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.
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.
On the eastern side of the Atlantic (Table 5), the diurnal inequality of the tide is small.
The diurnal inequality of tides along the North American seaboard shows the same tendency.
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).
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.