Light - year - wikipedia Light - year For other uses, see Light year ( disambiguation ). Light - year Map showing the stars that lie within 12.5 light - years of the Sun Unit information Unit system astronomy units Unit of length Symbol ly Unit conversions 1 ly in...... is equal to... metric ( SI ) units 7015946070000000000 ♠ 9.4607 × 10 m 7015946070000000000 ♠ 9.4607 Pm imperial & US units 7015946068963840000 ♠ 5.8786 × 10 mi astronomical units 7015946071894093870 ♠ 63 241 au 7015946068746477840 ♠ 0.3066 pc The light - year is a unit of length used to express astronomical distances and measures about 9.5 trillion kilometres or 5.9 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 ( 365.25 days ). Because it includes the word `` year '', the term light - year is sometimes misinterpreted as a unit of time, as a year is a unit of time equivalent to approximately 365 days. 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 ). Contents 1 Definitions 2 History 3 Usage of term 4 Related units 5 See also 6 Notes 7 References Definitions 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 ♠ 299 792 458 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 IAU recognized abbreviation for light - year is ly, although other standards like ISO 80000 use `` l.y. '' and localized symbols are frequent, such as `` al '' in French ( from année - lumière ) and Spanish ( from año luz ), `` Lj '' in German ( from Lichtjahr ), etc. 1 light - year = 7015946073047258080 ♠ 9460730472580800 metres ( exactly ) ≈ 7000946100000000000 ♠ 9.461 petametres ≈ 7000946100000000000 ♠ 9.461 trillion kilometres ≈ 7000587862500000000 ♠ 5.878 625 trillion miles ≈ 7004632410770000000 ♠ 63 241. 077 astronomical units ≈ 6999306601000000000 ♠ 0.306 601 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. 0 mean tropical year of 7007315569259747000 ♠ 31 556 925. 9747 ephemeris seconds and a speed of light of 7008299792500000000 ♠ 299 792. 5 km / s produced a light - year of 7015946053000000000 ♠ 9.460 530 × 10 m ( rounded to the seven significant digits in the speed of light ) found in several modern sources was probably derived from an old source such as C.W. Allen 's 1973 Astrophysical Quantities reference work, which was updated in 2000, including the IAU ( 1976 ) value cited above ( truncated to 10 significant digits ). Other high - precision values are not derived from a coherent IAU system. A value of 7015946053620700000 ♠ 9.460 536 207 × 10 m found in some modern sources is the product of a mean Gregorian year ( 365.2425 days or 7007315569520000000 ♠ 31 556 952 s ) and the defined speed of light ( 7008299792458000000 ♠ 299 792 458 m / s ). Another value, 7015946052840500000 ♠ 9.460 528 405 × 10 m, 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 ) `` Mly '' for a megalight - year ( 1,000,000 light years ) `` Gly '' for a gigalight - year ( 1,000,000,000 light years ) History 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 6.2 - inch ( 160 mm ) 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 ( 7011150000000000000 ♠ 1.50 × 10 km or 7011149668992000000 ♠ 9.30 × 10 mi ). In those terms, trigonometric calculations based on 61 Cygni 's parallax of 0.314 arcseconds, showed the distance to the star to be 7005660000000000000 ♠ 660 000 astronomical units ( 7016990000000000000 ♠ 9.9 × 10 km or 7016981699840000000 ♠ 6.1 × 10 mi ). Bessel added that light employs 10.3 years to traverse this 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, however, in 1851 in a German popular astronomical article by Otto Ule. 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. In 1868 an English journal labelled the light - year as a unit used by the Germans. Eddington called the light - year an inconvenient and irrelevant unit, which had sometimes crept from popular use into technical investigations. 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. List of orders of magnitude for length Scale ( ly ) Value Item 10 7008382213511092264 ♠ 40.4 × 10 ly Reflected sunlight from the Moon 's surface takes 1.2 -- 1.3 seconds to travel the distance to the Earth 's surface ( travelling roughly 7005350000000000000 ♠ 350 000 to 7005400000000000000 ♠ 400 000 kilometres ). 10 7011149479541466776 ♠ 15.8 × 10 ly One astronomical unit ( the distance from the Sun to the Earth ). It takes approximately 499 seconds ( 8.32 minutes ) for light to travel this distance. 7012120151277001776 ♠ 127 × 10 ly The Huygens probe lands on Titan off Saturn and transmits images from its surface, 1.2 billion kilometres from Earth. 7012476820815818072 ♠ 504 × 10 ly New Horizons encounters Pluto at a distance 4.7 billion kilometres, and the communication takes 4 hours 25 minutes to reach Earth. 10 7013192998901640648 ♠ 2.04 × 10 ly The most distant space probe, Voyager 1, was about 18 light - hours away from the Earth as of October 2014. It will take about 7004175000000000000 ♠ 17 500 years to reach one light - year ( 7015946073047258080 ♠ 1.0 × 10 ly ) at its current speed of about 17 km / s ( 7004169875200000000 ♠ 38 000 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. 10 7016151371687561292 ♠ 1.6 × 10 ly The Oort cloud is approximately two light - years in diameter. Its inner boundary is speculated to be at 7015747989353500000 ♠ 50 000 au, with its outer edge at 7016149597870700000 ♠ 100 000 au. 7016189214609451616 ♠ 2.0 × 10 ly Maximum extent of the Sun 's gravitational dominance ( Hill sphere / Roche sphere, 7016186997338375000 ♠ 125 000 au ). Beyond this is the deep ex-solar gravitational interstellar medium. 7016399242825942909 ♠ 4.22 × 10 ly The nearest known star ( other than the Sun ), Proxima Centauri, is about 4.22 light - years away. 7016813622820641948 ♠ 8.60 × 10 ly Sirius, the brightest star of the night sky. Twice as massive and 25 times more luminous than the Sun, it outshines more luminous stars due to its relative proximity. 7017112582692623711 ♠ 11.90 × 10 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. 7017193944974687906 ♠ 20.5 × 10 ly Gliese 581, a red - dwarf star with several detectable exoplanets. 7018293282644650004 ♠ 310 × 10 ly Canopus, second in brightness in the terrestrial sky only to Sirius, a type F supergiant 7004150000000000000 ♠ 15 000 times more luminous than the Sun. 10 7019283821914177424 ♠ 3 × 10 ly A0620 - 00, the nearest known black hole, is about 7003300000000000000 ♠ 3000 light - years away. 7020245978992287100 ♠ 26 × 10 ly The centre of the Milky Way is about 7004260000000000000 ♠ 26 000 light - years away. 7020946073047258079 ♠ 100 × 10 ly The Milky Way is about 7005100000000000000 ♠ 100 000 light - years across. 7021156102052797583 ♠ 165 × 10 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. 10 7022236518261814520 ♠ 2.5 × 10 ly The Andromeda Galaxy is approximately 2.5 million light - years away. 7022283821914177424 ♠ 3 × 10 ly The Triangulum Galaxy ( M33 ), at about 3 million light - years away, is the most distant object visible to the naked eye. 7023558183097882267 ♠ 59 × 10 ly The nearest large galaxy cluster, the Virgo Cluster, is about 59 million light - years away. 7008150000000000000 ♠ 150 × 10 -- 7024236518261814520 ♠ 250 × 10 ly The Great Attractor lies at a distance of somewhere between 150 and 250 million light - years ( the latter being the most recent estimate ). 10 7025113528765670969 ♠ 1.2 × 10 ly The Sloan Great Wall ( not to be confused with Great Wall and Her -- CrB GW ) has been measured to be approximately one billion light - years distant. 7025227057531341939 ♠ 2.4 × 10 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. 7026432355382596942 ♠ 45.7 × 10 ly The 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 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 7008299792458000000 ♠ 299 792 458 metres or ​ ⁄ 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. The Hayden Planetarium specifies the light month more precisely as 30 days of light travel time. Light travels approximately one foot in a nanosecond ; the term `` light - foot '' is sometimes used as an informal measure of time. See also Listen to this article ( info / dl ) This audio file was created from a revision of the article `` Light - year '' dated 27 June 2005, and does not reflect subsequent edits to the article. ( Audio help ) More spoken articles Look up light - year in Wiktionary, the free dictionary. 1 petametre ( examples of distances on the order of one light - year ) Einstein protocol Hubble length Orders of magnitude ( length ) Speed of light Distance measures ( cosmology ) Notes Jump up ^ One trillion here is taken to be 10 ( one million million, or billion in long scale ). Jump up ^ One Julian year is of exactly 365.25 days ( or 7007315576000000000 ♠ 31 557 600 s based on a day of exactly 7004864000000000000 ♠ 86 400 SI seconds ) Jump up ^ The speed of light is exactly 7008299792458000000 ♠ 299 792 458 m / s by definition of the metre. References Jump up ^ The Universe within 12.5 Light Years : The Nearest Stars ^ Jump up to : International Astronomical Union, Measuring the Universe : The IAU and Astronomical Units, retrieved 10 November 2013 Jump up ^ IAU Recommendations concerning Units, archived from the original on 2007 - 02 - 16 Jump up ^ `` Selected Astronomical Constants '' in Astronomical Almanac, p. 6. Jump up ^ ISO 80000 - 3 : 2006 Quantities and Units - Space and Time Jump up ^ IEEE / ASTM SI 10 - 2010, American National Standard for Metric Practice Jump up ^ P. Kenneth Seidelmann, ed. ( 1992 ), Explanatory Supplement to the Astronomical Almanac, Mill Valley, California : University Science Books, p. 656, ISBN 0 - 935702 - 68 - 7 Jump up ^ Basic Constants, Sierra College Jump up ^ Marc Sauvage, Table of astronomical constants, archived from the original on 2008 - 12 - 11 Jump up ^ Robert A. Braeunig, Basic Constants Jump up ^ C.W. Allen ( 1973 ), Astrophysical Quantities ( third ed. ), London : Athlone, p. 16, ISBN 0 - 485 - 11150 - 0 Jump up ^ Arthur N. Cox, ed. ( 2000 ), Allen 's Astrophysical Quantities ( fourth ed. ), New York : Springer - Valeg, p. 12, ISBN 0 - 387 - 98746 - 0 Jump up ^ Nick Strobel, Astronomical Constants Jump up ^ KEK B, Astronomical Constants Jump up ^ Thomas Szirtes ( 1997 ), Applied dimensional analysis and modeling, New York : McGraw - Hill, p. 60, ISBN 9780070628113 Jump up ^ Sun, Moon, and Earth : Light - year Jump up ^ Bessel, Friedrich ( 1839 ). `` On the parallax of the star 61 Cygni ''. London and Edinburgh Philosophical Magazine and Journal of Science. 14 : 68 -- 72. Retrieved 4 April 2014. Bessel 's statement that light employs 10.3 years to traverse the distance. Jump up ^ Ule, Otto ( 1851 ). `` Was wir in den Sternen lesen ''. Deutsches Museum : Zeitschrift für Literatur, Kunst und Öffentliches Leben. 1 : 721 -- 738. Retrieved 4 April 2014. Jump up ^ Diesterweg, Adolph Wilhelm ( 1855 ). Populäre Himmelskunde u. astronomische Geographie. p. 250. Jump up ^ The Student and Intellectual Observer of Science, Literature and Art. Retrieved 1 November 2014. Jump up ^ `` Stellar movements and the structure of the universe ''. Retrieved 1 November 2014. Jump up ^ `` Chapter 1, Table 1 - 1 '', IERS Conventions ( 2003 ) Jump up ^ WHERE ARE THE VOYAGERS ?, retrieved 14 October 2014 Jump up ^ NASA Spacecraft Embarks on Historic Journey Into Interstellar Space, retrieved 14 October 2014 Jump up ^ NASA, Cosmic Distance Scales - The Nearest Star Jump up ^ `` Proxima Centauri ( Gliese 551 ) '', Encyclopedia of Astrobiology, Astronomy, and Spaceflight Jump up ^ `` Tau Ceti 's planets nearest around single, Sun - like star ''. BBC News. 19 December 2012. Retrieved 1 November 2014. Jump up ^ Tuomi, Mikko ; Jones, Hugh R.A. ; Jenkins, James S. ; Tinney, Chris G. ; Butler, R. Paul ; Vogt, Steve S. ; Barnes, John R. ; Wittenmyer, Robert A. ; O'Toole, Simon ; Horner, Jonathan ; Bailey, Jeremy ; Carter, Brad D. ; Wright, Duncan J. ; Salter, Graeme S. ; Pinfield, David ( 2012 ). `` Signals embedded in the radial velocity noise : periodic variations in the τ Ceti velocities '' ( PDF ). arXiv : 1212.4277. Bibcode : 2013A&A... 551A... 79T. doi : 10.1051 / 0004 - 6361 / 201220509. Jump up ^ Eisenhauer, F. ; Schdel, R. ; Genzel, R. ; Ott, T. ; Tecza, M. ; Abuter, R. ; Eckart, A. ; Alexander, T. ( 2003 ), `` A Geometric Determination of the Distance to the Galactic Center '', The Astrophysical Journal, 597 ( 2 ) : L121, arXiv : astro - ph / 0306220, Bibcode : 2003ApJ... 597L. 121E, doi : 10.1086 / 380188 Jump up ^ McNamara, D.H. ; Madsen, J.B. ; Barnes, J. ; Ericksen, B.F. ( 2000 ), `` The Distance to the Galactic Center '', Publications of the Astronomical Society of the Pacific, 112 ( 768 ) : 202, Bibcode : 2000PASP... 112... 202M, doi : 10.1086 / 316512 Jump up ^ Fujisawa, K. ; Inoue, M. ; Kobayashi, H. ; Murata, Y. ; Wajima, K. ; Kameno, S. ; Edwards, P.G. ; Hirabayashi, H. ; Morimoto, M. ( 2000 ), `` Large Angle Bending of the Light - Month Jet in Centaurus A '', Publ. Astron. Soc. Jpn., 52 ( 6 ) : 1021 -- 26, Bibcode : 2000PASJ... 52.1021 F, doi : 10.1093 / pasj / 52.6. 1021, archived from the original on 2009 - 09 - 02 Jump up ^ Junor, W. ; Biretta, J.A. ( 1994 ), `` The Inner Light - Month of the M87 Jet '', in Zensus, J. Anton ; Kellermann ; Kenneth I., Compact Extragalactic Radio Sources, Proceedings of the NRAO workshop held at Socorro, New Mexico, February 11 -- 12, 1994, Green Bank, WV : National Radio Astronomy Observatory ( NRAO ), p. 97, Bibcode : 1994cers. conf... 97J Jump up ^ Light - Travel Time and Distance by the Hayden Planetarium Accessed October 2010. Jump up ^ David Mermin ( 2009 ). It 's About Time : Understanding Einstein 's Relativity. Princeton, New Jersey : Princeton University Press. p. 22. ISBN 978 - 0 - 691 - 14127 - 5. Units of length used in Astronomy Astronomical system of units Earth radius ( R or R ) Light - second ( ls ) Solar radius ( R ) gigametre ( Gm ) Astronomical unit ( AU ) terametre ( Tm ) light - year ( ly ) parsec ( pc ) kiloparsec ( kpc ) megaparsec ( Mpc ) gigaparsec ( Gpc ) See also Cosmic distance ladder Orders of magnitude ( length ) Conversion of units Retrieved from `` https://en.wikipedia.org/w/index.php?title=Light-year&oldid=842822897 '' Categories : Light Units of length Units of measurement in astronomy 1838 in science Hidden categories : Pages using deprecated image syntax Articles containing potentially dated statements from October 2014 All articles containing potentially dated statements Spoken articles Articles with hAudio microformats Talk Contents About Wikipedia Afrikaans አማርኛ Ænglisc Aragonés অসমীয়া Asturianu Azərbaycanca تۆرکجه বাংলা Bân - lâm - gú Беларуская Беларуская ( тарашкевіца ) ‎ Български Bosanski Brezhoneg Català Čeština Cymraeg Dansk Deutsch Eesti Ελληνικά Español Esperanto Estremeñu Euskara فارسی Fiji Hindi Français Frysk Gaeilge Galego 贛 語 ગુજરાતી 한국어 Հայերեն हिन्दी Hrvatski Ido Bahasa Indonesia Interlingua Íslenska Italiano עברית Basa Jawa ಕನ್ನಡ ქართული Қазақша Kiswahili Kreyòl ayisyen Kurdî ລາວ Latina Latviešu Lëtzebuergesch Lietuvių Ligure Limburgs Lumbaart Magyar Македонски Malagasy മലയാളം Malti मराठी مصرى Bahasa Melayu Монгол မြန်မာဘာသာ Nāhuatl Nederlands नेपाली 日本 語 Norsk Norsk nynorsk Occitan ଓଡ଼ିଆ Oromoo Oʻzbekcha / ўзбекча ਪੰਜਾਬੀ پنجابی ភាសា ខ្មែរ Piemontèis Plattdüütsch Polski Português Română Runa Simi Русиньскый Русский Саха тыла संस्कृतम् Scots Seeltersk Shqip Sicilianu Simple English سنڌي Slovenčina Slovenščina Soomaaliga کوردی Српски / srpski Srpskohrvatski / српскохрватски Suomi Svenska Tagalog தமிழ் Татарча / tatarça తెలుగు ไทย Türkçe Türkmençe Українська اردو Vèneto Tiếng Việt Volapük 文言 Winaray ייִדיש 粵語 Zazaki 中文 Kabɩyɛ Lingua Franca Nova 118 more Edit links This page was last edited on 24 May 2018, at 22 : 27. 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Question: how far does light travel in a year in kilometers
Answer: 9.5 trillion kilometres