# Astronomical unit

mean distance between Earth and the Sun, common length reference in astronomy

The astronomical unit (AU) is a unit of length derived from the Earth's orbit. It is the average distance the Earth gets from the Sun on the long axis of the ellipse. Its definition is: the length of the semi-major axis of the Earth's elliptical orbit around the Sun. "Semi-major" means half the long axis.

Astronomical unit The average distance between the Sun and the Earth is 1 AU
General information
Unit systemAstronomical system of units
(Accepted for use with the SI)
Unit oflength
Symbolau, ua, or AU
Conversions
1 au, ua, or AU in ...... is equal to ...
metric (SI) units   149597870700 m
imperial & US units   9.2956×107 mi
astronomical units   4.8481×10−6 pc
1.5813×10−5 ly

The AU is about 150 million kilometers or 93 million miles. Astronomers usually measure distances within the Solar System in astronomical units. Mars is about 1.4 AU from the Sun, Jupiter lies at roughly 5.2 AU, and Neptune is roughly 30 AU from the Sun. Light travels an AU in about 8.317 minutes.

## More accuracy

From 1976 to 2012 the AU was defined as “the radius of an unperturbed circular Newtonian orbit about the Sun of a particle having infinitesimal mass, moving with a mean motion of 0.01720209895 radians per day (known as the Gaussian constant)”.  In 2012, the IAU redefined it to be simply 149597870700 m.

In the IERS numerical standards, the speed of light in a vacuum is defined as c0 = 299792458 m/s, in accordance with the SI units. The time to cover an AU is τA = 499.0047838061±0.00000001 s, resulting in the astronomical unit in metres as c0τA = 149597870700±3 m. It is very roughly the distance from the Earth to the Sun.