Light-year

Apr 25, 2026 - 12:10
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Light-year

The right unit is meters

← Previous revision Revision as of 06:39, 25 April 2026
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A '''light-year''', alternatively spelled '''light year''' ('''ly''' or '''lyr'''{{cite journal | doi=10.1038/294236a0 | title=Milliarcsecond structure of BL Lac during outburst | date=1981 | last1=Mutel | first1=R. L. | last2=Aller | first2=H. D. | last3=Phillips | first3=R. B. | journal=Nature | volume=294 | issue=5838 | pages=236–238 | bibcode=1981Natur.294..236M | hdl=2027.42/62626 | hdl-access=free }}), is a [[unit of length]] used to express [[astronomical distance|astronomical distances]] and is equal to exactly {{val|9,460,730,472,580.8|u=km}}, which is approximately 9.46 trillion kilometres or 5.88 trillion miles. As defined by the [[International Astronomical Union]] (IAU), a light-year is the distance that [[Speed of light|light travels in vacuum]] in one [[Julian year (astronomy)|Julian year]] (365.25 days).{{citation | url = https://www.iau.org/public/themes/measuring/ | title = Measuring the Universe: The IAU and Astronomical Units | author = International Astronomical Union | access-date = 10 November 2013 | archive-date = 22 October 2009 | archive-url = https://web.archive.org/web/20091022182129/http://www.iau.org/public_press/themes/measuring/ | url-status = dead }} Despite its inclusion of the word "year", the term is not a [[unit of time]].{{cite web |author1=Bruce McClure |title=How far is a light-year? |url=https://earthsky.org/astronomy-essentials/how-far-is-a-light-year |publisher=EarthSky |access-date=15 October 2019 |date=31 July 2018}}
A '''light-year''', alternatively spelled '''light year''' ('''ly''' or '''lyr'''{{cite journal | doi=10.1038/294236a0 | title=Milliarcsecond structure of BL Lac during outburst | date=1981 | last1=Mutel | first1=R. L. | last2=Aller | first2=H. D. | last3=Phillips | first3=R. B. | journal=Nature | volume=294 | issue=5838 | pages=236–238 | bibcode=1981Natur.294..236M | hdl=2027.42/62626 | hdl-access=free }}), is a [[unit of length]] used to express [[astronomical distance|astronomical distances]] and is equal to exactly {{val|9,460,730,472,580.8|u=m}}, which is approximately 9.46 trillion kilometres or 5.88 trillion miles. As defined by the [[International Astronomical Union]] (IAU), a light-year is the distance that [[Speed of light|light travels in vacuum]] in one [[Julian year (astronomy)|Julian year]] (365.25 days).{{citation | url = https://www.iau.org/public/themes/measuring/ | title = Measuring the Universe: The IAU and Astronomical Units | author = International Astronomical Union | access-date = 10 November 2013 | archive-date = 22 October 2009 | archive-url = https://web.archive.org/web/20091022182129/http://www.iau.org/public_press/themes/measuring/ | url-status = dead }} Despite its inclusion of the word "year", the term is not a [[unit of time]].{{cite web |author1=Bruce McClure |title=How far is a light-year? |url=https://earthsky.org/astronomy-essentials/how-far-is-a-light-year |publisher=EarthSky |access-date=15 October 2019 |date=31 July 2018}}


The ''light-year'' is most often used when expressing distances to stars and other distances on a [[Galaxy|galactic]] scale, especially in [[public understanding of science|non-specialist]] contexts and [[popular science]] publications. The unit most commonly used in professional [[astronomy]] is the [[parsec]] (pc), approximately 3.26 light-years.
The ''light-year'' is most often used when expressing distances to stars and other distances on a [[Galaxy|galactic]] scale, especially in [[public understanding of science|non-specialist]] contexts and [[popular science]] publications. The unit most commonly used in professional [[astronomy]] is the [[parsec]] (pc), approximately 3.26 light-years.
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