The light-year, alternatively spelled lightyear, is a unit of length used to express astronomical distances and is equivalent to about 9.46 trillion kilometers (9.46×1012 km) or 5.88 trillion miles (5.88×1012 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). Because it includes the word "year", the term light-year is sometimes misinterpreted as a unit of time.
|Unit system||astronomy units|
|1 ly in ...||... is equal to ...|
|metric (SI) units|| 9.4607×1015 m|
|imperial & US units||5.8786×1012 mi|
|astronomical units|| 63241 au|
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. The unit most commonly used in professional astronomy is the parsec (symbol: pc, about 3.26 light-years) which derives from astrometry: it is the distance at which one astronomical unit subtends an angle of one second of arc.