Unit of measure used by astronomers to express distances to stars and galaxies. It is the distance at which the radius of Earth's orbit would subtend an angle of one second of arc, so an object one parsec away would have a parallax of one second. An object's distance in parsecs is the reciprocal of its parallax in seconds of arc. For example, Alpha Centauri, with a parallax of 0.76 second, is 1.33 parsecs from the Sun and Earth. One parsec equals 3.26 light-years, or 19.2 trillion mi (30.9 trillion km).