Astronomical Unit

The astronomical unit (AU or au or a.u. or sometimes ua) is a unit of length based on the distance from the Earth to the Sun. The precise value of the AU is currently accepted as 149,597,870,691 ± 30 metres (nearly 150 million kilometres or 93 million miles).

The symbol ua is recommended by the Bureau International des Poids et Mesures but in anglophone countries the reverse - au - is more common. The International Astronomical Union recommends au and international standard ISO 31-1 uses AU. In general, capital letters are only used for the symbols of units which are named after individual scientists, while au or a.u. can also mean atomic unit or even arbitrary unit; however, the use of AU to refer to the astronomical unit is widespread.

Originally, the AU was defined as the length of the semi-major axis of the Earth's elliptical orbit around the Sun. In 1976, the International Astronomical Union revised the definition of the AU for greater precision, defining it as the distance from the centre of the Sun at which a particle of negligible mass, in an unperturbed circular orbit, would have an orbital period of 365.2568983 days (one Gaussian year). This definition gives a value that is slightly less than the mean Earth-Sun distance.