One astronomical unit is the distance at which a massless particle in an unperturbed circular orbit about the Sun would have a mean daily motion of k radians per day, where k is the Gaussian constant, k ≡ 0.01720209895. The oft-quoted ‘definition’ of an astronomical unit as ‘the mean distance of the Earth from the Sun’ is merely an approximation.
In the 1976 IAU System of Astronomical Constants, introduced for use in the national almanacs from 1984, the meter, kilogram and second are the units of length, mass and time respectively, in the International System of Units (SI). For astronomical applications it would be inconvenient to use these units: for example, astronomical distances are too large to be measured accurately and conveniently in meters.
The astronomical unit of time is the day, a time interval of 86 400 SI seconds. The astronomical unit of mass is the mass of the Sun, M ⊙. The astronomical unit of length is the length for which k≡ 0.01720209895. This length is known as...
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Williams, G.V. (1997). Astronomical unit . In: Encyclopedia of Planetary Science. Encyclopedia of Earth Science. Springer, Dordrecht. https://doi.org/10.1007/1-4020-4520-4_31
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