parsec
The parsec is the distance at which one astronomical unit subtends an angle of one arcsecond, which since the IAU's 2015 resolution is defined as exactly 648,000/π astronomical units. That relation is definitional rather than measured, but the resulting value in metres is irrational, so the figure used here is a rounded 3.0857 × 10¹⁶ m. One parsec is about 3.26 light-years.
Watch out: The parsec is a distance, despite the famous line of dialogue that treats it as a time. It is also the unit professional astronomy actually uses, usually as the kiloparsec for galactic structure and the megaparsec for cosmology, while the light-year stays in popular writing.
| 1 pc | 3.0856776e+16 m |
The name is a contraction of "parallax second" and is attributed to Herbert Hall Turner, who proposed it in 1913. The unit exists because it makes the fundamental distance measurement trivial: a star with a measured annual parallax of one arcsecond is one parsec away, and one with a parallax of 0.1 arcsecond is ten parsecs away.
About the parsec
Everything about the parsec follows from one measurement technique. As the Earth orbits the Sun, a nearby star appears to shift slightly against the far background, tracing a tiny ellipse over the year. Half the angular width of that shift is the star's parallax, and distance is simply its reciprocal: \(d\,[\text{pc}] = 1/p\,[\text{arcsec}]\). No other distance unit turns a measurement into an answer by pressing the reciprocal key, which is exactly why astronomers kept it.
The angles involved are brutally small. Not one star has a parallax as large as one arcsecond, so no star is closer than one parsec: Proxima Centauri, the nearest, has a parallax of about 0.77 arcsec and sits at 1.30 pc. An arcsecond is 1/3600 of a degree, roughly the angle a coin subtends at 5 km, which is why parallax defeated astronomers for two centuries after Copernicus and its absence was taken as evidence against a moving Earth. Friedrich Bessel finally measured one in 1838. The Gaia spacecraft now measures parallaxes in the microarcsecond range, mapping distances across the galaxy in kiloparsecs, and cosmological distances are quoted in megaparsecs: the Hubble constant is given in km/s per megaparsec.