Units of Distance in Astronomy

pquoteThe standard unit of length (or distance) is the metre (symbol: m).

In Astronomy we work with very large distances as compared to distances on Earth. Therefore we commonly use other units for distance in Astronomy.

  • Astronomical Unit (AU) - average distance Earth - Sun
  • Light year (ly) – the distance light travels in one year
  • Parsec (pc)

The Astronomical Unit is used to express distances across the Solar System.

1 astronomical unit (AU) = average distance from the Earth to the Sun;
1 AU = 149,597,870,691 m = about 1.5 x 1011 m.

The Light Year is commonly used to express distances to objects, e.g. to stars. Note that the light year is not a measure of time!

1 light year (ly) = distance that light travels in one year
1 ly = 9.461 x 1015 m = 9.461 x 1012 km
1 ly = 63,240 AU.

To get an idea about how much a light year is, remember that light travels at about 3x108 m/s; this is about 7½ times around the Earth in one second.

For relatively short distances we use e.g.:
Distance to the Moon is in average 384,402 km or 1.282 Light second;
Distance to the Sun is 1 AU or about 8.3 Light minutes.

This unit also indicates how long ago the light that we see departed from the source.

The Parsec is a unit used to express distances outside our Solar system to stars and galaxies, etc.

1 parsec = 1 pc = 206,265 AU = 3.086×1016 m = 3.262 ly

This unit originates from determining the distance to a star by measuring parallax. It is the distance for which a star has a parallax of one arc second when the Earth moves 1 AU. We discuss this below.


Example: the Sombrero galaxy (M104) is at a distance of 50 Mly or 50,000,000 ly. It is also at a distance of 15.3 Mpc or 15,300,000 pc.


Readers who are not familair with the powers of ten notation (e.g. 105, 10-7) or with prefixes (e.g. Gly, Mpc) are referred to the handouts Scientific Notation and Prefixes & Symbols in the Physics Study Notes on this website.