definition 
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).

