light-year: Definition from Answers.com

Distances from the Earth to some celestial objects
Object	Distance from Earth (in light time)
Moon (mean)	1.3 s
Sun (mean)	8.3 min
Mars (closest)	3.1 min
Jupiter (closest)	33 min
Pluto (closest)	5.3 h
Nearest star (Proxima Centauri)	4.3 years
Andromeda Nebula (M31)	2,300,000 years

For other uses, see Light year (disambiguation).

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

Look up light year in Wiktionary, the free dictionary.

As defined by the International Astronomical Union (IAU), a light-year ( or light year) is the distance that light travels in a vacuum in one Julian year.^[1] The speed of light is 299,792,458 metres per second. Therefore, a light-year (symbol: ly) is a unit of length, equal to just under 10¹³ kilometres.

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]

1 Numerical value
- 1.1 Other values
2 Distances in light-years
3 See also
4 References

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:

exactly 9,460,730,472,580.8 km (about 10 Pm)
about 5,878,630,000,000 miles
about 63,241.1 astronomical units
about 0.306601 parsecs

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 Live Search value, 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)^[update]. 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 center of our galaxy, the Milky Way, is about 26 kilolight-years away.^[20]^[21]
10³	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.
10⁹	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.

References

^ ^a ^b The IAU and astronomical units, International Astronomical Union, http://www.iau.org/public_press/themes/measuring/, retrieved 5 July 2008
^ 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, http://sciencelinks.jp/j-east/article/200123/000020012301A0179284.php
^ 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, http://articles.adsabs.harvard.edu/full/1994cers.conf...97J
^ IAU Recommendations concerning Units
^ Astronomical Constants page K6 of the Astronomical Almanac.
^ USNO Circular 179 page 32.
^ P. Kenneth Seidelmann, ed., Explanatory Supplement to the Astronomical Almanac (Mill Valey, California: University Science Books, 1992) 656. ISBN 0-935702-68-7
^ Sierra College, Basic Constants
^ Marc Sauvage, Table of astronomical constants
^ Robert A. Braeunig, Basic Constants
^ C. W. Allen, Astrophysical Quantities (third edition, London: Athlone, 1973) 16. ISBN 0-485-11150-0
^ Arthur N. Cox, ed., Allen's Astrophysical Quantities (fourth edition, New York: Springer-Valeg, 2000) 12. ISBN 0-387-98746-0
^ Nick Strobel, Astronomical Constants
^ KEKB Astronomical Constants
^ Microsoft's Live Search convert light year to meters
^ IERS Conventions (2003), Chapter 1, Table 1-1.
^ NASA pressrelease (05-131) 24 May 2005: Voyager Mission Operations Status Report Week Ending March 9, 2007
^ NASA: Cosmic Distance Scales - The Nearest Star
^ Proxima Centauri (Gliese 551), Encyclopedia of Astrobiology, Astronomy, and Spaceflight
^ F. Eisenhauer, et al., "A Geometric Determination of the Distance to the Galactic Center" (pdf, 93KB), Astrophysical Journal 597 (2003) L121-L124
^ 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.

v • d • e Units of length used in Astronomy

Earth radius ( $\scriptstyle R_\oplus$ or RE) · Astronomical unit (AU) · light-year (ly) · parsec (pc) · kiloparsec (kpc) · megaparsec (Mpc) · gigaparsec (Gpc)

See Also: Cosmic distance ladder · Orders of magnitude (length) · Conversion of units

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Contents

Numerical value

Other values

Distances in light-years

See also

References