Astronomical unit

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.

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)". Resolution No. 10 of the XVIth General Assembly of the International Astronomical Union , Grenoble, 1976 In 2012, the IAU redefined it to be the exact value .

In the IERS numerical standards, the speed of light in a vacuum is defined as c0 = , in accordance with the SI units. The time to cover an AU is τA = , resulting in the astronomical unit in metres as c0τA = . For complete document see  It is very roughly the distance from the Earth to the Sun.
 References 

Category:Astronomical distances
Category:Units of length