# Astronomical unit

The **astronomical unit (AU)** is a measure of distance widely used by astronomers when considering distances in the Solar System. It is approximately 92,957,000 miles (149,600,000 km).

It was originally defined as the average distance of the Earth from the Sun. This was xconvenient as relative distances within the Solar System were known far more accurately than absolute ones until well into the 20th century, so a distance could be given with much higher precision in AU than in miles.

However, there was a problem because the mean distance is changing slowly over time. A more rigorous definition was adopted in 1900: it is the radius of a circular orbit about the Sun such that a body of infinitesimal mass would orbit at a rate of 0.01720209895 radians per day.^{[1]} (Such a body would take about 365.2568983 days to complete an orbit.) With that definition, the mean distance of the Earth from the Sun is not necessarily exactly 1 AU.