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An analysis of the differences between observed nutation angles and the 1980 IAU nutation theory shows the existence of currently unmodeled effects. An empirical set of corrections to the 1980 IAU nutation theory is presented and compared with current geophysical models. A retrograde periodic variation (period about 418 days), which may be related to the theoretical free core nutation, is seen.
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McCarthy et al. (1991) studied this question.
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