Paris Observatory Geodetic VLBI CenterParis Observatory Geodetic VLBI Center
This page displays the information relevant to the empirical modeling of the free nutation of the Earth's figure axis associated with the free rotational mode of the fluid core inside the viscoelastic mantle (free core nutation or FCN). The empirical model is adjusted to the complex-valued nutation dX + i dY measured by VLBI using a least-square fit of a retrograde circular term of period -430.21 days and constant offsets, let
dX + i dY = A eiσt + X0 + i Y0
where A is the searched complex amplitude of the FCN. Yearly values of A are obtained by fitting over a sliding window of width 7 years and displaced by steps of half a year. The fit is applied to various nutation series submitted to or computed by the IERS Earth Orientation Center. The models are updated regularly, typically every week as series are updated by the analysis centers. For each series, we make available a table of the yearly amplitudes from which one can derive the nutation offsets at any epoch.
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