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{{Other uses2|Light year}}

   {{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).


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= |title=IAU Recommendations concerning Units |deadurl=yes |archiveurl= |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 – Space and TimeIEEE/ASTM SI 10-2010, American National Standard for Metric Practice and localized abbreviations are frequent, such as "al" in French (from année-lumière) and Spanish (from año 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=,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= |title=Basic Constants|publisher=Sierra College}}{{Citation|author=Marc Sauvage|url=|title=Table of astronomical constants|deadurl=yes|archiveurl=|archivedate=2008-12-11|df=}}{{Citation|author=Robert A. Braeunig|url=|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=,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= |title=Astronomical Constants}}{{Citation|author=KEKB |url=|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= |title=Applied dimensional analysis and modeling|location=New York|publisher= McGraw-Hill|date= 1997|page= 60|isbn=9780070628113}}{{Citation|url= |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= |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= |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= |title=Encyclopedia of Distances |edition=fourth |year=2016 |publisher=Springer |page=620 |isbn=978-3-662-52843-3}}


The light-year unit appeared a few years after the first successful measurement of the distance to a star other than the Sun, by Friedrich Bessel in 1838. The star was 61 Cygni, and he used a {{convert|6.2|in|adj=on}} heliometer designed by Joseph von Fraunhofer. The largest unit for expressing distances across space at that time was the astronomical unit, equal to the radius of the Earth's orbit {{nowrap|({{val|1.50|e=8|u=km}}}} or {{nowrap|{{val|9.30|e=7|u=mi}}).}} In those terms, trigonometric calculations based on 61 Cygni's parallax of 0.314 arcseconds, showed the distance to the star to be {{val|660000}} astronomical units ({{val|9.9|e=13|u=km}} or {{val|6.1|e=13|u=mi}}). Bessel added that light employs 10.3 years to traverse this distance.JOURNAL,weblink On the parallax of the star 61 Cygni, Friedrich, Bessel, London and Edinburgh Philosophical Magazine and Journal of Science, 14, 68–72, 1839, Bessel's statement that light employs 10.3 years to traverse the distance. He recognized that his readers would enjoy the mental picture of the approximate transit time for light, but he refrained from using the light-year as a unit. He may have resented expressing distances in light-years because it would deteriorate the accuracy of his parallax data due to multiplying with the uncertain parameter of the speed of light. The speed of light was not yet precisely known in 1838; its value changed in 1849 (Fizeau) and 1862 (Foucault). It was not yet considered to be a fundamental constant of nature, and the propagation of light through the aether or space was still enigmatic.The light-year unit appeared in 1851 in a German popular astronomical article by w:.JOURNAL,weblink Was wir in den Sternen lesen, Otto, Ule, Deutsches Museum: Zeitschrift für Literatur, Kunst und Öffentliches Leben, 1, 721–738, 1851, The paradox of a distance unit name ending on "year" was explained by Ule by comparing it to a hiking road hour (Wegstunde). A contemporary German popular astronomical book also noticed that light-year is an odd name.BOOK, Diesterweg, Adolph Wilhelm, Populäre Himmelskunde u. astronomische Geographie, 1855, 250,weblink In 1868 an English journal labelled the light-year as a unit used by the Germans.BOOK,weblink The Student and Intellectual Observer of Science, Literature and Art, 1868, Eddington called the light-year an inconvenient and irrelevant unit, which had sometimes crept from popular use into technical investigations.WEB,weblink Stellar movements and the structure of the universe, 1 November 2014, Although modern astronomers often prefer to use the parsec, light years are also popularly used to gauge the expanses of interstellar and intergalactic space.

Usage of term

Distances expressed in light-years include those between stars in the same general area, such as those belonging to the same spiral arm or globular cluster. Galaxies themselves span from a few thousand to a few hundred thousand light-years in diameter, and are separated from neighbouring galaxies and galaxy clusters by millions of light-years. Distances to objects such as quasars and the Sloan Great Wall run up into the billions of light-years.{| class="wikitable"|+List of orders of magnitude for length!Scale (ly)!Value!Item|10−9{{vale=-8|u=ly}}Moon's surface takes 1.2–1.3 seconds to travel the distance to the Earth's surface (travelling roughly {{val>350000}} to {{val|400000}} kilometres).10−6{{vale=-5|u=ly}}astronomical unit (the distance from the Sun to the Earth). It takes approximately 499 seconds (8.32 minutes) for light to travel this distance.{{Citation>chapter-url= chapter=Chapter 1, Table 1-1}}{{vale=-4|u=ly}}Huygens probe lands on Titan (moon)>Titan off Saturn and transmits images from its surface, 1.2 billion kilometres from Earth.{{vale=-4|u=ly}}|New Horizons encounters Pluto at a distance 4.7 billion kilometres, and the communication takes 4 hours 25 minutes to reach Earth.10−3{{vale=-3|u=ly}}space probe, Voyager 1, was about 18 light-hours away from the Earth {{As of>October 2014title=WHERE ARE THE VOYAGERS? accessdate=14 October 2014}} It will take about {{val1.0u=ly}}) at its current speed of about 17 km/s ({{valu=mph}}) relative to the Sun. On September 12, 2013, NASA scientists announced that Voyager 1 had entered the interstellar medium of space on August 25, 2012, becoming the first manmade object to leave the Solar System.{{Citation url= |accessdate=14 October 2014}}{{vale=-3|u=ly}}|Voyager 1 as of October 2018, nearly 20 light-hours from the Earth 100{{vale=0|u=ly}}Oort cloud is approximately two light-years in diameter. Its inner boundary is speculated to be at {{val>50000100000|u=au}}.{{vale=0|u=ly}}Sun's gravitational dominance (Hill sphere/Roche sphere, {{val>125000|u=au}}). Beyond this is the deep ex-solar gravitational interstellar medium.{{vale=0|u=ly}}star (other than the Sun), Proxima Centauri, is about 4.22 light-years away.{{Citation>author=NASA weblink >title=Cosmic Distance Scales – The Nearest Star}}{{Citationchapter=Proxima Centauri (Gliese 551)|title=Encyclopedia of Astrobiology, Astronomy, and Spaceflight}}{{vale=0|u=ly}}Sirius, the brightest star of the night sky. Twice as massive and 25 times more Luminosity#In astronomy>luminous than the Sun, it outshines more luminous stars due to its relative proximity.{{vale=1|u=ly}}HD 10700 e, an extrasolar candidate for a habitable planet. 6.6 times as massive as the earth, it is in the middle of the habitable zone of star Tau Ceti.HTTPS://WWW.BBC.CO.UK/NEWS/SCIENCE-ENVIRONMENT-20770103TITLE=TAU CETI'S PLANETS NEAREST AROUND SINGLE, SUN-LIKE STARACCESSDATE=1 NOVEMBER 2014, MIKKO >LAST1=TUOMI LAST2=JONES LAST3=JENKINS LAST4=TINNEY LAST5=BUTLER LAST6=VOGT LAST7=BARNES LAST8=WITTENMYER LAST9=O'TOOLE LAST10=HORNER LAST11=BAILEY LAST12=CARTER LAST13=WRIGHT LAST14=SALTER LAST15=PINFIELD TITLE=SIGNALS EMBEDDED IN THE RADIAL VELOCITY NOISE: PERIODIC VARIATIONS IN THE Ï„ CETI VELOCITIES URL=HTTP://STAR-WWW.HERTS.AC.UK/~HRAJ/TAUCETI/PAPER.PDF BIBCODE = 2013A&A...551A..79T VOLUME=551, A79, {{vale=1|u=ly}}|Gliese 581, a red-dwarf star with several detectable exoplanets.{{vale=2|u=ly}}Canopus, second in brightness in the terrestrial sky only to Sirius, a type A9 bright giant {{val>10700}} times more luminous than the Sun. 103{{vale=3|u=ly}}A0620-00, the nearest known black hole, is about {{val>3000}} light-years away.{{vale=4|u=ly}}galactic center>centre of the Milky Way is about {{valdoi= 10.1086/380188date= 2003first1= F.first2= R.first3= R.first4= T.first5= M.first6= R.first7= A.first8= T.volume= 597pages= L121bibcode = 2003ApJ...597L.121E }}{{citationtitle=The Distance to the Galactic Centerlast1=McNamaraauthorlink1=D. Harold McNamarafirst2=J. B.first3=J.first4=B. F.volume=112pages=202 |bibcode = 2000PASP..112..202M }}{{vale=5|u=ly}}Milky Way is about {{val>100000}} light-years across.{{vale=5|u=ly}}|R136a1, in the Large Magellanic Cloud, the most luminous star known at 8.7 million times the luminosity of the Sun, has an apparent magnitude 12.77, just brighter than 3C 273. 106{{vale=6|u=ly}}|The Andromeda Galaxy is approximately 2.5 million light-years away.{{vale=6|u=ly}}Triangulum Galaxy (Messier object>M33), at about 3 million light-years away, is the most distant object visible to the naked eye.{{vale=7|u=ly}}|The nearest large galaxy cluster, the Virgo Cluster, is about 59 million light-years away.{{vale=8}} – {{vale=8|u=ly}}|The Great Attractor lies at a distance of somewhere between 150 and 250 million light-years (the latter being the most recent estimate). 109{{vale=9|u=ly}}Sloan Great Wall (not to be confused with Great Wall (astronomy)>Great Wall and Her–CrB GW) has been measured to be approximately one billion light-years distant.{{vale=9|u=ly}}|3C 273, optically the brightest quasar, of apparent magnitude 12.9, just dimmer than R136a1. 3C 273 is about 2.4 billion light-years away.{{vale=10|u=ly}}comoving distance from the Earth to the edge of the visible universe is about 45.7 billion light-years in any direction; this is the comoving radius of the observable universe. This is larger than the age of the universe dictated by the cosmic background radiation; see Observable universe#Misconceptions on its size>here for why this is possible.

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= |journal=Publications of the Astronomical Society of Japan|volume=52 |issue=6 |pages=1021–26 |bibcode=2000PASJ...52.1021F |doi=10.1093/pasj/52.6.1021 |deadurl=yes |archiveurl= |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 11–12, 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

See also





External links

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

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