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 9,460,730,472,580.8 km (Scientific notation: 9.4607304725808 × 1012 km), which is approximately 5.88 trillion mi. As defined by the International Astronomical Union (IAU), a light-year is the distance that light travels in a vacuum in one Julian year (365.25 days).[2] Because it includes the word "year", the term is sometimes 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]