The Astronomical Unit (AU) is a unit of measurement based on the average distance between what two bodies?

The Astronomical Unit is based on the average distance between the Earth and the Sun.

One astronomical unit, usually abbreviated AU or au, is approximately 149.6 million kilometres, or about 93 million miles. It provides a convenient scale for describing distances within the Solar System, where kilometres quickly become unwieldy. Earth’s average orbital distance from the Sun is defined as roughly 1 AU.

Earth does not travel in a perfect circle: its orbit is slightly elliptical. The planet is closer to the Sun at perihelion, in early January, and farther away at aphelion, in early July. That is why the AU is best understood as a standardised average scale rather than a distance that remains constant every day.

Since 2012, the International Astronomical Union has defined the astronomical unit exactly as 149,597,870,700 metres. Astronomers use it to compare planetary orbits; for example, Mars averages about 1.52 AU from the Sun, while Jupiter averages about 5.2 AU.