N2

N2 is the larger lunar elliptic semidiurnal tidal constituent, period 12.6583 hours and speed 28.4397 degrees per hour, the third-largest constituent in many semidiurnal regimes after M2 and S2. It accounts for the variation in the lunar tide caused by the Moon’s elliptical orbit, beating against M2 over the 27.55-day anomalistic month to produce the perigean-to-apogean swing in range. When N2 reinforces M2 near perigee the semidiurnal range grows; when it opposes M2 near apogee the range shrinks. N2 is essential to predicting the monthly modulation of tidal range and to reproducing perigean spring tides in harmonic tide tables.