ON NON-RIGID EARTH NUTATION THEORY
proposed by Joint Discussion N. 3
and endorsing the conclusions of the IAU-IUGG Working Group
The XXIIIrd International Astronomical Union General Assembly
that the International Astronomical Union and the International Union of Geodesy and Geophysics Working Group (IAU-IUGG WG) on Non-rigid Earth Nutation Theory has met its goal by identifying the remaining geophysical and astronomical phenomena that must be modeled before an accurate theory of nutation for a non-rigid Earth can be adopted, and
that, as instructed by IAU Recommendation C1 in 1994, the International Earth Rotation Service (IERS) has published in the IERS Conventions (1996) an interim precession-nutation model that matches the observations with an uncertainty of +/- 1 milliarcsecond (mas),
the conclusions of the IAU-IUGG WG on Non-rigid Earth Nutation Theory given in the appendix,
the IAU-IUGG WG on Non-rigid Earth Nutation Theory to present a detailed report to the next IUGG General Assembly (August 1999), at which time the WG will be discontinued,
the scientific community to address the following questions in the future:
- completion of a new rigid Earth nutation series with the additional terms necessary for the theory to be complete to within +/- 5 microarcseconds, and
- completion of a new non-rigid Earth transfer function for an Earth initially in non-hydrostatic equilibrium, incorporating mantle inelasticity and a Free Core Nutation period in agreement with the observations, and taking into account better modeling of the fluid parts of the planet, including dissipation.
The WG on Non-rigid Earth Nutation Theory has quantified the problems in the nutation series adopted by the IAU in 1980 by noting:
(1) that there is a difference in the precession rate of about -3.0 milliarcseconds per year (mas/year) between the value observed by Very Long Baseline Interferometry (VLBI) and Lunar Laser Ranging (LLR) and the adopted value, (
2) that the obliquity has been observed (by VLBI and LLR) to change at a rate of about -0.24 mas/year, although there is no such change implied by the 1980 precession-nutation theory,
(3) that, in addition to these trends, there are observable peak-to-peak differences of up to 20 milliarcseconds (mas) between the nutation observed by VLBI and LLR and the nutation adopted by the IAU in 1980,
(4) that these differences correspond to spectral amplitudes of up to several mas, and
(5) that the differences between observation and theory are well beyond the present observational accuracy.
The WG has recognized the improvements made in the modeling of these quantities, and recommends, in order to derive a more precise nutation model, at the mas level in spectral amplitudes and at a few mas level in the peak to peak analysis, the use of models:
(1) based on a new non-rigid Earth transfer function for an Earth initially in non-hydrostatic equilibrium, incorporating mantle inelasticity, a core-mantle-boundary flattening giving a Free Core Nutation (FCN) period in agreement with the observed value, and a global Earth dynamical flattening in agreement with the observed precession, and
(2) based on a new rigid Earth nutation series which takes into account the following perturbing effects:
1. in lunisolar ephemerides: indirect planetary effects, lunar inequality, J2-tilt, planetary-tilt, secular variations of the amplitudes, effects of precession and nutation,
2. in the perturbing bodies to be considered: in addition to the Moon and the Sun, the direct planetary effects of Venus, Jupiter, Mars, and Saturn, should be included,
3. in the order of the external potential to be considered: J3 and J4 effects for the Moon, and
4. in the theory itself: effects of the tri-axiality of the Earth, relativistic effects and second order effects.
The WG recognizes that this new generation of models still has some imperfections, the principal one being poor modeling of the dissipation in the core and of certain effects of the ocean and the atmosphere, and urges the scientific community to address these questions in the future.
The WG recognizes that, due to the remaining imperfections of the present theoretical nutation models, the nutation series published in the IERS Conventions (1996), following 1994 IAU recommendation C1, still provides the users with the best nutation series. This IERS model being based on observations of the celestial pole offset, the WG supports the recommendation that the scientific community continue VLBI and LLR observations to provide accurate estimations of nutation, precession and rate of change in obliquity.