Why the tide comes in twice a day
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Most coastlines see two high tides and two low tides roughly every twenty-four hours and fifty minutes. The Moon’s gravity pulls the ocean on the near side of the Earth toward it, raising a bulge of water. On the far side, the Earth itself is pulled toward the Moon slightly more than the water there, which leaves a second bulge behind.
As the planet turns beneath these two bulges, any given stretch of shore passes through both of them, which is why the tide rises twice. The extra fifty minutes comes from the Moon moving along its own orbit while the Earth spins, so each day the planet has to rotate a little further to catch up.
The Sun adds its own, smaller pull. When Sun and Moon line up at new and full moon, their effects stack and produce spring tides with the largest range. A week later, when they pull at right angles, the range shrinks to the gentler neap tides.
Local geography then reshapes everything. Narrow bays can funnel the incoming water into enormous ranges, while enclosed seas such as the Mediterranean barely move at all. That is why tide tables are calculated port by port rather than from a single global formula.
Modern predictions combine dozens of these periodic effects, each with its own rhythm, and add them together. The result is accurate enough to plan shipping schedules years in advance, though storms and air pressure can still push the real water level away from the forecast.