Two-epoch expressions of the precession quantities in the IAU 2000 and post IAU 2000 precession-nutation models

According to one of the resolutions of the 24th IAU General Assembly, the IAU 2000 precession-nutation model should be adopted from 2003 January 1 on. The one-epoch expressions of seven frequently used precession quantities of the model are given in the IERS Conventions (in 2003). In 2003, Capitaine et al. improved on the model and the improved model is known as the post IAU 2000 precession-nutation model. Based on this work, the two-epoch expressions of the precession quantities in the two models are obtained. When the two-epoch expressions are applied to the coordinate transformation, the accuracies over the years 1800 to 2200 are in accordance with the original models, reaching 1 mas and 1 μas, respectively. Since the expressions of the precession quantities have the same form as the IAU 1976 precession model except as regards the order of expansion, it provides a convenient tool for practical applications. The results have been applied to the calculation and compilation of the Chinese Astronomical Ephemeris.