually expressed in seconds of arc. The reciprocal of trigonometric parallax is the star's distance in parsecs. From Earth, accurate parallaxes can be obtained only out to about 50 l.y.; the Hipparcos satellite extended this to several hundred light years and future satellites such as Gaia will extend this even farther. The first star to have its parallax measured was *61 Cygni. See also annual parallax.
parallax: The apparent change in position of a star as seen from diametrically opposite points of the Earth's orbit
Subjects: Astronomy and Astrophysics.