Published online by Cambridge University Press: 14 August 2015
In order to discuss accurately the motions of the Earth in space, it is necessary to define rigorously two readily accessible reference systems. The conception and the realization of celestial absolute systems and terrestrial coordinate systems are discussed. It is suggested that these systems of reference ought to be defined with a minimum of theoretical or observational constraints. Examples of such ideal reference systems are given, together with some desirable properties for intermediate systems.