A non-rotating origin on the instantaneous equator

Abstract

In order to give an exact and clear description of the angle of rotation of the Earth, we propose to use, as the reference point in space, a “non-rotating origin” (Guinot 1979) such that its hour angle, reckoned from the origin of the longitudes (or “non-rotating origin” in the Earth), represents strictly the specific rotation of the Earth. The position of this origin on the instantaneous equator depends only on the trajectory of the pole of rotation. We show that the estimation of the deduced angle of rotation is not critically affected by the precision with which this trajectory is known. We give therefore the formulae to obtain the non-rotating origin, at any date t, from a chosen fixed reference, and we propose a definition of the Universal Time which will remain valid, even if the adopted model for the precession and the nutation is revised. We show that the use of the non-rotating origin also simplifies the transformation of coordinates between the terrestrial and celestial reference systems.