what is an astronomical unit

An astronomical unit (AU) is a unit of length used by astronomers to describe distances within the Solar System or around other stars. It is defined as the average distance between the Earth and the Sun, which is approximately 93 million miles or 150 million kilometers. The actual distance from Earth to the Sun varies by about 3% as Earth orbits the Sun, from a maximum (aphelion) to a minimum (perihelion) and back again once each year. The astronomical unit was originally conceived as the average of Earths aphelion and perihelion, but since 2012 it has been defined as exactly 149,597,870,700 meters. Astronomical units are used primarily for measuring distances within the Solar System or around other stars, and they are less commonly used to measure distances outside of our Solar System. One light-year equals 63,240 AU, so it is easier to use light-years to measure distances to stars or other galaxies, but not for measuring distances within our own Solar System.