Modeling of nutation‐precession: Very long baseline interferometry results

Abstract

Analysis of over 20 years of very long baseline interferometry data (VLBI) yields estimates of the coefficients of the nutation series with standard deviations ranging from 5 microseconds of arc (μas) for the terms with periods <400 days to 38 μas for the longest‐period terms. The largest deviations between the VLBI estimates of the amplitudes of terms in the nutation series and the theoretical values from the Mathews‐Herring‐Buffett (MHB2000) nutation series are 56 ± 38 μas (associated with two of the 18.6 year nutations). The amplitudes of nutational terms with periods <400 days deviate from the MHB2000 nutation series values at the level standard deviation. The estimated correction to the IAU‐1976 precession constant is −2.997 ± 0.008 mas yr −1 when the coefficients of the MHB2000 nutation series are held fixed and is consistent with that inferred from the MHB2000 nutation theory. The secular change in the obliquity of the ecliptic is estimated to be −0.252 ± 0.003 mas yr −1 . When the coefficients of the largest‐amplitude terms in the nutation series are estimated, the precession constant correction and obliquity rate are estimated to be −2.960 ± 0.030 and −0.237 ± 0.012 mas yr −1 . Significant variations in the freely excited retrograde free core nutation mode are observed over the 20 years. During this time the amplitude has decreased from ∼300 ± 50 μas in the mid‐1980s to nearly zero by the year 2000. There is evidence that the amplitude of the mode in now increasing again.