### Parsecs

**Abbreviation/Symbol:**

- pc

**Unit of:**

- Astronomical length/distance

**Worldwide use:**

Global

**Description:**

The parsec is a unit of length equivalent to around 20 trillion (20,000,000,000,000) miles, 31 trillion kilometres, or 206,264 times the distance from the earth to the sun.

A parsec is also equivalent to approximately 3.26 light years (the journey distance if you travelled at the speed of light for three years and three months).

**Definition:**

Astronomers used trigonometry to calculate the distance to stars long before the term parsec was coined, but the new unit made it easier to conceptualise unfathomable distances.

A parsec is the distance from the sun to an astronomical object which has a parallax angle of one arcsecond (1/3600 of a degree). The parallax angle is found by measuring the parallax motion (or apparent movement of a star relative to stable, more distant stars) when the star is observed from opposite sides of the Sun (an interval of six months on Earth). The parallax angle is obtained by halving the angular difference in measurements.

Once the parallax angle is established you can calculate the distance to a star using trigonometry, because we know Earth’s distance from the Sun. The distance from the Sun of a body with a parallax angle of 1 arcsecond was thus defined as a unit and, thanks to Turner, named the parsec.

With the parsec defined, deriving and describing huge distances became easy, since a distance in parsecs can be calculated as the reciprocal of the parallax angle in arcseconds (if the parallax angle is 1 arcsecond, the object is 1 pc from the sun. 0.5 arcseconds means the object is 2 pc distant).

**Origin:**

The term parsec was coined by British astronomer Herbert Hall Turner in 1913. A unit of distance useful in astronomy had been defined but was without a name, and the Astronomer Royal appealed for suggestions. Turner’s was accepted – parsec being derived from the definition of the unit as the distance from the sun to an astronomical object which has a parallax angle of one arcsecond.

**Common references:**

- Proxima Centauri – the nearest star to the Earth other than the Sun, is 1.29 parsecs away.
- The centre of the Milky Way is over 8kpc from the earth.

**Usage context:**

Astronomy – despite the enormous distance it describes, the parsec is a relatively small unit in astronomical terms. The megaparsec (Mpc) is commonly used to describe distances of one million parsecs.

**Component units:**

- None

**Multiples:**

- kiloparsec (kpc) – 1,000 pc
- megaparsec (Mpc) – 1,000,000 pc
- gigaparsec (Gpc) – 1,000,000,000 pc