The
astronomical unit (symbol
au,
AU or
ua) 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, 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.