Equivalently , one au is the radius of an unperturbed circular Newtonian orbit about the sun of a particle having infinitesimal mass , moving with an angular frequency of 0.01720209895 radians per day ; or alternatively that length such that , when used to describe the positions of the objects in the Solar System , the heliocentric gravitational constant ( the product GM ☉ ) is equal to ( 0.01720209895 ) 2 au3\/d2 .
From 1976 to 2012 the AU was defined as `` 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 ( known as the Gaussian constant ) '' .