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A light-year, also light year or lightyear (symbol: ly) is a unit of length, equal to just under 10 trillion kilometres (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 (approximately 3.26 light-years), because it can be more easily derived from, and compared with, observational data.^[1]

Numerical value[link]

1 light-year = 9460730472580800 metres (exactly)

  ≈ 5878625 milllion miles

  ≈ 63241 astronomical units

  ≈ 0.306601 parsecs

The figures above are based on a Julian year (not Gregorian year) of exactly 365.25 days (each of exactly 86400 SI seconds, totalling 31557600 seconds)^[2] and a defined speed of light of 299792458 m/s, both included in the IAU (1976) System of Astronomical Constants, used since 1984.^[3]

Other values[link]

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 31556925.9747 ephemeris seconds and a speed of light of 299792.5 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 C. W. Allen's 1973 Astrophysical Quantities reference work,^[8] which was updated in 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 (31556952 s) and the defined speed of light (299792458 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.

History[link]

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 thousand astronomical units (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 10210 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[link]

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 light-years—one of the largest distance measures used. Gigalight-years are typically used to measure distances to supergalactic structures, including quasars and the Sloan Great Wall.

Related units[link]

Other units of length can similarly be formed 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.^[20][21] The Hayden Planetarium specifies the light month more precisely as 30 days of light travel time.^[22]

See also[link]

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• Einstein protocol
• 1 petametre (examples of distances on the order of one light-year)
• Orders of magnitude (length)
• Hubble length

References[link]

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