What is an Astronomical Unit?

An astronomical unit, or AU, is a unit of distance that's roughly based on the distance between the Earth and the Sun. This is approximately 93 million miles or 150 million kilometers.

The Earth and the Sun are at different distances over the course of a year, so the average is used. The exact average is surprisingly difficult to calculate and has changed a few times over the years. An Astronomical Unit is currently standardized to 149,597,870,700 meters.

Astronomical Units are a rare example of a measurement that is not at human scale. That is to say that most standard measurements evolved from things that humans directly experience. Astronomical Units are far beyond typical human experience and are used to explain distances that humans can only imagine. For example, the distance from the Sun to the center of the Milky Way is 1,700,000,000 AU.

Example Distances in Astronomical Units
Light-minute	0.12 AU
Light-year	63,241 AU
Distance to Sun	1 AU
Proxima Centauri The nearest star outside our solar system	268,000 AU

Overview: Astronomical Unit
Area	Space
Definition	A unit of length used to measure distances in space defined by the average distance of the Sun from the Earth over a year.
Kilometers	149.597 Million
Miles	92.955 Million
Symbol	AU, au, ua
Related Concepts	Fermi Paradox Spacecraft Human Scale