The basic unit of length in the solar system. The astronomical unit (AU) is also used to a limited extent for interstellar distances through the definition of the parsec (1 pc = 206,265 AU). It is nearly equal to the mean distance a between the center of mass of the Sun and the center of mass of the Earth-Moon system (a = 1.00 000 23 AU), and for that reason it is often convenient to think of it as the mean distance between the Sun and Earth.