An astronomical unit (AU) is a commonly used unit in astronomy, but what exactly is its value?
Until recently 1 AU was defined to be “the radius of an unperturbed circular Newtonian orbit about the Sun of a particle having infinitesimal mass, moving with a mean motion of 0.01720209895 radians per day”  (the value 0.01720209895 radians per day is also known as the Gaussian constant).
There are three main problems with that definition:
- It is confusing,
- Einstein doesn’t like it (shifting your reference frame from the Earth to Jupiter changes the value by over a thousand metres),
- The Gaussian constant used in the calculation is dependent upon Solar mass, which is decreasing, causing a change in AU at the same time.
Although AUs are rarely used professionally, it is used fundamentally in the definition in the more commonly used ‘parsec’ – thus the variable definition that had previously been defined needed fixing.
This problem was addressed at the 2012 International Astronomical Union meeting (the same group that took Pluto as a planet away from us). The result?
1 AU = 149,597,870,700 metres 
The IAU decided to define 1 AU exactly, making sure there are no more arguments, and solving the (admittedly infrequently asked) problem from here on in.
1. International Bureau of Weights and Measures (2006), The International System of Units (SI) (8th ed.), p. 126, ISBN 92-822-2213-6.
2. “RESOLUTION B2 on the re-definition of the astronomical unit of length”, RESOLUTION B2, Beijing, Kina: International Astronomical Union, 31 August 2012.