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The parsec (symbol: pc) is the distance at which the parallax relative to the earth's orbit of an object is equal to one arc second, or alternatively, the distance at which an angle of one arc second subtends a line one astronomical unit (AU). As the length of the astronomical unit is approximately 1.4959787×1011m, and the inverse tangent of one arc second is approximately 206264.8, the length of a parsec is approximately 3.085678×1016m.

Due to the size of the parsec, it is primarily useful in measuring distances between stars. The distance to a star in parsecs is equal to the reciprocal of the annual parallax of the star in arcseconds. The nearest star, Proxima Centauri, has a parallax of about 0.7687 arc seconds[1], and is thus 1.301 parsecs from the sun.

Value in SI units

1 parsec = 3.085678×1016m

Comparison to other units

Distances in parsecs

  • the nearest star, Proxima Centauri, has a distance of about 1.30 parsecs
  • the first star whose parallax was measured, 61 Cygni, is at a distance of about 3.48 parsecs
  • the distance to the Andromeda galaxy (M31) is about 800,000 parsecs, or 0.8 megaparsec.