Celestial motion of the pole: precession and nutation
The celestial pole offsets, Dpsi, Depsilon give the offsets in longitude and in obliquity of the celestial pole with respect to its position defined by the conventional IAU precession/nutation models.
The observed variations reflect the difference of the actual celestial motion of the pole with the one predicted by the conventional IAU precession and nutation models. Most of these variations are explained by errors in a few terms of the IAU models: secular term, periodic terms (mainly 18.6, 1.0, 0.5 years and 14 days). These terms are included in the IERS conventions (1996) correction model. The remaining variations after the IERS (1996) model is taken out may be attributed to the free nutation of the earth that can be modelled as
a sin 2Pi (t-to)/P + b cos 2Pi (t-to)/P,
where P is set to 433 days, t is the date (MJD) and to = 51544.5. Estimated values and of a and b over two-years intervals from 1983 on are available.
The IERS Conventions (1996) correction model matches the observations within +/- 0.001".