The Moon's orbital plane wobbles relative to the Earth's equator over 18.61 years, changing the Moon's maximum declination and therefore the strength of lunar constituents. M2 swings about 4 percent either side of its mean over the cycle and the diurnal constituents by much more. Predictions apply nodal factors to correct each constituent for the year being predicted, and tidal datums are averaged over 19 years so the cycle cancels out.
We're near the top of the nodal cycle, so high tide flooding counts will jump even without any sea level change.
When comparing flooding counts between years, allow for the nodal cycle, which can make a run of years look better or worse than the underlying sea level trend.
Also heard as
- lunar nodal cycle
- 18.6-year cycle
Related terms
Tidal datum epoch
The fixed 19-year period over which water levels are averaged to compute official tidal datums.
LAT
Lowest Astronomical Tide: the lowest level that tide predictions reach when the weather is held at average.
O1
The principal lunar diurnal constituent, period about 25.82 hours, created by the Moon's declination.
Declination
The angle of the Moon or Sun north or south of the celestial equator, which makes the two daily tides unequal.