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.
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.
|9.461×1012 km||9.461×1015 m|
|63.24×103 AU||0.3066 pc|
|US customary / Imperial units|
|5.879×1012 mi||31.04×1015 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) 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. The DE405 value of the astronomical unit, 149,597,870,691 m, 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.).
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 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 was probably derived from an old source such as a reputable 1973 reference which was not updated until 2000.
Other high precision values are not derived from a coherent IAU system. A value of 9.460536207 × 1015 metres found in some modern sources 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, is the product of the J1900.0 mean tropical year and the defined speed of light.
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 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.
|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.|
|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 . It took that space probe 30 years to cover that distance, 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.|
|103||26 × 103 ly||The centre of our galaxy, the Milky Way, is about 26 kilolight-years away.|
|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.|