An astronomical unit (abbreviated as AU, au, a.u., or sometimes ua) is a unit of length roughly equal to the mean distance between the Earth and the Sun. It is approximately 150 million kilometres (93 million miles). The symbol ua is recommended by the International Bureau of Weights and Measures, but au is more common in Anglosphere countries.

The International Astronomical Union recommends au, while international standard ISO 31-1 uses AU. In general, capital letters are only used for the symbols of units which are named after individual scientists, while au or a.u. can also mean atomic unit or even arbitrary unit; however, the use of AU to refer to the astronomical unit is widespread. The astronomical constant whose value is one astronomical unit is referred to as unit distance and given the symbol A.

Originally, the AU was defined as the length of the semi-major axis of the Earth's elliptical orbit around the Sun. In 1976, the International Astronomical Union revised the definition of the AU for greater precision, defining it as that length for which the Gaussian gravitational constant (k) takes the value 0.017 202 098 95 when the units of measurement are the astronomical units of length, mass and time. An equivalent definition is the radius of an unperturbed circular Newtonian orbit about the Sun of a particle having infinitesimal mass, moving with a mean motion of 0.017 202 098 95 radians per day, or that length for which the heliocentric gravitational constant (the product GM☉) is equal to (0.017 202 098 95)2 AU3/d2.

It is approximately equal to the mean Earth–Sun distance.

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