Light-year
| Light-year | |
|---|---|
Map showing the stars that lie within 12.5 light-years of the Sun
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| Unit system | astronomy units |
| Unit of | length |
| Symbol | ly |
| 1 ly in ... | ... is equal to ... |
| metric (SI) units | 9.4607×1015 m |
| imperial & US units | 5.8786×1012 mi |
| astronomical units |
6.3241×104 au 0.3066 pc |
A light-year is a unit of length used to express astronomical distances. It is about 9 trillion kilometres or 6 trillion miles. As defined by the International Astronomical Union (IAU), a light-year is the distance that light travels in vacuum in one Julian year (365.25 days). Because it includes the word "year", the term light-year is sometimes misinterpreted as a unit of time.
The light-year is most often used when expressing distances to stars and other distances on a galactic scale, especially in nonspecialist and popular science publications. The unit usually used in professional astrometry is the parsec (symbol: pc, about 3.26 light-years; the distance at which one astronomical unit subtends an angle of one second of arc).
As defined by the IAU, the light-year is the product of the Julian year (365.25 days as opposed to the 365.2425-day Gregorian year) and the speed of light (299792458 m/s). Both these values are included in the IAU (1976) System of Astronomical Constants, used since 1984. From this, the following conversions can be derived. The IAU recognized abbreviation for light-year is ly, although other standards like ISO 80000 uses "l.y." and localized symbols are frequent, such as "al" in French (from année-lumière) and Spanish (from año luz), "Lj" in German (from Lichtjahr), etc.
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