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Sailing reference
The YouSail sailing glossary
Clear explanations of the language used in navigation, pilotage, weather, safety and life under sail.
Loading this page…
Sailing reference
Clear explanations of the language used in navigation, pilotage, weather, safety and life under sail.
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The time between the Moon crossing your meridian on successive days
Averages 24 hours 50.5 minutes because the Moon advances roughly 12.2 degrees eastward along its orbit each solar day, so the Earth must rotate a little further to bring the Moon back to the same meridian. This governs the daily retardation of tidal times, commonly about 50 minutes, though the actual interval at a given port varies with the Moon's changing orbital speed and declination.
Read definition →How the Moon's height above the equator and distance change tidal range
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.
Read definition →Sailing reference
Clear explanations of the language used in navigation, pilotage, weather, safety and life under sail.