The Earth's rotation: Definition of the pole and of the angle of sidereal rotation

The use of new techniques for measuring the Earth's orientation in space and the intrinsic qualities of their attached terrestrial and celestial reference frames have now raised the accuracy of the computed polar coordinates and the angle of sidereal rotation to a level usually better than 0.001. The conceptual and conventional definitions of the Earth's pole of rotation and of the Universal Time UT1 must accordingly be given with the same order of precision. This paper gives a review of the past and present definitions of the celestial pole and UT1 as well as an evaluation of their deficiencies. Some necessary improvements in these definitions are proposed.