Definition:Astronomical Distance Units/Light Year
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.
Sources
- 1964: Milton Abramowitz and Irene A. Stegun: Handbook of Mathematical Functions ... (previous) ... (next): $2$. Physical Constants and Conversion Factors: Table $2.4$ Miscellaneous Conversion Factors
- 1998: David Nelson: The Penguin Dictionary of Mathematics (2nd ed.) ... (previous) ... (next): light year
- 2008: David Nelson: The Penguin Dictionary of Mathematics (4th ed.) ... (previous) ... (next): light year
- 2014: Christopher Clapham and James Nicholson: The Concise Oxford Dictionary of Mathematics (5th ed.) ... (previous) ... (next): light year