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The astronomical unit (AU) is a unit adopted for measurement of interplanetary scale distances.

1 AU = 149597870.691 kilometers.

One astronomical unit is defined as equal to the semi-major axis of a circular orbit with a period of one Earth year. Its value is only 1500 kilometers less than the semi-major axis of Earth's actual orbit - which is elliptical, not circular.

The AU is accepted as a non-standard SI unit by the international General Conference on Weights and Measures.