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astronomical unit
The astronomical unit (symbol au, AU〔〔 or ua〔ISO 80000-3, Quantities and units - Space and time〕) is a unit of length, roughly the distance from the Earth to the Sun. However, that distance varies as the Earth orbits the Sun, from a maximum (aphelion) to a minimum (perihelion) and back again once a year. Originally conceived as the average of Earth's aphelion and perihelion, it is now defined as exactly metres (about 150 million kilometres, or 93 million miles). The astronomical unit is used primarily as a convenient yardstick for measuring distances within the Solar System or around other stars. However, it is also a fundamental component in the definition of another unit of astronomical length, the parsec.
==Symbol usage==
A variety of unit symbols and abbreviations are in use for the astronomical unit. In a 1976 resolution, the International Astronomical Union (IAU) used the symbol ''A'' for the AU.〔 In 2006, the International Bureau of Weights and Measures (BIPM) recommended ''ua'' as the symbol for the unit. In 2012, the IAU, noting "that various symbols are presently in use for the astronomical unit", recommended the use of the symbol "au".〔 In the 2014 revision of the SI Brochure, the BIPM used the unit symbol "au". The symbol "AU"〔 and abbreviation ''a.u.'' are also used. In ISO 80000-3, the symbol of the astronomical unit is "ua".
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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