Definition
See also: Earth's orbitThe AU was originally defined as the length of the semi-major axis of the Earth's elliptical orbit around the Sun. In 1976 the International Astronomical Union (IAU) revised the definition of the AU for greater precision, defining it as that length for which the Gaussian gravitational constant (k) takes the value 0.01720209895 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 an angular frequency of 0.01720209895 radians per day; or that length such that, when used to describe the positions of the objects in the Solar System, the heliocentric gravitational constant (the product GM☉) is equal to (0.01720209895)2 AU3/d2. This value of the astronomical unit had to be obtained experimentally and so it was not known exactly. Furthermore, it was subject to relativity and thus was not constant for all observers, therefore, in 2012, the IAU redefined it again to just simply be 149,597,870,700 m.
In the IERS numerical standards, the speed of light in a vacuum is defined as c0 = 299,792,458 m/s, in accordance with the SI units. The time to traverse an AU is found to be τA = 499.0047838061±0.00000001 s, resulting in the astronomical unit in metres as c0τA = 149,597,870,700±3 m. It is approximately equal to the distance from the earth to the sun.
Read more about this topic: Astronomical Unit
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