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Concept# Light-year

Summary

A light-year, alternatively spelled light year, is a unit of length used to express astronomical distances and is equivalent to about 9.46 trillion kilometers (9.46e12km), or 5.88 trillion miles (5.88e12mi). 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). Because it includes the word "year", the term 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 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 (au) subtends an angle of one second of arc.
As defined by the International Astronomical Union (IAU), the light-year is the product of the Julian year (365.25 days, as opposed to the 365.2425-day Gregorian year or the 365.24219-day Tropical year that both approximate) and the speed of light (299792458m/s). Both of 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 The IAU-recognized abbreviation for light-year is "ly", although other standards like ISO 80000:2006 (now superseded) have used "l.y." and localized abbreviations are frequent, such as "al" in French (from année-lumière), Spanish (from año luz), Italian (from anno luce), "Lj" in German (from Lichtjahr), etc.
{|
|-
|rowspan=6 valign=top|1 light-year
|= 9460730472580800 metres (exactly)
|-
|≈ 9.461 petametres
|-
|≈ 9.461 trillion kilometres (5.879 trillion miles)
|-
|≈ 63241.077 astronomical units
|-
|≈ 0.306601 parsecs
|}
Before 1984, the tropical year (not the Julian year) and a measured (not defined) speed of light were included in the IAU (1964) System of Astronomical Constants, used from 1968 to 1983. The product of Simon Newcomb's J1900.

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