The coordinate dependence of the definition of light deflexion is discussed and the concepts of “natural” and “proper” reference frame are introduced in order to derive astrometric observables from coordinate quantities. Confusion in published formulations of VLBI data analysis is discussed in the context of the IAU definition of the coordinate time scale TDB. It is pointed out that with this definition, the unit of TDB differs from that of TDT and it is suggested that the speed of light in barycentric units be adopted as a defining constant in the system of astronomical constants.