Parallax
Stellar distance
Solar parallax
Stellar parallax
The apparent difference in the position of a celestial object as seen by an observer from two widely separated locations. The parallax of an object can be used to derive its distance. The relationship between the parallax angle p (measured in seconds of arc) and the distance d (measured in astronomical units) is given by d = 206,264 / p. For a parallax angle p = 1'', the distance to the star would correspond to 206,264 AU. By convention, the distance unit parsec is defined to be equivalent to 206,264 AU. Therefore, the parallax relation takes the much simpler form: d (in pc) = 1/p (in seconds of arc). The first star whose parallax was measured was 61 Cygni (Bessel, 1838).