Definitions for

**"A.U"**Abbr. See astronomical unit.

Astronomical unit. the radius of a circular orbit in which a body of negligible mass, and free of perturbations, would revolve around the Sun in 2 /k days, where k is the Gaussian gravitational constant. This is slightly less than the semimajor axis of the Earth's orbit.

Astronomical Unit. An Astronomical Unit is the mean distance between the Earth and the Sun, i.e. the Earth orbits at a distance of 1 AU from the Sun. It is equal to 149,597,870.691 kilometers.