Parsec (pc)

AstronomicalCurrent unit

About 3.26 light-years, defined as 648,000/π astronomical units. The standard unit for distances to stars.

1 parsec = 3.085677581×10¹⁶ meters (approximately).

About the parsec

British astronomer Herbert Hall Turner coined the word in 1913 as a contraction of the distance at which a parallax equals one arcsecond. Rivals were Frank Dyson's astron and Carl Charlier's siriometer. The idea rests on stellar parallax, first measured successfully by Friedrich Bessel in 1838 for 61 Cygni, which he put near 3.5 parsecs. With the angle in arcseconds, the distance in parsecs is simply its reciprocal.

Since 2015 the IAU has defined the parsec as exactly 648,000 divided by pi astronomical units, which with the 2012 exact au gives about 30,856,775,814,913,673 meters, or 3.0857 times ten to the sixteenth power. That is roughly 3.26 light-years. Proxima Centauri lies about 1.3 parsecs away. Professional astronomers still prefer the parsec to the light-year, and ESA's Gaia satellite, launched in 2013, measures parallaxes to microarcsecond precision.

Sources: Wikipedia: Parsec; NIST SP 811 Appendix B.8: factors for units outside the SI

Parsec conversion chart

1 parsec expressed in every other length unit:

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