IAU(1991) RECOMMENDATION IX
Nutation angles: IAU conventional models and IERS observational results
The XXIst General Assembly of the International Astronomical Union,
that a generally accepted non rigid Earth theory of nutation, including all known effects at the one tenth milliarcsecond level, is not yet available,
1. that those satisfied with accuracy of the nutation angles [Epsilon or Psi.sin(Epsilon_0)] numerically greater than +/- 0.002" (one sigma rms) may continue to use the 1980 IAU Nutation Theory (P.K. Seidelmann, Celestial Mechanics, 27, 79, 1982),
2. that those requiring values of the nutation angles more accurate than +/- 0.002" (one sigma rms) should make use of the Bulletins of the IERS which publish observations and predictions of the celestial pole offsets accurate to about +/- 0.0006" (one sigma rms) for a period of up to six months in advance,
3. that the IUGG be encouraged to develop and adopt an appropriate Earth model to be used as the basis for a new IAU Theory of Nutation.