What is the standard unit for the Earth-Sun distance?

The standard unit for the Earth–Sun distance is the astronomical unit, abbreviated au or AU. One astronomical unit is defined as exactly 149,597,870,700 metres.

The unit was created because kilometres or miles are awkward for describing distances within the Solar System. Earth’s average distance from the Sun is about one astronomical unit, while Mars averages roughly 1.52 au from the Sun and Jupiter about 5.2 au. Because planetary orbits are elliptical, the Earth–Sun distance changes slightly during the year.

Historically, the astronomical unit was understood as an average Earth–Sun distance and was connected to efforts to measure the Solar System’s scale. In 2012, the International Astronomical Union gave it its exact modern definition in metres, separating the unit from small changes caused by orbital dynamics and the Sun’s mass loss.

The astronomical unit is not the same as a light-year or a parsec. A light-year measures the distance light travels in one year and is used mainly for stars and galaxies. A parsec is an even larger astronomical distance based on stellar parallax. A solar radius, by contrast, measures the Sun’s size rather than the distance between Earth and the Sun.