# Parsec

Physical unit
Unit name Parsec
Unit symbol ${\ displaystyle \ mathrm {pc}}$
Physical quantity (s) length
Formula symbol ${\ displaystyle l}$
dimension ${\ displaystyle {\ mathsf {L}}}$
system Astronomical units of measure
In SI units ${\ displaystyle 3 {,} 0857 \ cdot 10 ^ {16} \, \ mathrm {m}}$
Named after par allax sec ond
Derived from Astronomical unit , arcsecond
Star parallax (not to scale)

The parsec (abbreviation from English parallax second ), unit symbol pc , partly translated as loan parallax second , is an astronomical measure of length . It is the distance from which the mean earth orbit radius (= 1 AU, astronomical unit ), i.e. the mean distance between the sun and the earth, appears at an angle of one arc second , and corresponds to about 3.26  light years or about 206,000 astronomical units or about 3.09 · 10 16  meters (30.9 trillion kilometers ).

The unit parsec has its origin in the distance measurement via star parallaxes . The parallax angle of the closest stars, based on the Earth's orbit radius, is just under one arc second. Stars that are further away have smaller parallaxes. The reciprocal of the star parallax in arc seconds is the distance in parsecs. In technical language, information in parsec is far more common than in light years. In particular, the kiloparsec (kpc) is used for galactic distances and the megaparsec (Mpc).

## calculation

If a star were 1 parsec away from the sun, it would move back and forth around its mean position by 1 arc second when viewed from the earth due to the movement of the earth around the sun against the background of distant objects. From the picture you can read that the ratio of 1 astronomical unit to 1 parsec corresponds to the radian measure of 1 arc second, 1 °3600  = 360 × 3600  rad ≈ 4.848 µrad. 1 parsec is thus 360 × 3600  ≈ 206265 times as large as 1 astronomical unit.

In 2012, the astronomical unit was defined by the IAU resolution B2 as exactly 149.5978707 million kilometers. Consequently, followed by the IAU Resolution B2, 2016, the 24th General in August 2015, the definition of 1 pc = 648,000 / π AE and thus approximately 3,085.677.581 × 10 16 meters.