Definition of the Celestial Ephemeris Origin and of UT1 in the International Celestial Reference Frame

The adoption of the international Celestial Reference System ICRS, and of t he corresponding Frame, ICRF by the 23rd General Assembly of the Internatio nal Astronomical Union, calls for a redefinition of the departure point on the true equator. Several possibilities have been suggested. This paper con siders the use of the non-rotating origin (Guinot 1979). The "Celestial Eph emeris Origin" (CEO) is defined here as the non-rotating origin on the equa tor of the Celestial Ephemeris Pole (CEP). Developments valid at the microa rcsecond, based on the best model for precession, nutation and pole offset at J2000.0 with respect to the pole of ICRF, are provided for computing the CEP coordinates and the position of the CEO. It is shown that an operation al definition of UT1 based on the CEO leads to values which are insensitive at the microarcsecond level to future improvements of this model.