Astronomical Unit

Astronomical unit, abbreviated as \(AU\) or \(au\), is a unit of length used in astronomy to express distances within the Solar System.  The International Astronomical Union (IAU) defines 1 astronomical unit as exactly 149,597,870,700 meters, which is equal to about 149.6 million kilometers (92.96 million miles). The astronomical unit was based on the average distance between Earth and the Sun.  Because Earth's orbit is slightly elliptical, the actual Earth-Sun distance changes throughout the year, so the AU was originally an approximation of the mean Earth-Sun distance.  To eliminate ambiguity and improve measurement precision, the IAU adopted a fixed definition in 2012, making the astronomical unit an exact unit of length rather than a quantity that depends on Earth's changing orbital position.