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Parsec
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API
stellar parallax motion from annual parallax
Unit information
Unit system
astronomical units
Unit of
length
Symbol
pc
Unit conversions
1 pc in...
is equal to...
SI units
3.0857×1016 m
imperial & US units
1.9174×1013 mi
other astronomical
2.0626×105 AU
units
3.26156 ly
A parsec (symbol: pc) is an astronomical unit of distance derived by the theoretical annual parallax (or heliocentric parallax) of one arc second, and is found as the inverse of that measured parallax. In astronomical terms, parallaxes are the apparent measured difference in the position of a star as seen from Earth and another hypothetical observer at the Sun. As the distance is the inverse of the parallax, the smaller the measured parallax the larger the celestial object's distance. One parsec equals about 3.26 light-years (30.9 trillion kilometres or 19.2 trillion miles). All known stars (other than the Sun) lie more than one parsec away, with Proxima Centauri showing the largest parallax of 0.7687 arcsec, making the distance 1.3009 parsecs (4.243 light years). Most of the visible stars in the nighttime sky lie within 500 parsecs of the Sun. The parsec was introduced to make quick calculations of astronomical distances without the need for more complicated conversions, i.e. knowing the true speed of light to calculate light years. Parsec is named from the abbreviation of the parallax of one arcsecond, and was likely first suggested by British astronomer Herbert Hall Turner in 1913.Usage of parsecs is preferred in astronomy and astrophysics, though popular science texts commonly use light-years, probably because it is much easier to understand the amount of time it takes light to travel from the source. Multiples of parsecs are commonly used for scales in the universe, including parsecs for the visible stars, kiloparsecs for galactic objects and megaparsecs for nearby and more distant galaxies.
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