4. Preliminary proposals

The answers and comments to the Newsletter 2 as well as the considerations above lead to the preliminary proposals :

1) The IAU 1980 conceptual and conventional definition of the CEP must be abandoned,

2) the reference pole has not to be defined by its realization but by a clear concept not dependent on further improvements in the model,

3) this pole must be defined such that it can be realizable by a model as accurately as possible,

4) the change from the CEP to the new definition has to be as less as possible in its practical realization,

5) a change of name could be considered as the ``Celestial Reference Pole" (CRP), or the ``Celestial Intermediate Pole" (CIP),

6) concerning the motion with respect to the CRS, the choice should be to specify this motion by a model including only the components with periods longer than 2 days (such a model will automatically include all the geophysical perturbations such as the retrograde diurnal motion due to ocean tides),

7) a tentative conceptual definition is ``the Pole of the intermediate equator of which motion with respect to the CRS is produced by the luni-solar and planetary torque" (or ... ``by the external gravitational forces acting on the Earth"),

as the high frequency motion is not considered in the nutation model, it may be possible to add to the conceptual definition that ``the motion of the equator with respect to the CRS is considered for an Earth with an equatorial symmetry", or that ``the motion is considered after filtering out terms of period shorter than 2 days",

8) the prograde diurnal terms in nutation should be considered as long periodic terms of the polar motion and the prograde semi-diurnal terms in nutation should be considered as prograde diurnal terms of the polar motion,

9) concerning the motion with respect to the TRS, the choice should be to sharpen the definition of the pole of reference by taking into account the prograde diurnal variations as well as the prograde and retrograde semi-diurnal variations as a predictable part of the polar motion which can be realized by a model,

10) the processing of the observations should include the estimation of the celestial pole offsets wrt the model for precession-nutation as well as the corrections to an empirical model for polar motion,

11) a ``Mean CRP" (MCRP) could be defined including only the long periodic part of the motion both in the TRS and the CRS, and then a ``true CRP" as obtained by adding the high frequency components to the polar motion of the MCRP.

12) a conventional procedure must be given for estimating the high frequency components in polar motion in order to provide the best realization of the pole in the processing of observations : C3 is proposed when possible or C2 in the more general case.