Light-year#Distances in light-years

{{about|the unit of length|other uses|Light year (disambiguation)}}

{{Infobox unit

| standard = astronomy units

| quantity = length

| symbol = ly

| units1 = metric (SI) units

| inunits1 = {{ubl|{{val|9.4607|e=15|ul=m}}|{{val|9.46073|ul=Pm}}}}

| units2 = imperial and US units

| inunits2 = {{ubl|{{val|5.8786|e=12|ul=mi}}}}

| units3 = astronomical units

| inunits3 = {{ubl|{{val|63,241|ul=au}}|{{val|0.3066|ul=pc}}}}

| caption = Map showing stars and star systems lying within 12.5 light-years of the Sun{{Cite web|url=http://www.atlasoftheuniverse.com/12lys.html|title=The Universe within 12.5 Light Years – The Nearest stars|website=www.atlasoftheuniverse.com|accessdate=2 April 2022}}|image = 300px

}}

A light-year, alternatively spelled light year (ly or lyr{{cite journal | url=https://www.nature.com/articles/294236a0 | 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 distances and is equal to exactly {{val|9,460,730,472,580.8|u=km}}, which is approximately 9.46 trillion km or 5.88 trillion mi. 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 = 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}} Despite its inclusion of the word "year", the term should not be misinterpreted as 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 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).

Definitions

As defined by the International Astronomical Union (IAU), the light-year is the product of the Julian year{{refn|One Julian year is 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 |url-status=dead |archive-url=https://web.archive.org/web/20070216041250/http://www.iau.org/Units.234.0.html |archive-date=16 February 2007 }}|group=note}} (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 ({{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."[http://asa.usno.navy.mil/static/files/2009/Astronomical_Constants_2009.pdf Selected Astronomical Constants] {{Webarchive|url=https://web.archive.org/web/20140726132053/http://asa.usno.navy.mil/static/files/2009/Astronomical_Constants_2009.pdf |date=2014-07-26 }}" in Astronomical Almanac, p. 6. From this, the following conversions can be derived:

rowspan=6 valign=top|1 light-year

|= {{val|9460730472580800}} metres (exactly)

{{Circa}} {{val|9.461}} petametres

{{Circa}} {{val|9.461}} trillion (short scale) kilometres ({{val|5.879}} trillion miles)

{{Circa}} {{val|63241.077}} astronomical units

{{Circa}} {{val|0.306601}} parsec

The abbreviation used by the IAU for light-year is "ly", International standards like ISO 80000:2006 (now superseded) have used "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, Spanish, and Italian (from année-lumière, año luz and anno luce, respectively), "Lj" in German (from Lichtjahr), etc.

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.{{Citation|editor=P. Kenneth Seidelmann|url=https://books.google.com/books?id=uJ4JhGJANb4C&pg=PP1 |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|url-status=dead|archive-url=https://web.archive.org/web/20081211001523/http://marc.sauvage.free.fr/astro_book/Cts_pages/astro.html|archive-date=11 December 2008}}{{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|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 |access-date=5 November 2008 |archive-url=https://web.archive.org/web/20070909131655/http://www-acc.kek.jp/kekb/Introduction/misc/astronomical_constant.html |archive-date=9 September 2007 |url-status=dead }} 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=978-0-07-062811-3}} is the product of the J1900.0 mean tropical year and the defined speed of light.

{{anchor|abbr}}Abbreviations used for light-years and multiples of light-years are:

"ly" for one light-year
"kly" {{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}} or "klyr"[https://www.sciencedirect.com/science/article/abs/pii/0146641094900094 Viollier 1994] for a kilolight-year (1,000 light-years)
"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"{{citation |last1=Deza |first1=Michel Marie |last2=Deza |first2=Elena |author2-link=Elena Deza|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}} or "Glyr"[https://www.progress-in-physics.com/2022/PP-63-11.PDF Sanchez et al. 2022] for a gigalight-year ({{val|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 {{convert|6.2|in|adj=on|order=flip}} heliometre 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 at {{convert|150|e6km|e6mi|abbr=off}}. In those terms, trigonometric calculations based on 61 Cygni's parallax of 0.314 arcseconds, showed the distance to the star to be {{convert|660,000|AU|km mi|comma=gaps}}. Bessel added that light takes 10.3 years to traverse this distance.{{Cite journal |url=https://books.google.com/books?id=UdChCg32-a0C&q=%22light+employs%22&pg=PA71|title=On the parallax of the star 61 Cygni|first=Friedrich |last=Bessel |journal=London and Edinburgh Philosophical Magazine and Journal of Science |volume=14 |pages= 68–72 |date=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 resisted expressing distances in light-years because it would reduce 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; the estimate of 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 Otto Ule.{{Cite journal |url=https://books.google.com/books?id=QtBGAAAAcAAJ&q=lichtjahre&pg=PA728|title=Was wir in den Sternen lesen |first=Otto |last=Ule |journal=Deutsches Museum: Zeitschrift für Literatur, Kunst und Öffentliches Leben |volume=1 |pages= 721–738 |date=1851}} Ule explained the oddity of a distance unit name ending in "year" by comparing it to a walking hour (Wegstunde).

A contemporary German popular astronomical book also noticed that light-year is an odd name.{{cite book |last=Diesterweg |first=Adolph Wilhelm |title=Populäre Himmelskunde u. astronomische Geographie |date=1855 |pages=250 |url=https://books.google.com/books?id=0QIrAAAAcAAJ&q=lichtjahr&pg=PA250 }} In 1868 an English journal labelled the light-year as a unit used by the Germans.{{cite book |url=https://books.google.com/books?id=Dko5AQAAMAAJ&pg=PA240 |title=The Student and Intellectual Observer of Science, Literature and Art |volume=1 |year=1868 |page=240 |publisher=Groombridge and Sons |location=London }} Eddington called the light-year an inconvenient and irrelevant unit, which had sometimes crept from popular use into technical investigations.{{cite web|url=https://archive.org/stream/stellarmovements00eddirich#page/14/mode/2up/search/light+year|title=Stellar movements and the structure of the universe|access-date=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

|align=right|{{val|4.04|e=-8|u=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 {{val|350000}} to {{val|400000}} kilometres).

rowspan=3|10⁻⁶

|align=right|{{val|1.58|e=-5|u=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.{{Citation|chapter-url=http://www.iers.org/MainDisp.csl?pid=46-25776 |title=IERS Conventions (2003)| chapter=Chapter 1, Table 1-1}}

align=right|{{val|1.27|e=-4|u=ly}}

|The Huygens probe lands on Titan off Saturn and transmits images from its surface, 1.2 billion kilometres from Earth.

align=right|{{val|5.04|e=-4|u=ly}}

|New Horizons encounters Pluto at a distance of 4.7 billion kilometres, and the communication takes 4 hours 25 minutes to reach Earth.

rowspan="2" |10⁻³

|align=right|{{val|2.04|e=-3|u=ly}}

|The most distant space probe, Voyager 1, was about 18 light-hours (130 au,19.4 billion km, 12.1 billion mi) away from the Earth {{As of|October 2014|lc=on}}.{{citation |title=WHERE ARE THE VOYAGERS? |url=http://voyager.jpl.nasa.gov/where/index.html |access-date=14 October 2014}} It will take about {{val|17500}} years to reach one light-year at its current speed of about 17 km/s ({{val|38000|u=mph}}, 61 200 km/h) relative to the Sun. On 12 September 2013, NASA scientists announced that Voyager 1 had entered the interstellar medium of space on 25 August 2012, becoming the first manmade object to leave the Solar System.{{Citation |title=NASA Spacecraft Embarks on Historic Journey Into Interstellar Space |url=http://www.nasa.gov/mission_pages/voyager/voyager20130912.html#.VD37P410yCg |access-date=14 October 2014}}

align=right|{{val|2.28|e=-3|u=ly}}

|Voyager 1 as of October 2018, nearly 20 light-hours (144 au, 21.6 billion km, 13.4 billion mi) from the Earth.

rowspan=7| 10⁰

|align=right|{{val|1.6|e=0|u=ly}}

|The Oort cloud is approximately two light-years in diameter. Its inner boundary is speculated to be at {{val|50000|u=au}} ≈ 0.8 ly, with its outer edge at {{val|100000|u=au}} ≈ 1.6 ly.

align=right|{{val|2.0|e=0|u=ly}}

|Approximate maximum distance at which an object can orbit the Sun (Hill sphere/Roche sphere, {{val|125000|u=au}}). Beyond this is the deep ex-solar gravitational interstellar medium.

align=right|{{val|4.24|e=0|u=ly}}

|The nearest known star (other than the Sun), Proxima Centauri, is about 4.24 light-years away.{{Citation|author=NASA |url= http://heasarc.gsfc.nasa.gov/docs/cosmic/nearest_star_info.html |title=Cosmic Distance Scales – The Nearest Star}}{{Citation|chapter-url=http://www.daviddarling.info/encyclopedia/P/ProximaCen.html|chapter=Proxima Centauri (Gliese 551)|title=Encyclopedia of Astrobiology, Astronomy, and Spaceflight}}

align=right|{{val|8.6|e=0|u=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.

align=right|{{val|1.19|e=1|u=ly}}

|Tau Ceti 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.{{cite web|url=https://www.bbc.co.uk/news/science-environment-20770103|date=19 December 2012 |title=Tau Ceti's planets nearest around single, Sun-like star|work=BBC News|access-date=1 November 2014}}{{cite journal |first1=Mikko |last1=Tuomi |first2=Hugh R. A. |last2=Jones |first3=James S. |last3=Jenkins |first4=Chris G. |last4=Tinney |first5=R. Paul |last5=Butler |first6=Steve S. |last6=Vogt |first7=John R. |last7=Barnes |first8=Robert A. |last8=Wittenmyer |first9=Simon |last9=O'Toole |first10=Jonathan |last10=Horner |first11=Jeremy |last11=Bailey |first12=Brad D. |last12=Carter |first13=Duncan J. |last13=Wright |first14=Graeme S. |last14=Salter |first15=David |last15=Pinfield |date=March 2013 |title=Signals embedded in the radial velocity noise: periodic variations in the τ Ceti velocities |doi=10.1051/0004-6361/201220509 |url=http://star-www.herts.ac.uk/~hraj/tauceti/paper.pdf |arxiv = 1212.4277 |bibcode = 2013A&A...551A..79T |journal=Astronomy & Astrophysics |volume=551|pages=A79 |s2cid=2390534 }}

align=right|{{val|2.05|e=1|u=ly}}

|Gliese 581, a red-dwarf star with several detectable exoplanets.

align=right|{{val|3.1|e=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.

rowspan=4| 10³

|align=right|{{val|3|e=3|u=ly}}

|A0620-00, the second-nearest known black hole, is about {{val|3000}} light-years away.

align=right|{{val|2.6|e=4|u=ly}}

|The centre of the Milky Way is about {{val|26000}} light-years away.{{Citation|doi= 10.1086/380188|title= A Geometric Determination of the Distance to the Galactic Center|date= 2003|last1= Eisenhauer|first1= F.|last2= Schdel|first2= R.|last3= Genzel|first3= R.|last4= Ott|first4= T.|last5= Tecza|first5= M.|last6= Abuter|first6= R.|last7= Eckart|first7= A.|last8= Alexander|first8= T.|journal= The Astrophysical Journal|volume= 597|issue= 2|pages= L121|arxiv = astro-ph/0306220 |bibcode = 2003ApJ...597L.121E |s2cid= 16425333}}{{citation|doi=10.1086/316512|title=The Distance to the Galactic Center|date=2000|last1=McNamara|first1=D. H.|author-link1=D. Harold McNamara|last2=Madsen|first2=J. B.|last3=Barnes|first3=J.|last4=Ericksen|first4=B. F.|journal=Publications of the Astronomical Society of the Pacific|volume=112|issue=768|pages=202 |bibcode = 2000PASP..112..202M |doi-access=free}}

align=right|{{val|1|e=5|u=ly}}

|The Milky Way is about {{val|100000}} light-years across.

align=right|{{val|1.65|e=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.

rowspan=4| 10⁶

|align=right|{{val|2.5|e=6|u=ly}}

|The Andromeda Galaxy is approximately 2.5 million light-years away.

align=right|{{val|3|e=6|u=ly}}

|The Triangulum Galaxy (M33), at about 3 million light-years away, is the most distant object visible to the naked eye.

align=right|{{val|5.9|e=7|u=ly}}

|The nearest large galaxy cluster, the Virgo Cluster, is about 59 million light-years away.

align=right|{{val|1.5|e=8}} – {{val|2.5|e=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).

rowspan=3| 10⁹

|align=right|{{val|1.2|e=9|u=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.

align=right|{{val|2.4|e=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.

align=right|{{val|4.57|e=10|u=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 {{val|299792458}} metres or {{sfrac|{{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=1021–26 |bibcode=2000PASJ...52.1021F |doi=10.1093/pasj/52.6.1021 |url-status=dead |archive-url=https://web.archive.org/web/20090902035920/http://sciencelinks.jp/j-east/article/200123/000020012301A0179284.php |archive-date=2 September 2009 |doi-access=free }}{{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.[http://www.haydenplanetarium.org/universe/duguide/app_light_travel_time_dista.php 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.

{{cite book

|author=David Mermin

|title=It's About Time: Understanding Einstein's Relativity

|url=https://books.google.com/books?id=rKFhqlzjv-IC&pg=PA22

|page=22

|year=2009

|location=Princeton, New Jersey

|publisher=Princeton University Press

|isbn=978-0-691-14127-5

}}