Astronomical Unit
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Astronomical Unit
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>n^{2}a^{3} =
k^{2}(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.
References
This article is based on NASA's Dictionary of Technical Terms for Aerospace Use