Using a simple nonlinear analysis method (E. M. Blanter et al., submitted manuscript, 2001), we are able to observe in the length of day the 18.6-year beating of the two main fortnightly lunar tides and to measure the amplitude of the smallest one with a great accuracy. We can thus estimate the associated zonal response coefficient κ for this period. We find κ = 0.311 ¿ 0.010.