- published: 09 Oct 2014
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A light-year (or light year, abbreviation: ly) is a unit of length used informally to express astronomical distances. It is well over 9 trillion kilometres (or about 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. 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 non-specialist and popular science publications. The unit usually used in professional astrometry is the parsec (symbol: pc, approximately 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 (7008299792458000000♠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 internationally recognized abbreviation for light-year is "l.y.").
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