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light-year
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{{valul=Pm}}Imperial units>imperial & US units5.8786ul=mi}}astronomical system of units>astronomical units63,2410.3066|ul=pc}}12.5 light-years of the SunThe Universe within 12.5 Light Years: The Nearest Stars>image = (File:12lightyears.gif|300px)}}The light-year is a unit of length used to express astronomical distances and measures about 9.46 trillion kilometres (9.46 x 1012 km) or 5.88 trillion miles (5.88 x 1012 mi).One trillion here is taken to be 1012 (one million million, or billion in long scale). 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).{{citation| url =weblink |title = Measuring the Universe: The IAU and Astronomical Units | author = International Astronomical Union | accessdate=10 November 2013}} 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 nonspecialist and popular science publications. The unit most commonly used in professional astrometry is the parsec (symbol: pc, about 3.26 light-years; the distance at which one astronomical unit subtends an angle of one second of arc).

## Definitions

As defined by the IAU, the light-year is the product of the Julian year{{refn|One Julian year is of exactly 365.25 days (or {{val|31557600|u=s}} based on a day of exactly {{val|86400}} SI seconds){{Citation|url=http://www.iau.org/Units.234.0.html |title=IAU Recommendations concerning Units |deadurl=yes |archiveurl=https://web.archive.org/web/20070216041250weblink |archivedate=2007-02-16 |df= }}|group=note}} (365.25 days as opposed to the 365.2425-day Gregorian year) and the speed of light ({{val|299792458|u=m/s}}).The speed of light is precisely {{val|299792458|u=m/s}} by definition of the metre. Both of these values are included in the IAU (1976) System of Astronomical Constants, used since 1984."Selected Astronomical Constants" in Astronomical Almanac, p. 6. From this, the following conversions can be derived. The IAU recognized abbreviation for light-year is ly, although other standards like ISO 80000 use "l.y."ISO 80000-3:2006 Quantities and Units â€“ Space and TimeIEEE/ASTM SI 10-2010, American National Standard for Metric Practice and localized abbreviations are frequent, such as "al" in French (from annÃ©e-lumiÃ¨re) and Spanish (from aÃ±o luz), "Lj" in German (from Lichtjahr), etc.
{|
1 light-year   9460730472580800}} metres (exactly)9.461}} petametres9.461}} trillion kilometres5.878625}} trillion miles63241.077}} astronomical units0.306601}} parsecsBefore 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.{{Citation|editor=P. Kenneth Seidelmann|url=https://books.google.com/?id=uJ4JhGJANb4C&pg=PP1&lpg=PP1#PPA656,M1 |title=Explanatory Supplement to the Astronomical Almanac|location=Mill Valley, California|publisher= University Science Books|date= 1992|page=656|isbn= 978-0-935702-68-2}} The product of Simon Newcomb's J1900.0 mean tropical year of {{val|31556925.9747}} ephemeris seconds and a speed of light of {{val|299792.5|u=km/s}} produced a light-year of {{val|9.460530|e=15|u=m}} (rounded to the seven significant digits in the speed of light) found in several modern sources{{Citation|url=http://astronomy.sierracollege.edu/Resources/Reference/basic%20Contants%20app2.htm |title=Basic Constants|publisher=Sierra College}}{{Citation|author=Marc Sauvage|url=http://marc.sauvage.free.fr/astro_book/Cts_pages/astro.html|title=Table of astronomical constants|deadurl=yes|archiveurl=https://web.archive.org/web/20081211001523weblink|archivedate=2008-12-11|df=}}{{Citation|author=Robert A. Braeunig|url=http://www.braeunig.us/space/constant.htm|title= Basic Constants}} was probably derived from an old source such as C. W. Allen's 1973 Astrophysical Quantities reference work,{{Citation|author=C. W. Allen|title=Astrophysical Quantities|edition=third|location= London|publisher= Athlone|date= 1973|page= 16|isbn=978-0-485-11150-7}} which was updated in 2000, including the IAU (1976) value cited above (truncated to 10 significant digits).{{Citation|editor=Arthur N. Cox|url=https://books.google.com/books?id=w8PK2XFLLH8C&pg=PP1&lpg=PP1#PPA12,M1|title= Allen's Astrophysical Quantities|edition=fourth |location= New York|publisher= Springer-Valeg|date= 2000|page= 12|isbn= 978-0-387-98746-0}}Other high-precision values are not derived from a coherent IAU system. A value of {{val|9.460536207|e=15|u=m}} found in some modern sources{{Citation|author=Nick Strobel|url=http://www.astronomynotes.com/tables/tablesa.htm |title=Astronomical Constants}}{{Citation|author=KEKB |url=http://www-acc.kek.jp/kekb/Introduction/misc/astronomical_constant.html|title= Astronomical Constants}} is the product of a mean Gregorian year (365.2425 days or {{val|31556952|u=s}}) and the defined speed of light ({{val|299792458|u=m/s}}). Another value, {{val|9.460528405|e=15|u=m}},{{Citation|author=Thomas Szirtes|url=https://books.google.com/books?id=8Fk__-TUdCEC&pg=PA60 |title=Applied dimensional analysis and modeling|location=New York|publisher= McGraw-Hill|date= 1997|page= 60|isbn=9780070628113}}{{Citation|url=http://www.scienceclarified.com/everyday/Real-Life-Chemistry-Vol-7/Sun-Moon-and-Earth.html |title=Sun, Moon, and Earth: Light-year}} is the product of the J1900.0 mean tropical year and the defined speed of light.Abbreviations used for light years and multiples of light years are
• "ly" for one light year
• "kly" for a kilolight-year (1,000 light years){{citation |last=Comins |first=Neil F. |url=https://books.google.com/books?id=qK_4mNve1DYC&pg=PA365 |title=Discovering the Essential Universe |edition=fifth |year=2013 |publisher=W. H. Freeman |page=365 |isbn=978-1-4292-5519-6}}
• "Mly" for a megalight-year (1,000,000 light years){{citation |last=Hassani |first=Sadri |url=https://books.google.com/books?id=oypZ_a9pqdsC&pg=PA445 |title=From Atoms to Galaxies |publisher=CRC Press |year=2010 |page=445 |isbn=978-1-4398-0850-4}}
• "Gly" for a gigalight-year (1,000,000,000 light years){{citation |last1=Deza |first1=Michel Marie |last2=Deza |first2=Elena |url=https://books.google.com/books?id=KQHdDAAAQBAJ&pg=PA620 |title=Encyclopedia of Distances |edition=fourth |year=2016 |publisher=Springer |page=620 |isbn=978-3-662-52843-3}}

## Related units

Distances between objects within a star system tend to be small fractions of a light year, and are usually expressed in astronomical units. However, smaller units of length can similarly be formed usefully by multiplying units of time by the speed of light. For example, the light-second, useful in astronomy, telecommunications and relativistic physics, is exactly {{val|299792458}} metres or {{frac|{{val|31557600}}}} of a light-year. Units such as the light-minute, light-hour and light-day are sometimes used in popular science publications. The light-month, roughly one-twelfth of a light-year, is also used occasionally for approximate measures.{{citation|author1=Fujisawa, K. |author2=Inoue, M. |author3=Kobayashi, H. |author4=Murata, Y. |author5=Wajima, K. |author6=Kameno, S. |author7=Edwards, P. G. |author8=Hirabayashi, H. |author9=Morimoto, M. |date=2000 |title=Large Angle Bending of the Light-Month Jet in Centaurus A |url=http://sciencelinks.jp/j-east/article/200123/000020012301A0179284.php |journal=Publications of the Astronomical Society of Japan|volume=52 |issue=6 |pages=1021â€“26 |bibcode=2000PASJ...52.1021F |doi=10.1093/pasj/52.6.1021 |deadurl=yes |archiveurl=https://web.archive.org/web/20090902035920weblink |archivedate=2009-09-02 |df= }}{{citation |author1=Junor, W. |author2=Biretta, J. A. | date = 1994 | contribution = The Inner Light-Month of the M87 Jet | bibcode = 1994cers.conf...97J | title = Compact Extragalactic Radio Sources, Proceedings of the NRAO workshop held at Socorro, New Mexico, February 11â€“12, 1994 |editor1=Zensus, J. Anton |editor2=Kellermann |editor3=Kenneth I. | location = Green Bank, WV | publisher = National Radio Astronomy Observatory (NRAO)| page = 97}} The Hayden Planetarium specifies the light month more precisely as 30 days of light travel time.Light-Travel Time and Distance by the Hayden Planetarium Accessed October 2010.Light travels approximately one foot in a nanosecond; the term "light-foot" is sometimes used as an informal measure of time.BOOK
, David Mermin
, It's About Time: Understanding Einstein's Relativity
, 22
, 2009
, Princeton, New Jersey
, Princeton University Press
, 978-0-691-14127-5
,

{{NoteFoot}}

## References

{{Reflist}}

{{Spoken Wikipedia|Light year.ogg|27 June 2005}}
• {{wiktionary-inline}}
{{Units of length used in Astronomy}}

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