ABSTRACT
The movements of the Sun and the Moon were used to define units of time. Horizon, equator and ecliptic are three great circles on the celestial sphere used to define different astronomical reference systems. All stars will describe their parallel of declination in the clockwise direction. Therefore, each site has associated with it a system of fixed cardinal points and great circles, with respect to which the celestial sphere is in continuous rotation. Dynamical considerations led to the introduction of the geoid, to indicate the equilibrium surface the free water of the oceans would have under the sole influence of gravity and rotation. In other words, all points on the geoid have the same potential. Such surface is extended by convention through the continental lands. The chapter considers the time variation of the coordinates due to the diurnal rotation of the Earth.
