Definition:Astronomical Distance Units/Light Year

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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


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.