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Outer shell is one light year from Sun and left yellow line is Comet 1910 A1's orbit. Inner shell is one light month.

A light-year, also light year or lightyear, (symbol: ly) is a unit of length, equal to just under 10 trillion kilometres (i.e. 1016 metres). 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]

The related unit of the light-month, roughly one-twelfth of a light-year, is also used occasionally for approximate measures.[2][3]


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

A 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)[4] 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.[5] The DE405 value of the astronomical unit, 149,597,870,691 m,[6] is used for the light-year in astronomical units and parsecs.

The light-month does not have a precise definition or value, as there is no unique definition of a month in astronomical or SI units: however it may be understood to be approximately one-twelfth of a light-year, in line with the various mundane definitions of the month (hollow month, average Julian month, etc.).


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.[7] 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 metres (rounded to the seven significant digits in the speed of light) found in several modern sources[8][9][10] was probably derived from an old source such as a reputable 1973 reference[11] which was not updated until 2000.[12]

Other high precision values are not derived from a coherent IAU system. A value of 9.460536207 × 1015 metres found in some modern sources[13][14] 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). The value given by Microsoft's Bing, 9.460528405 × 1015 metres,[15] is the product of the J1900.0 mean tropical year and the defined speed of light.

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, or 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, or about 306,600 parsecs. Megalight-years are typically used to measure distances between neighboring galaxies and galaxy clusters.

One gigalight-year, abbreviation "Gly", is one billion light-years—one of the largest distance measures used. One gigalight-year is about 306.6 million parsecs. Gigalight-years are typically used to measure distances to supergalactic structures, including quasars and the Great Wall.

List of orders of magnitude for length
Factor (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.[16]
10-3 3.2 × 10−3 ly The most distant space probe, Voyager 1, was about 14 light-hours away from Earth as of 9 March 2007 (2007 -03-09). It took that space probe 30 years to cover that distance,[17] and will take over 18,000 years to reach one light-year at the same speed.
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.[18][19]
103 26 × 103 ly The centre of our galaxy, the Milky Way, is about 26 kilolight-years away.[20][21]
100 × 103 ly The Milky Way is about 100,000 light-years across.
106 2.5 × 106 ly The Andromeda Galaxy is approximately 2.5 megalight-years away.
3.14 × 106 ly The Triangulum Galaxy (M33), at 3.14 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.
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.

See also

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  1. ^ a b The IAU and astronomical units, International Astronomical Union,, retrieved 2008-07-05 
  2. ^ 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, 
  3. ^ 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, 
  4. ^ IAU Recommendations concerning Units
  5. ^ Astronomical Constants page K6 of the Astronomical Almanac.
  6. ^ USNO Circular 179 page 32.
  7. ^ P. Kenneth Seidelmann, ed., Explanatory Supplement to the Astronomical Almanac (Mill Valey, California: University Science Books, 1992) 656. ISBN 0-935702-68-7
  8. ^ Sierra College, Basic Constants
  9. ^ Marc Sauvage, Table of astronomical constants
  10. ^ Robert A. Braeunig, Basic Constants
  11. ^ C. W. Allen, Astrophysical Quantities (third edition, London: Athlone, 1973) 16. ISBN 0-485-11150-0
  12. ^ Arthur N. Cox, ed., Allen's Astrophysical Quantities (fourth edition, New York: Springer-Valeg, 2000) 12. ISBN 0-387-98746-0
  13. ^ Nick Strobel, Astronomical Constants
  14. ^ KEKB Astronomical Constants
  15. ^ Microsoft's Bing convert lightyear to metres
  16. ^ IERS Conventions (2003), Chapter 1, Table 1-1.
  17. ^ NASA pressrelease (05-131) 24 May 2005: Voyager Mission Operations Status Report Week Ending March 9, 2007
  18. ^ NASA: Cosmic Distance Scales - The Nearest Star
  19. ^ Proxima Centauri (Gliese 551), Encyclopedia of Astrobiology, Astronomy, and Spaceflight
  20. ^ F. Eisenhauer, et al., "A Geometric Determination of the Distance to the Galactic Center" (pdf, 93KB), Astrophysical Journal 597 (2003) L121-L124
  21. ^ 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.

Simple English

A light year (or light-year or lightyear) is not a length of time, but the distance that light will travel in one Julian year (365.25 days), going at the speed of light.[1] One light year is about 6 trillion miles, or 10 trillion kilometers. Because the universe is so big, some things are hundreds, thousands or millions of light years away. Because light that leaves a star 100 light years away will take 100 years to get to us, this means that when we see the star's light, we are actually seeing that star as it was 100 years ago. However, the length of time taken for light to reach the earth from the star varies due to the fact that some stars are further away from the earth than others.


  1. The IAU and astronomical units, International Astronomical Union,, retrieved 2008-07-05 
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