Fr.: parallaxe stellaire
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