ABSTRACT
The stars will provide an ideally fixed reference frame against which the complex movements of the equator, the ecliptic and their intersections can be determined. For fundamental dynamical reasons, the ecliptic plane is much more stable in the inertial system than the equator, whose movements are larger and known with some residual imprecision even today. Hence, the efforts made, not only by geophysicists but also by astronomers, to know the movements of the terrestrial observer with ever greater precision. The discussion of these movements developed in this chapter is based on the traditional approach to this intricate subject. 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.
