# Definition:Astronomical Distance Units/Light Year

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## Definition

The light year is a derived unit of length.

It is derived as the distance travelled by light in a vacuum over the course of $1$ Julian year ($365.25$ days).

It is defined as being exactly $9 \, 460 \, 730 \, 472 \, 580 \, 800$ metres.

The light year is a standard unit of measurement used by astronomers when discussing distances between stars and galaxies, particularly in popular and non-scientific literature.

Serious astronomers use the term parsec instead.

### Conversion Factors

 $\ds$  $\ds 1$ light year $\ds$ $=$ $\ds 9 \, 460 \, 730 \, 472 \, 580 \, 800$ metres $\ds$ $\approx$ $\ds 5 \cdotp 8786 \times 10^{12}$ miles $\ds$ $\approx$ $\ds 632 \, 41 \cdotp 077$ astronomical units $\ds$ $\approx$ $\ds 0 \cdotp 3066$ parsec

### Symbol

There are a number of symbols which are in use for the light year:

$\mathrm {ly}$ (as approved by the International Astronomical Union)
$\mathrm {l.y.}$ (the ISO 80000 standard)

Language-specific versions can be seen in particular localities, for example:

$\mathrm {al}$ in French (from année-lumière), Spanish (from año luz) and Italian (from anno luce)
$\mathrm {Lj}$ in German (from Lichtjahr)

The $\LaTeX$ code for $\mathrm {ly}$ is \mathrm {ly} .

The $\LaTeX$ code for $\mathrm {l.y.}$ is \mathrm {l.y.} .

The $\LaTeX$ code for $\mathrm {Lj}$ is \mathrm {Lj} .

The $\LaTeX$ code for $\mathrm {al}$ is \mathrm {al} .

## Also known as

The term light year can also be seen expressed as light-year or lightyear.