  Neither astronomical technique, including VLBI, can measure nutation
directly. Estimates of parameters of the nutation model are produced by solving
the LSQ problem of adjusting millions parameters using estimates of group
delay. The choice of the mathematical model for nutation used in the estimation
process of analysis of group delays affects our ability to interpret the
results. Ignoring these subtleties and using parameters of the nutation model
either in the form of time series, or in the form of empirical expansion as
"VLBI measurement of nutation", opens a room for misinterpretation and
mistakes. Detailed analysis of the problem reveals that the separation of
forced nutations, atmospheric nutations, ocean nutations, and the retrograde
free core nutation requires invoking some hypotheses, and beyond a specific
level becomes uncertain. This sets a limit of our ability to make an inference
about the free core nutation.
