In particular, there is a huge quantity of very precise data relating to the positions of objects within the Solar System that cannot conveniently be expressed or processed in SI units.
Through a number of modifications, the astronomical system of units now explicitly recognizes the consequences of general relativity, which is a necessary addition to the International System of Units in order to accurately treat astronomical data.
The associated astronomical constants also fix the different frames of reference that are needed to report observations.
In practice, the masses of celestial bodies appear in the dynamics of the Solar System only through the products GM, where G is the constant of gravitation.
In the past, GM of the Sun could be determined experimentally with only limited accuracy.
An equivalent formulation of the old definition of the astronomical unit is the radius of an unperturbed circular Newtonian orbit about the Sun of a particle having infinitesimal mass, moving with a mean motion of 0.01720209895 radians per day.
The astronomical unit of length was determined by the condition that the measured data in the ephemeris match observations, and that in turn decides the transit time τA.