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        Gazette IERS Gazette IERS Gazette IERS Gazette IERS Gazette
                                    _______________________________________
        No 18, 16 July 1997        /
__________________________________/                 Contact: iers@obspm.fr
                                     ftp:  hpiers.obspm.fr (145.238.100.28)
                              WWW:  http://hpiers.obspm.fr
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        Title:   Using the IERS precession-nutation model with the ICRS
        Authors: M. Feissel, D. Gambis, D. McCarthy


   The IERS Conventions (1996) provide the definition of the International 
Celestial Reference System, ICRS (Chapter 1), as well as procedures to be 
used to transform between the celestial and terrestrial reference frames 
(Chapter 5). Chapter 5 also specifies a nutation model and improved numerical 
values for the precession rate of the equator in longitude and in obliquity.

   The nutation model and the rate corrections are referred to the mean 
pole at J2000.0.  As explained in Chapter 1, the mean pole at J2000.0 is 
slightly offset from the ICRS pole. In order to refer the IERS nutation 
model to the ICRS pole, one must therefore use additive constants in longitude 
and obliquity.  This means that, after calculating the total displacement
of the celestial pole using the corrected precession rate and the improved
nutation model, a bias in both longitude and obliquity must be added in order
to refer to the ICRS pole.

   The constant terms to use with the IERS (1996) precession-nutation model 
are as follows.
            
              In longitude       dPsi     = - 0.0431"
              In obliquity       dEpsilon = - 0.0051"