An astronomical unit (abbreviated AU) is a unit of distance, originally defined as the semimajor axis of the earth's orbit, which is the mean distance of the earth from the sun. For greater precision, the IAU in 1976 defined it as as the distance from the sun at which a particle of negligible mass in an unperturbed orbit would have an orbital period of 365.2568983 days (a Gaussian year). The precise value is 149,597,870.66 km. For the layman, the distance is approximately that of the Earth to the sun.
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