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| Light-year | |
|---|---|
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| General information | |
| Unit system | astronomy units |
| Unit of | length |
| Symbol | ly[2] |
| Conversions | |
| 1 ly[2] in ... | ... is equal to ... |
| metric (SI) units | |
| imperial and US units |
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| astronomical units | |
A light-year, alternatively spelled light year (ly or lyr[3]), is a unit of length used to express astronomical distances and is equal to exactly 9460730472580.8 km, which is approximately 9.46 trillion km or 5.88 trillion mi. 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).[2] Despite its inclusion of the word "year", the term should not be misinterpreted as a unit of time.[4]
The light-year is most often used when expressing distances to stars and other distances on a galactic scale, especially in non-specialist contexts and popular science publications.[4] The unit most commonly used in professional astronomy is the parsec (symbol: pc, about 3.26 light-years).[2]
- ^ "The Universe within 12.5 Light Years – The Nearest stars". www.atlasoftheuniverse.com. Retrieved 2 April 2022.
- ^ a b c International Astronomical Union, Measuring the Universe: The IAU and Astronomical Units, retrieved 10 November 2013
- ^ Mutel, R. L.; Aller, H. D.; Phillips, R. B. (1981). "Milliarcsecond structure of BL Lac during outburst". Nature. 294 (5838): 236–238. Bibcode:1981Natur.294..236M. doi:10.1038/294236a0. hdl:2027.42/62626.
- ^ a b Bruce McClure (31 July 2018). "How far is a light-year?". EarthSky. Retrieved 15 October 2019.
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