astronomical unit (AU or a.u.)

A unit of measurement used mainly for distances within the solar system. It is effectively the mean distance between the Earth and the Sun, though it now has a formal definition independent of the Earth's orbit. Its value is 149,597,870 kilometres (92,955,730 miles), which is slightly less than the semimajor axis of the Earth's orbit. There are about 63,240 astronomical units in a light year.