Page "Astronomical unit" Paragraph 8

With the definitions used before 2012, the astronomical unit was dependent on the heliocentric gravitational constant, that is the product of the gravitational constant G and the solar mass M < sub >☉</ sub >.

Neither G nor M < sub >☉</ sub > can be measured to high accuracy in SI units, but the value of their product is known very precisely from observing the relative positions of planets ( Kepler's Third Law expressed in terms of Newtonian gravitation ).

Only the product is required to calculate planetary positions for an ephemeris, which explains why ephemerides are calculated in astronomical units and not in SI units.