Astronomical Unit

Distances in the Solar System are very large! To compare the average distances between the Sun and the planets, it’s convenient to do it in terms of the average Earth-Sun separation.

A very useful approximate definition of the astronomical unit (AU) is:

1 AU = average distance between Sun and Earth = 1.496 × 108km


More accurately, the IAU has defined the AU as:

“equal to the distance from the centre of the Sun at which a particle of negligible mass, in an unperturbed circular orbit, would have an orbital period of 365.2568983 days.”

This is slightly less than the mean Sun-Earth distance.

See also: parsec.

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