<Tr> <Th> imperial & US units </Th> <Td> 7011149598180864000 ♠ 9.2956 × 10 mi </Td> </Tr> <Tr> <Th> astronomical units </Th> <Td> 7011149596734827110 ♠ 4.8481 × 10 pc 7011149602530962920 ♠ 1.5813 × 10 ly </Td> </Tr> <P> The astronomical unit (symbol: au, ua, or AU) is a unit of length, roughly the distance from Earth to the Sun . However, that distance varies as 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, since 2012 it has been defined as exactly 7011149597870700000 ♠ 149 597 870 700 metres or about 150 million kilometres (93 million miles). The astronomical unit is used primarily 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 . </P> <P> A variety of unit symbols and abbreviations have been in use for the astronomical unit . In a 1976 resolution, the International Astronomical Union (IAU) used the symbol A for the astronomical unit . In the astronomical literature, the symbol AU was (and remains) common . In 2006, the International Bureau of Weights and Measures (BIPM) recommended ua as the symbol for the unit . In the non-normative Annex C to ISO 80000 - 3 (2006), the symbol of the astronomical unit is "ua". 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". </P>

The mean distance between the earth and the sun is called the