An astronomical unit, abbreviated AU, is the average distance between Earth and the Sun, defined exactly as 149,597,870,700 meters, or close to a hundred and fifty million kilometers. It is the natural yardstick for the solar system: Jupiter orbits at about five AU, Neptune at thirty, and the Kuiper Belt stretches to roughly fifty. Light crosses one AU in a little over eight minutes. Beyond the planets the unit becomes unwieldy, and astronomers switch to light-years and parsecs.