Astronomical unit One Astronomical unit (AU) is defined as 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 1 parsec 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 Mega parsec (Mpc) = 3.26x10^{6} light years = 3.097x10^{22} m |