Three variants of geophysical excitations and seven different VLBI solutions of celestial pole offsets (CPO) are used to determine period and Q-factor of Free Core Nutation (FCN). Brzeziński’s broad-band Liouville equations (Brzeziński, 1994) are numerically integrated to derive geophysical effects in nutation in time domain. Possible effect of geomagnetic jerks (GMJ) is also considered. Best-fitting values of FCN parameters are estimated by least-squares fit to observed CPO, corrected for the differences between the FCN parameters used in IAU 2000 model of nutation and newly estimated ones; MHB transfer function is used to compute these corrections. It is demonstrated that different VLBI solutions lead to FCN parameters that agree on the level of their formal uncertainties, but different models of geophysical excitations change the results more significantly. Using GMJ excitations always brings improvement of the fit between integrated and observed CPO. The obtained results show that the best fit is achieved when only GMJ excitations are used. Our conclusion is that GMJ are very probably more important for exciting FCN than the atmosphere and oceans. Empirical Sun-synchronous correction, introduced in the present IAU 2000 nutation model, cannot be explained by diurnal atmospheric tidal effects., Jan Vondrák and Cyril Ron., and Obsahuje bibliografii
The present definition of U.T.1 is a complex one which introduces the old concept of the ”Fictitious Mean Sun” which has been
suggested by Newcomb (1895). The conventional right ascension of the Fictitious Mean Sun brings the basic relationship between Sidereal Time, arising directly from observations, and U.T.1, as it is internationally adopted. Unfortunately, this basic relationship needs some effort of understanding for the common user. It is the reason
why B. Guinot (1979) proposed to adopt another point instead of the vernal equinox on the celestial equator, that he called the
‘non-rotating origin σ’. This point obeys to a clear kinematical
concept. Moreover, it should bring a new conceptual definition of U.T.1 very easy to understand. The position of σ on the celestial sphere can be easily determined by the Eulerian angles ψ and θ which are positionning the instantaneous axis of rotation of the Earth relatively to an inertial plane of reference. It can also be realized by the way of a quantity 's’ depending on the only motion
of the instantaneous equator. Formulation and developpement of ‘s’ are successively given.