What is an Astronomical Unit? An Astronomical Unit (A.U.) is the average distance between Earth and the sun, which is about 93 million miles or 150 million kilometers. Astronomical units are usually used to measure distances within our solar system. For example, the planet Mercury is about 1/3 of an A.U. from the sun, while the farthest planet, Pluto, is about 40 A.U. from the sun (that's 40 times as far away from the sun as Earth is).