1. A unit of length, usually defined as the distance from the earth to the sun, 149,599,000 kilometers.
This value for the AU was derived from radar observations of the distance of Venus. The value given in astronomical ephemerides, 149,500,000 kilometers, was derived from observations of the minor planet Eros.
2. The unit of distance in terms of which, in the Kepler Third Law,<math>n2a3 = k2(1+m)</math>, the semimajor axis a of an elliptical orbit must be expressed in order that the numerical value of the Gaussian constant k may be exactly 0.01720209895 when the unit of time is the ephemeris day.
In astronomical units, the mean distance of the earth from the sun, calculated by the Kepler law from the observed mean motion n and adopted mass m, is 1.00000003.
This article is based on NASA's Dictionary of Technical Terms for Aerospace Use