Nodal Modulation

Nodal modulation is the slow 18.61-year variation in the amplitude and phase of tidal constituents caused by the regression of the Moon’s ascending node, which swings lunar declination between about 18.3 and 28.6 degrees over that cycle. Harmonic analysis handles it through the nodal factor f and the nodal angle u applied to each constituent, scaling the constant amplitude H to the value for the analysis epoch. For M2 the f factor varies by roughly plus or minus 3.7 percent and for K1 by about plus or minus 11 percent over the 18.61-year period. Tide-table producers must apply these corrections, and a tidal-datum epoch is set to a Metonic-length 19-year span to average the cycle out.