the mean distance between the earth and the sun is called the
Astronomical unit The astronomical unit (symbol: au,[1][2][3] ua,[4] or AU) is a unit of length, roughly the distance from Earth to the Sun. However, that distance varies as Earth orbits the Sun, from a maximum (aphelion) to a minimum (perihelion) and back again once a year. Originally conceived as the average of Earth's aphelion and perihelion, since 2012 it has been defined as exactly 7011149597870700000♠149597870700 metres or about 150 million kilometres (93 million miles).[5] The astronomical unit is used primarily for measuring distances within the Solar System or around other stars. However, it is also a fundamental component in the definition of another unit of astronomical length, the parsec.
Perihelion and aphelion The perihelion (/ˌpɛrɪˈhiːliən/) of any orbit of a celestial body about the Sun is the point where the body comes nearest to the Sun. It is the opposite of aphelion (/æpˈhiːliən/), which is the point in the orbit where the celestial body is farthest from the Sun.[1]