In 1983, the International Bureau of Weights and Measures defined the metre as the distance travelled by light in a vacuum in 1⁄299,792,458 of a second. That makes a metre a fixed unit of length.
For astronomers, however, distance is rather more malleable. In astronomy, distance is measured in astronomical units. Astronomers think of an au as the distance between the Earth and the Sun.
But since the Earth’s orbit is elliptical, this distance isn’t constant. At one time an au was defined as the length of the semi-major axis of the Earth’s orbit. But that isn’t constant either.
So in 1976, the Bureau International des Poids et Mesures in France defined the au to be the distance from the centre of the Sun at which a particle of negligible mass would orbit in 365.2568983 days.
The trouble with this definition is that it depends both on the mass of the Sun and the gravitational constant G.
That’s not good say, Nicole Capitaine and Bernard Guinot at the Observatoire de Paris. The gravitational constant and the mass of the sun can only be measured with limited accuracy. And who’s to say their value isn’t changing anyway? It makes no sense to define a fundamental unit of length in these terms.
They say astronomers desperately need a new definition and point out there is an obvious choice: define an au as some suitable multiple of a metre.
That would bring astronomy into line with SI units and making various calculations much more straightforward.
So what are they waiting for?
Ref: arxiv.org/abs/0812.2970: The Astronomical Units