Tides
What causes tides?
Tides are the regular rise and fall of sea level. They are caused mainly by differences in the Moon’s gravitational pull across Earth, with the Sun contributing too. The shape of the coast affects local tide times and heights.
The Sun joins in
The Sun does the same stretch at a little under half the Moon’s strength. When Sun, Moon and Earth line up — new moon and full moon — the bulges stack: spring tides. When they pull at right angles — first and last quarter — they argue: neap tides.
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Why not exactly 24 hours
The Moon has moved on while Earth turned, so the bulge returns in about 24 hours 50 minutes. Two high tides in that span is the common pattern. Coasts, bays and the shape of the basin then scramble the simple picture, which is why a real tide chart is a prediction, not this diagram.