ABSTRACT
The stars will provide an ideally fixed reference frame against which determine the complex movements of the equator, the ecliptic, and their intersections. For fundamental dynamical reasons, in the inertial system the ecliptic plane is much more stable than the equator, whose movements are larger, and known with some residual imprecision. Much of treatment can be traced back to the books of Newcomb and Danjon quoted in the Bibliography. Since 1984, the International Astronomical Union (IAU) has enforced a profound revision of several fundamental constants and variables, including time. The procedure will allow the mean coordinates at any date to be derived, starting from the known mean coordinates at a given fundamental epoch. A planetary nutation must also be present; however, the movements of the ecliptic with respect to the equator do not change the stellar declinations, but only the common origin of the right ascensions, so that the planetary nutation goes unnoticed in differential measurements.
