A non-rotating origin on the instantaneous equator: Definition, properties and use

Abstract

The exact description of the Earth's rotation raises the problem of the choice of a reference point on the instantaneous equator both in space and in 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 sidereal rotation of the Earth. Such an origin on the instantaneous equator depends only on the motion of the pole of rotation; it is practically realizable from a chosen fixed reference and we give the formulae to obtain it in space and in the Earth. We show that the estimation of the sidereal rotation is not critically affected by the precision with which the trajectory of the pole is known. We therefore 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. An additional simplification of this transformation would be obtained when using, in the precession and nutation matrixes the development of the celestial coordinates of the pole as function of time in place of the various usual equatorial and ecliptic parameters. The use of the non-rotating origin instead of the equinox would thus have advantages for both conceptual and practical reasons.