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Unit system astronomical units
Unit of length
Symbol ly
Unit conversions
1 ly in... is equal to...
   SI units    9.4607×1015 m
   imperial & US units    5.8786×1012 mi
   other astronomical    6.3241×104 AU
      units    0.3066 pc

A light-year (symbol: ly), sometimes written light year or lightyear is a unit of length used in astronomy, though it is often mistaken for a unit of time. It is equal to just under 10 trillion kilometres (or about 6 trillion miles).note 1 As defined by the International Astronomical Union (IAU), a light-year is the distance that light travels in vacuum in one Julian year.1

The light-year is most often used when expressing distances to stars and other distances on a galactic scale, especially in non-specialist and popular science publications. The unit usually used in professional astrometry is the parsec (symbol: pc, approximately 3.26 light-years; the distance at which one astronomical unit subtends an angle of one second of arc).1


As defined by the IAU, the light-year is the product of the Julian yearnote 2 (365.25 days as opposed to the 365.2425-day Gregorian year) and the speed of light (299792458 m/s).note 3 Both these values are included in the IAU (1976) System of Astronomical Constants, used since 1984.3 From this the following conversions can be derived.

1 light-year = 9460730472580800 metres (exactly)
5.878625 trillion miles
63241.077 astronomical units
0.306601 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 1983.4 The product of Simon Newcomb's J1900.0 mean tropical year of 31556925.9747 ephemeris seconds and a speed of light of 299792.5 km/s produced a light-year of 9.460530×1015 m (rounded to the seven significant digits in the speed of light) found in several modern sources567 was probably derived from an old source such as C. W. Allen's 1973 Astrophysical Quantities reference work,8 which was updated in 2000.9clarification needed

Other high-precision values are not derived from a coherent IAU system. A value of 9.460536207×1015 m found in some modern sources1011 is the product of a mean Gregorian year (365.2425 days or 31556952 s) and the defined speed of light (299792458 m/s). Another value, 9.460528405×1015 m,1213 is the product of the J1900.0 mean tropical year and the defined speed of light.


The first successful measurement of the distance to a star other than our Sun was made 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 (AU), equal to the radius of the Earth's orbit (1.50×108 km or 9.30×107 mi). In those terms, trigonometric calculations based on 61 Cygni's parallax of 0.314 arcseconds, showed the distance to the star to be 660000 astronomical units (9.9×1013 km or 6.1×1013 mi).

James Bradley stated in 1729 that light travelled 10210 times faster than the Earth in its orbit. In 1769, a transit of Venus revealed the distance of the Earth from the Sun, and this, together with Bradley's figure, allowed the speed of light to be calculated as 3.01×108 m/s, very close to the modern value. Bessel used this speed to work out how far light would travel in a year, and announced that it would take light 10.3 years to cross the distance to 61 Cygni.14

Although modern astronomers often prefer to use the parsec, light years are also popularly used to gauge the expanses of interstellar and intergalactic space.

Distances in light-years

Distances measured in fractions of a light-year (or in light-months, etc.) usually involve objects within a star system. Distances measured in light-years include distances between nearby stars, such as those in the same spiral arm or globular cluster.

One kilolight-year, abbreviated "kly", is one thousand light-years (about 307 parsecs). Kilolight-years are typically used to measure distances between parts of a galaxy.

One megalight-year, abbreviated "Mly", is one million light-years (about 307 kiloparsecs). Megalight-years are typically used to measure distances between neighbouring galaxies and galaxy clusters.

One gigalight-year, abbreviation "Gly", is one billion light-years (about 307 megaparsecs)—one of the largest distance measures used. Gigalight-years are typically used to measure distances to supergalactic structures, including quasars and the Sloan Great Wall.

List of orders of magnitude for length
Scale (ly) Value Item
10−9 40.4×10−9 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 350000 to 400000 kilometres).
10−6 15.8×10−6 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.15
127×10−6 ly The Huygens probe lands on Titan off Saturn and transmits images from its surface 1.2 billion kilometres to the Earth.
10−3 1.95×10−3 ly The most distant space probe, Voyager 1, was about 17 light-hours away from the Earth as of December 2012. It will take about 17500 years to reach one light-year (1.0×100 ly) at its current speed of about 17 km/s (38000 mph) relative to the Sun.16 On June 15, 2012, NASA scientists stated that Voyager 1 may be very close to entering the interstellar medium of space, becoming the first manmade object to leave the Solar System and is expected to do so before 2015.
100 1.6×100 ly The Oort cloud is approximately two light-years in diameter. Its inner boundary is speculated to be at 50000 AU, with its outer edge at 100000 AU.
2.0×100 ly Maximum extent of the Sun's gravitational dominance (hill sphere/roche sphere, 125000 AU). Beyond this is the deep ex-solar gravitational interstellar medium.
4.22×100 ly The nearest known star (other than our Sun), Proxima Centauri, is about 4.22 light-years away.1718
8.60×100 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.
11.90×100 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.1920
20.5×100 ly Gliese 581g, the first discovered extrasolar candidate for habitable planet. Three or four times as massive as the Earth, it is in the middle of the habitable zone of star Gliese 581.21
310×100 ly Canopus, second in brightness in the terrestrial sky only to Sirius, a type F supergiant 15000 times more luminous than the Sun.
103 26×103 ly The centre of our galaxy, the Milky Way, is about 26 kilolight-years away.2223
100×103 ly The Milky Way is about 100000 light-years across.
165×103 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 2.5×106 ly The Andromeda Galaxy is approximately 2.5 megalight-years away.
3×106 ly The Triangulum Galaxy (M33), at about 3 megalight-years away, is the most distant object visible to the naked eye.
59×106 ly The nearest large galaxy cluster, the Virgo Cluster, is about 59 megalight-years away.
150×106250×106 ly The Great Attractor lies at a distance of somewhere between 150 and 250 megalight-years (the latter being the most recent estimate).
109 1.2×109 ly The Sloan Great Wall (not to be confused with the Great Wall) has been measured to be approximately one gigalight-year distant.
2.4×109 ly 3C 273, optically the brightest quasar, of apparent magnitude 12.9, just dimmer than R136a1.
45.7×109 ly The comoving distance from the Earth to the edge of the visible universe is about 45.7 gigalight-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 size of the universe: misconceptions for why this is possible.

Related units

Other units of length can similarly be formed by multiplying units of time by the speed of light. For example, the light-second, useful in astronomy, telecommunications and relativistic physics, is exactly 299792458 metres or 131557600 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.2425 The Hayden Planetarium specifies the light month more precisely as 30 days of light travel time.26

Light travels approximately one foot in a nanosecond; the term "light-foot" is sometimes used as an informal measure of time, and is about a nanosecond.

See also


  1. ^ One trillion here is taken to be 1012 (one million million).
  2. ^ One Julian year is of exactly 365.25 days (or 31557600 s based on a day of exactly 86400 SI seconds)2
  3. ^ The speed of light is exactly 299792458 m/s by definition of the metre.


  1. ^ a b International Astronomical Union, "Measuring the Universe: The IAU and Astronomical Units", retrieved November 10, 2013 
  2. ^ IAU Recommendations concerning Units 
  3. ^ Astronomical Constants  page K6 of the Astronomical Almanac.
  4. ^ P. Kenneth Seidelmann, ed. (1992), Explanatory Supplement to the Astronomical Almanac, Mill Valey, California: University Science Books, p. 656, ISBN 0-935702-68-7 
  5. ^ Basic Constants, Sierra College 
  6. ^ Marc Sauvage, Table of astronomical constants 
  7. ^ Robert A. Braeunig, Basic Constants 
  8. ^ C. W. Allen (1973), Astrophysical Quantities (third ed.), London: Athlone, p. 16, ISBN 0-485-11150-0 
  9. ^ Arthur N. Cox, ed. (2000), Allen's Astrophysical Quantities (fourth ed.), New York: Springer-Valeg, p. 12, ISBN 0-387-98746-0 
  10. ^ Nick Strobel, Astronomical Constants 
  11. ^ KEKB, Astronomical Constants 
  12. ^ Thomas Szirtes (1997), Applied dimensional analysis and modeling, New York: McGraw-Hill, p. 60, ISBN 9780070628113 
  13. ^ Sun, Moon, and Earth: Light-year 
  14. ^ "A brief history of light years". National Geographic. Retrieved 14 August 2013. 
  15. ^ "Chapter 1, Table 1-1", IERS Conventions (2003) 
  16. ^ Voyager Mission Operations Status Report # 2011-02-04, Week Ending February 4, 2011 
  17. ^ NASA, Cosmic Distance Scales - The Nearest Star 
  18. ^ "Proxima Centauri (Gliese 551)", Encyclopedia of Astrobiology, Astronomy, and Spaceflight 
  19. ^ BBC News 19 December 2012 Tau Ceti's planets nearest around single, Sun-like star
  20. ^ "Signals embedded in the radial velocity noise — Periodic variations in the τ Ceti velocities" M. Tuomi, H. R. A. Jones, J. S. Jenkins, C. G. Tinney, R. P. Butler, S. S. Vogt, J. R. Barnes, R. A. Wittenmyer, S. O’Toole, J. Horner, J. Bailey, B. D. Carter, D. J. Wright, G. S. Salter, and D. Pinfield of various of University of Hertfordshire, Centre for Astrophysics Research, Science and Technology Research Institute, College Lane, AL10 9AB, Hatfield, UK University of Turku, Tuorla Observatory, Department of Physics and Astronomy, V¨ais¨al¨antie 20, FI-21500, Piikki¨o, Finland Departamento de Astronomıa, Universidad de Chile, Camino del Observatorio 1515, Las Condes, Santiago, Chile; School of Physics, University of New South Wales, 2052, Sydney, Australia, Australian Centre for Astrobiology (ibid) Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC 20015, USA; UCO Lick Observatory, Department of Astronomy and Astrophysics, University of California at Santa Cruz, Santa Cruz, CA 95064, December 14, 2012]
  21. ^ Confirmed Planets Catalogue
  22. ^ 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 
  23. ^ 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 
  24. ^ 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...52.1021F 
  25. ^ 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 
  26. ^ Light-Travel Time and Distance by the Hayden Planetarium Accessed October 2010.

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