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 In the non-normative Annex C to ISO 80000-3:2006 (now withdrawn), the symbol of the astronomical unit was also ua. In 2006, the International Bureau of Weights and Measures (BIPM) had recommended ua as the symbol for the unit, from the French "unité astronomique". In the astronomical literature, the symbol AU was (and remains) common. In a 1976 resolution, the International Astronomical Union (IAU) had used the symbol A to denote a length equal to the astronomical unit. ![]() History of symbol usage Ī variety of unit symbols and abbreviations have been in use for the astronomical unit. It is also a fundamental component in the definition of another unit of astronomical length, the parsec. The astronomical unit is used primarily for measuring distances within the Solar System or around other stars. The astronomical unit was originally conceived as the average of Earth's aphelion and perihelion however, since 2012 it has been defined as exactly 149 597 870 700 m. The actual distance from Earth to the Sun varies by about 3% as Earth orbits the Sun, from a maximum ( aphelion) to a minimum ( perihelion) and back again once each year. The astronomical unit (symbol: au, or AU or AU) is a unit of length, roughly the distance from Earth to the Sun and approximately equal to 150 million kilometres (93 million miles) or 8.3 light-minutes. The grey line indicates the Earth–Sun distance, which on average is about 1 astronomical unit.
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