Astronomical unit

The astronomical unit (symbol: au, or AU) is a unit of length, roughly the distance from Earth to the Sun and approximately equal to 150 billion metres (93 million miles) or 8.3 light-minutes. 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 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.

This article is about the unit of length. For constants, see astronomical constant. For units in astronomy, see astronomical system of units. For other uses of "AU", see Au.

Astronomical unit
The grey line indicates the Earth–Sun distance, which on average is about 1 astronomical unit.
General information
Unit systemAstronomical system of units
(Accepted for use with the SI)
Unit oflength
Symbolau or AU or AU
Conversions
1 au or AU or AU in ...... is equal to ...
   metric (SI) units   1.495978707×1011 m
   imperial & US units   9.2956×107 mi
   astronomical units   4.8481×10−6 pc
   1.5813×10−5 ly
   215.03 R☉

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.

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