Image showing the scale of a light-year (outer shell) and a light month (inner shell) surrounding the Sun compared to Comet 1910 A1's orbit (yellow line) and Comet Hyakutake's orbit (orange line).

A light-year, also light year or lightyear (symbol: ly) is a unit of length, equal to just under 10 trillion kilometres (10×1015 metres, 10 petametres or about 6 trillion miles). As defined by the International Astronomical Union (IAU), a light-year is the distance that light travels in a vacuum in one Julian year.[1]

The light-year is often used to measure distances to stars and other distances on a galactic scale, especially in non-specialist and popular science publications. The preferred unit in astrometry is the parsec, because it can be more easily derived from, and compared with, observational data. The parsec is defined as the distance at which an object will appear to move one arcsecond of parallax when the observer moves one astronomical unit perpendicular to the line of sight to the observer, and is equal to approximately 3.26 light-years.[1]


Numerical value

1 light-year =
SI units
9.461×10^12 km 9.461×10^15 m
Astronomical units
63.24×10^3 AU 0.3066 pc
US customary / Imperial units
5.879×10^12 mi 31.04×10^15 ft

One light-year is equal to:

The figures above are based on a Julian year (not Gregorian year) of exactly 365.25 days (each of exactly 86,400 SI seconds, totalling 31,557,600 seconds)[2] and a defined speed of light of 299,792,458 m/s, both included in the IAU (1976) System of Astronomical Constants, used since 1984.[3]

Other values

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 31,556,925.9747 ephemeris seconds and a speed of light of 299,792.5 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 sources[5][6][7] was probably derived from an old source such as a reputable 1973 reference[8] which was not updated until 2000.[9]

Other high-precision values are not derived from a coherent IAU system. A value of 9.460536207×1015 m found in some modern sources[10][11] is the product of a mean Gregorian year of 365.2425 days (31,556,952 s) and the defined speed of light (299,792,458 m/s). Another value, 9.460528405×1015 m,[12][13] 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 superlative 6.2-inch (160 mm) heliometer designed by Joseph von Fraunhofer. The largest unit for measuring distances across space at that time was the Astronomical Unit (AU), equal to the radius of the Earth's orbit (1.50×108 km; 9.30×107 mi). The use of this unit in trigonometric calculations based on 61 Cygni's parallax of 0.314 arcseconds, gave the distance to the star as 660,000 AU (9.9×1013 km; 6.1×1013 mi). Bessel realised that a much larger unit of measurement was needed to make the vast interstellar distances comprehensible.

James Bradley had stated in 1729 that light travelled 10,210 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 the distance to 61 Cygni was 10.3 light-years. This was the first appearance of the light-year as a measurement of distance, and, although modern astronomers prefer the parsec, it is 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) 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 (109) light-years—one of the largest distance measures used. Gigalight-years are typically used to measure distances to supergalactic structures, including quasars and the 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. (The surface of the Moon is roughly 376,300 kilometres from the surface of the Earth, on average. 376,300 km ÷ 300,000 km/s (roughly the speed of light) ≈ 1.25 seconds)
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.[14]
127×10−6 ly The Huygens probe lands on Titan and transmits images from its surface 1200 million kilometres to the Earth.
10−3 3.2×10−3 ly The most distant space probe, Voyager 1, was about 16 light-hours away from the Earth as of 2011. It will take about 17,500 years to reach one light-year at its current speed of about 17 km/s (38,000 mph) relative to the Sun.[15]
100 1.6×100 ly The Oort cloud is approximately two light-years in diameter. Its inner boundary is speculated to be at 50,000 AU, with its outer edge at 100,000 AU.
2.0×100 ly Maximum extent of the Sun's gravitational dominance (hill sphere/roche sphere, 125,000 AU). Beyond this is the true interstellar medium.
4.22×100 ly The nearest known star (other than the Sun), Proxima Centauri, is about 4.22 light-years away.[16][17]
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.
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.
310×100 ly Canopus, second in brightness in the terrestrial sky only to Sirius, a type F supergiant 15,000 times more luminous than the Sun.
103 26×103 ly The centre of our galaxy, the Milky Way, is about 26 kilolight-years away.[18][19]
100×103 ly The Milky Way is about 100,000 light-years across.
165×103 ly R136a1, in the Large Magellanic Cloud, the most luminous star known at 8,700,000 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.
150×106 – 250×106 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.
46.5×109 ly The comoving distance from the Earth to the edge of the visible universe is about 46.5 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.


The related unit of the light-month, roughly one-twelfth of a light-year, is also used occasionally for approximate measures.[20][21] The Hayden Planetarium specifies the light month more precisely as 30 days of light travel time.[22]

See also


  1. ^ a b The IAU and astronomical units, International Astronomical Union,, retrieved 2008-07-05 
  2. ^ IAU Recommendations concerning Units
  3. ^ Astronomical Constants page K6 of the Astronomical Almanac.
  4. ^ P. Kenneth Seidelmann, ed., Explanatory Supplement to the Astronomical Almanac (Mill Valey, California: University Science Books, 1992) 656. ISBN 0-935702-68-7
  5. ^ Sierra College, Basic Constants
  6. ^ Marc Sauvage, Table of astronomical constants
  7. ^ Robert A. Braeunig, Basic Constants
  8. ^ C. W. Allen, Astrophysical Quantities (third edition, London: Athlone, 1973) 16. ISBN 0-485-11150-0
  9. ^ Arthur N. Cox, ed., Allen's Astrophysical Quantities (fourth edition, New York: Springer-Valeg, 2000) 12. ISBN 0-387-98746-0
  10. ^ Nick Strobel, Astronomical Constants
  11. ^ KEKB Astronomical Constants
  12. ^ Thomas Szirtes, Applied dimensional analysis and modeling (New York: McGraw-Hill, 1997) 60.
  13. ^ Sun, Moon, and Earth: Light-year
  14. ^ IERS Conventions (2003), Chapter 1, Table 1-1.
  15. ^ Voyager Mission Operations Status Report # 2011-02-04, Week Ending February 4, 2011
  16. ^ NASA: Cosmic Distance Scales - The Nearest Star
  17. ^ Proxima Centauri (Gliese 551), Encyclopedia of Astrobiology, Astronomy, and Spaceflight
  18. ^ F. Eisenhauer, et al., "A Geometric Determination of the Distance to the Galactic Center" (pdf, 93KB), Astrophysical Journal 597 (2003) L121-L124
  19. ^ McNamara, D. H., et al., "The Distance to the Galactic Center" (pdf, 298KB), The Publications of the Astronomical Society of the Pacific, 112 (2000), pp. 202–216.
  20. ^ 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, 
  21. ^ 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 
  22. ^ Light-Travel Time and Distance by the Hayden Planetarium Accessed October 2010.

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