A parsec (symbol: pc) is a unit of length used in astronomy, defined as the distance at which one astronomical unit (AU) subtends an angle of one arcsecond. It is derived from the method of stellar parallax, by which astronomers estimate the distances to nearby stars by measuring the apparent shift in a star's position against the background of more distant objects as Earth orbits the Sun. One parsec equals approximately 3.086 × 10¹³ kilometers, or about 3.26 light-years. The kiloparsec (kpc), megaparsec (Mpc), and gigaparsec (Gpc) are commonly used for galactic and cosmological distances.

The parsec is formally defined in terms of the astronomical unit. If a star exhibits a parallax angle of exactly one arcsecond when observed from opposite ends of a one-AU baseline (i.e., from two positions in Earth's orbit separated by one AU), that star lies at a distance of one parsec. Because no star other than the Sun is close enough to show a parallax of a full arcsecond, all stellar distances measured in parsecs are greater than one. The nearest star system, Alpha Centauri, lies at approximately 1.34 pc.

The term “parsec” is generally credited to the astronomer Herbert Hall Turner, who coined it around 1913. It was in informal use by the early twentieth century and was formalized as a standard unit of astronomical measurement. The International Astronomical Union (IAU) adopted the parsec as part of the standard system of astronomical units. In 2012, the IAU redefined the astronomical unit as exactly 149,597,870,700 meters, which fixed the parsec at approximately 3.085677581 × 10¹³ km.

The parsec is the preferred distance unit among professional astronomers for most extragalactic and stellar contexts, in part because parallax measurements yield distances directly in parsecs without conversion. Ground-based parallax measurements are reliable to roughly 100 pc; space-based astrometry missions have extended reliable parallax distances considerably further. The Hipparcos mission (1989-1993) measured parallaxes for over 100,000 stars to milliarcsecond precision. Its successor, the Gaia mission (2013-), has extended precise parallax measurements to distances of several kiloparsecs for hundreds of millions of stars.

For cosmological distances, megaparsecs and gigaparsecs are standard. The Hubble constant, which describes the rate of expansion of the universe, is conventionally expressed in units of kilometers per second per megaparsec (km/s/Mpc).

The definition, derivation, and value of the parsec reflect scientific consensus in astronomy and metrology. There is no substantive scientific dispute regarding the unit itself or its relationship to the astronomical unit and arcsecond geometry.

• Stellar Parallax - the observational method from which the parsec is derived
• Astronomical Unit - the baseline unit on which the parsec definition depends
• Light-Year - an alternative distance unit commonly used in popular science
• Hubble Constant - a cosmological quantity conventionally expressed per megaparsec
• Gaia Mission - ESA astrometry mission extending parallax measurements to large distances
• Stellar Distance Measurement - broader overview of distance ladder methods

1. International Astronomical Union, IAU 2012 Resolution B2, defining the astronomical unit as exactly 149,597,870,700 m.
2. Perryman, M. A. C., et al., “The Hipparcos Catalogue,” Astronomy & Astrophysics 323 (1997): L49-L52.
3. Gaia Collaboration, Brown, A. G. A., et al., “Gaia Early Data Release 3: Summary of the contents and survey properties,” Astronomy & Astrophysics 649 (2021): A1.
4. Cox, A. N., ed., Allen's Astrophysical Quantities, 4th ed. (New York: Springer, 2000), 12-13.