Astronomy Answers
From the Astronomical Dictionary


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the parsec

'par‧sec, plural parsecs; parallax + second

The parsec is approximately the distance at which the radius of the orbit of the Earth around the Sun covers an angle of 1 second of arc. A star at a distance of 1 parsec shows an annual parallax of 1 second of arc. A parsec corresponds to 648,000/π = about 206,264.8062 AU or 30,856,780,000,000 km or 19,173,560,000,000 miles or 3.085678 × 1013 m or 30.085678 Pm or 3.2616 lightyears. A parsec may be abbreviated to pc. In astronomy, distances occur that are large even when measured in parsecs. Therefore standard abbreviations with SI-prefixes are used, too, such as kpc for 1000 pc, Mpc for 1000 kpc, and even Gpc for 1000 Mpc.