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 Earth is about 147.1 million kilometers (91.4 million miles) from the Sun at perihelion around January 3, in contrast to about 152.1 million kilometers (94.5 million miles) at aphelion around July 4 â€” a difference of about 5.0 million kilometers (3.1 million miles). (These dates change over time due to precession and other orbital factors, which follow cyclical patterns known as Milankovitch cycles. For a table of these dates for various years, see Apsis.)