The observability of the celestial pole and its nutations

The expected motion characteristics of the real earth are systematically analysed based on available dynamical theories for the rigid model, the elastic earth model and the earth model with liquid core. The various axes which are implicit in the dynamical theories are investigated regarding observability on the basis of astronomical observations and suitability for defining reference directions. The observational insignificance of the ''diurnal polar motion'' is demonstrated. A special effort is made to clarify customarily used terminology.