ABSTRACT
This chapter is dedicated to a very simplified dynamical analysis of the Earth's lunisolar precession and nutation and free nutation. The immediate observational consequence of this wobble of the rotation axis would be a periodic alteration of the astronomical latitude of each observatory, so that the phenomenon is also called variation of the astronomical latitudes. The reasoning can be applied to the Moon, taking into account the different mass, distance, and position with respect to the equator. The International Earth Rotation Service (IERS) has selected a catalogue of 608 extragalactic radio-sources that constitute the International Celestial Reference Frame (ICRF) to be used for very long baseline radio-interferometers (VLBI) observations. This free-body nutation is superimposed to the forced lunisolar precession and nutation, whose presence forces the motion of M with respect to the fixed stars, and slightly alters the previous considerations by a minute variation of the apertures of the cones, at the level of few hundredths of arcsec.
