Until 1998 the reference frame was defined in terms of the Solar System and the dynamics induced by the motions of Solar System bodies and their shapes. The two reference planes were the equator of the Earth and the ecliptic, the mean plane of the Earth's orbit. The intersection of these two planes, the equinox, was the fiducial point. Since both the equator and the ecliptic move owing to solar and lunar gravitational forces on the shape of the Earth and perturbations by the planets on the Earth's motion, the equinox moves with time. Therefore, each dynamical reference frame had to be defined for a specific epoch and observations and predictions were transformed to and from standard epochs to the chosen times of observations.

We have presented, in Chapter 7, the new adopted International Celestial Reference System (ICRS) based on the extragalactic radio sources. Now, the fundamental reference frame is fixed for all times. The IAU introduced in 2000 two space fixed systems. The Barycentric Celestial Reference System (BCRS) and the Geocentric Celestial Reference System (GCRS) have been defined in terms of metric tensors and the generalized Lorentz transformation between them, which contains the acceleration of the geocenter and the gravitational potential. The ICRS is to be understood as defining the orientation of the axes of both these systems for each of the origins. The International Celestial Reference Frame, (ICRF), determined from VLBI observations is the realization of the ICRS and can similarly be geocentric and barycentric.