astronomical unit
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An astronomical unit (AU) is the average distance between Earth and the Sun, which astronomers use as a standard measurement for distances within our solar system. This reference point makes it easier to describe and compare how far planets, asteroids, and other objects are from the Sun without using extremely large numbers.
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The astronomical unit (symbol: au or AU) is a unit of length defined as 149597870700 m. Historically, the astronomical unit was conceived as the average Earth-Sun distance (the average of Earth's aphelion and perihelion), before its modern redefinition in 2012.
The astronomical unit is used primarily for measuring distances within the Solar System or around other stars. It is also a fundamental component in the definition of another unit of astronomical length, the parsec. One au is approximately equivalent to 499 light-seconds.