Why Are Some High Tides Higher? Two Pulls Lining Up

The moon raises the larger tide because it is far closer, and the sun raises a smaller one that is roughly half as strong despite its enormous mass, since tidal effect falls off extremely steeply with distance. Both are present continuously and their effects add. When the sun, the Earth and the moon lie approximately in a line, which happens at new moon and at full moon, the two effects reinforce and the difference between high and low water is unusually large. When the sun and moon are at right angles as seen from Earth, which happens at the quarter moons, they partly cancel and the range is unusually small.

The pattern repeats roughly every fortnight:

• •Largest ranges around new moon and full moon
• •Smallest ranges around the first and last quarters
• •Two large and two small periods in each lunar month
• •A lag of a day or two between the alignment and the largest tide
• •Ranges also vary with the moon's distance, which changes monthly
• •The very largest occur when alignment and closest approach coincide

The term causes constant confusion because it suggests the season and refers to nothing of the kind. It derives from an older sense of the word meaning to rise or to leap up, as a spring of water springs from the ground and as a person springs to their feet, so the name describes the water springing up rather than the time of year. The opposing small tides carry a name derived from a word meaning scanty or lacking. Both terms are old, predate any confusion with the season, and are used year round, so a large tide occurs in every month and has nothing to do with March or April.

Several longer cycles modulate the fortnightly pattern and combine to produce the extremes. The moon's orbit is elliptical, so its distance varies over about twenty seven days and its tidal effect varies with it, which is why a large tide coinciding with the moon's closest approach is larger still. The Earth's orbit is also elliptical, so the solar contribution peaks around early January. The alignment of the moon's orbit with the equator varies over about eighteen and a half years, which produces a long cycle in the extremes. Combining all of these, the very largest tides recur at intervals of years and are predictable decades in advance, which is why tide tables can be published far ahead.

The astronomical cause is identical everywhere and the resulting range differs enormously by place, which is entirely a matter of local geography. Tides are long waves travelling around ocean basins, and their behaviour depends on the shape and depth of those basins, with water piling up where a funnelling coastline concentrates it and barely moving where it does not. Some basins resonate with the tidal period, amplifying the range dramatically, which is why the Bay of Fundy reaches over fifteen metres while parts of the Mediterranean move by a few tens of centimetres. Some places have one tide a day rather than two because of how the waves interfere. Predicting local tides therefore requires local measurement rather than astronomy alone.

The moon raises the larger tide and the sun a smaller one, and they reinforce at new and full moon to give the largest ranges and partly cancel at the quarters to give the smallest, repeating roughly every fortnight. The name derives from an old sense of springing up rather than from the season. Local range depends on basin shape and resonance, which is why it varies from centimetres to over fifteen metres.