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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