|Unit system||astronomical units|
|1 ly in ...||... is equal to ...|
|metric (SI) units||9.4607×1015 m|
|imperial & US units||5.8786×1012 Script error: No such module "convert".|
6.3241×104 Script error: No such module "convert".|
0.3066 Script error: No such module "convert".
|40x40px||Look up light year in Wiktionary, the free dictionary.|
A light-year (abbreviation: l.y.), sometimes written light year, is a unit of length used informally to express astronomical distances. It is approximately 9 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. Because it includes the word year, the term light-year is sometimes misinterpreted as a unit of time.
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).
As defined by the IAU, the light-year is the product of the Julian year[note 2] (365.25 days as opposed to the 365.2425-day Gregorian year) and the speed of light (299792458Lua error: Unmatched close-bracket at pattern character 67.).[note 3] Both these values are included in the IAU (1976) System of Astronomical Constants, used since 1984. From this the following conversions can be derived. The internationally recognized abbreviation for light-year is "l.y.").
1 light-year = Script error: No such module "Gaps". metres (exactly) ≈ 9.461 petametres ≈ 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. The product of Simon Newcomb's J1900.0 mean tropical year of 31556925.9747 ephemeris seconds and a speed of light of 299792.5Lua error: Unmatched close-bracket at pattern character 67. produced a light-year of 9.460530×1015Lua error: Unmatched close-bracket at pattern character 67. (rounded to the seven significant digits in the speed of light) found in several modern sources was probably derived from an old source such as C. W. Allen's 1973 Astrophysical Quantities reference work, which was updated in 2000.[clarification needed]
Other high-precision values are not derived from a coherent IAU system. A value of 9.460536207×1015Lua error: Unmatched close-bracket at pattern character 67. found in some modern sources is the product of a mean Gregorian year (365.2425 days or 31556952Lua error: Unmatched close-bracket at pattern character 67.) and the defined speed of light (299792458Lua error: Unmatched close-bracket at pattern character 67.). Another value, 9.460528405×1015Lua error: Unmatched close-bracket at pattern character 67., is the product of the J1900.0 mean tropical year and the defined speed of light.
The light-year unit appeared a few years after the first successful measurement of the distance to a star other than our Sun, by Friedrich Bessel in 1838. The star was 61 Cygni, and he used a Script error: No such module "convert". heliometer 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 (1.50×108Lua error: Unmatched close-bracket at pattern character 67. or 9.30×107Lua error: Unmatched close-bracket at pattern character 67.). 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 (9.9×1013Lua error: Unmatched close-bracket at pattern character 67. or 6.1×1013Lua error: Unmatched close-bracket at pattern character 67.). Bessel added that light employs 10.3 years to traverse this 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 resented expressing distances in light-years because it would deteriorate 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; 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, however, in 1851 in a German popular astronomical article by Otto Ule. The paradox of a distance unit name ending on year was explained by Ule by comparing it to a hiking road hour (Wegstunde). A contemporary German popular astronomical book also noticed that light-year is an odd name. In 1868 an English journal labelled the light-year as a unit used by the Germans. Eddington called the light-year an inconvenvient and irrelevant unit, which had sometimes crept from popular use into technical investigations.
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.
|10−9||40.4×10−9Lua error: Unmatched close-bracket at pattern character 67.||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||15.8×10−6Lua error: Unmatched close-bracket at pattern character 67.||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.|
|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||2.04×10−3Lua error: Unmatched close-bracket at pattern character 67.||The most distant space probe, Voyager 1, was about 18 light-hours away from the Earth as of October 2014[update]. It will take about 17500 years to reach one light-year (1.0×100Lua error: Unmatched close-bracket at pattern character 67.) at its current speed of about 17 km/s (38000Lua error: Unmatched close-bracket at pattern character 67.) relative to the Sun. On September 12, 2013, NASA scientists announced that Voyager 1 had entered the interstellar medium of space on August 25, 2012, becoming the first manmade object to leave the Solar System.|
|100||1.6×100Lua error: Unmatched close-bracket at pattern character 67.||The Oort cloud is approximately two light-years in diameter. Its inner boundary is speculated to be at 50000Lua error: Unmatched close-bracket at pattern character 67., with its outer edge at 100000Lua error: Unmatched close-bracket at pattern character 67..|
|2.0×100Lua error: Unmatched close-bracket at pattern character 67.||Maximum extent of the Sun's gravitational dominance (Hill sphere/Roche sphere, 125000Lua error: Unmatched close-bracket at pattern character 67.). Beyond this is the deep ex-solar gravitational interstellar medium.|
|4.22×100Lua error: Unmatched close-bracket at pattern character 67.||The nearest known star (other than our Sun), Proxima Centauri, is about 4.22 light-years away.|
|8.60×100Lua error: Unmatched close-bracket at pattern character 67.||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.|
|11.90×100Lua error: Unmatched close-bracket at pattern character 67.||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.|
|20.5×100Lua error: Unmatched close-bracket at pattern character 67.||Gliese 581, a red-dwarf star with several detectable exoplanets.|
|310×100Lua error: Unmatched close-bracket at pattern character 67.||Canopus, second in brightness in the terrestrial sky only to Sirius, a type F supergiant 15000 times more luminous than the Sun.|
|103||26×103Lua error: Unmatched close-bracket at pattern character 67.||The centre of our galaxy, the Milky Way, is about 26000 light-years away.|
|100×103Lua error: Unmatched close-bracket at pattern character 67.||The Milky Way is about 100000 light-years across.|
|165×103Lua error: Unmatched close-bracket at pattern character 67.||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||2.5×106Lua error: Unmatched close-bracket at pattern character 67.||The Andromeda Galaxy is approximately 2.5 million light-years away.|
|3×106Lua error: Unmatched close-bracket at pattern character 67.||The Triangulum Galaxy (M33), at about 3 million light-years away, is the most distant object visible to the naked eye.|
|59×106Lua error: Unmatched close-bracket at pattern character 67.||The nearest large galaxy cluster, the Virgo Cluster, is about 59 million light-years away.|
|150×106 – 250×106Lua error: Unmatched close-bracket at pattern character 67.||The Great Attractor lies at a distance of somewhere between 150 and 250 million light-years (the latter being the most recent estimate).|
|109||1.2×109Lua error: Unmatched close-bracket at pattern character 67.||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.|
|2.4×109Lua error: Unmatched close-bracket at pattern character 67.||3C 273, optically the brightest quasar, of apparent magnitude 12.9, just dimmer than R136a1.|
|45.7×109Lua error: Unmatched close-bracket at pattern character 67.||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.|
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 299792458 metres or 1⁄31557600 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. The Hayden Planetarium specifies the light month more precisely as 30 days of light travel time.
Unknown extension tag "indicator"
- 1 petametre (examples of distances on the order of one light-year)
- Einstein protocol
- Hubble length
- Orders of magnitude (length)
- One trillion here is taken to be 1012 (one million million).
- One Julian year is of exactly 365.25 days (or 31557600Lua error: Unmatched close-bracket at pattern character 67. based on a day of exactly 86400 SI seconds)
- The speed of light is exactly 299792458Lua error: Unmatched close-bracket at pattern character 67. by definition of the metre.
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