Definition
When lunar declination is large, the two bulges are displaced away from the equator, strengthening the diurnal constituents and increasing diurnal inequality at mid-latitude ports. Because the tidal force varies as the inverse cube of distance, perigean tides, when the Moon is nearest at about 356,000 km against 406,000 km at apogee, are noticeably larger. A perigean spring tide combines both effects and produces the year's greatest ranges.
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