Changing declinations of the Sun and the Moon with respect to the plane of the ecliptic cause diurnal variations in the strength of the tides, a phenomenon called diurnal inequality. Usually, both the two High Waters and the two Low Waters during a day are affected, their respective levels being unequal.
Nor is the Moon able to produce diurnal inequality when it crosses the plane of the Earth's equator.
The diurnal inequality between high waters and low waters is nil when the Moon moves through the plane of Earth's equator, and as seen from Earth, reaches its peak when the Moon is in its most northerly or southerly position in the sky.
This is because of the diurnal inequality of the tides, which causes High Water to be nearly the same each day and Low Water level to vary.
The strongest diurnal inequality is possible when spring tides occur during the solstices when both celestial bodies are near their maximum declination, and acting together.
The peak of diurnal inequality will occur around 21:00 hours, or about 9 hours later than the theoretical time (see also Desplanque and Mossman 2001b).
When the Moon is exactly above the Equator, as happens every 13.6 days, there will be no difference in strength of the two daily tides (no diurnal inequality).
Therefore the maximum diurnal inequality is centred around spring tides in June and December, and the weakest inequality during neap tides in March and September.
This is due to an extreme case of diurnal inequality when the Moon has the greatest degree of declination and the local semidiurnal components of the tide are weak.
Furthermore the 18.6 year cycle is based on the period of revolution of the Moon's nodes and during this period the diurnal inequality of the tides varies in strength.
This is the case in regions like the Bay of Fundy where diurnal inequality of the tides is of such minor importance that it can be virtually ignored among variations caused by the coincidence of perigean and spring tides.
Diurnal inequality. The difference in height between the two High Waters or the two Low Waters of each tidal day.