Astronomical unit

One Astronomical unit (AU) is the mean distance of the Earth from the Sun (1.5x10^{11} m)

The light year

This is the distance that light travels in free space in one year = 9.5x10^{15}m

The Parsec

This is the distance at which an object subtends an angle of one second using the radius of the Earth’s orbit as the baseline.

1 Parsec = 3.06x10^{16} m = 2.04x10^{5} AU = 3.26 light years

1 Megaparsec = 3.26x10^{6} light years = 3.097x10^{22} m