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.