Tides
The twice-daily rise and fall of the sea, driven by the Moon and Sun; it is largest at spring tides and smallest at neap tides.
Tides are the regular rise and fall of the sea caused mainly by the Moon's gravity, with help from the Sun's. The Moon pulls slightly harder on the side of Earth facing it than on Earth's centre, and harder on the centre than on the far side. The difference stretches the oceans into two bulges, one towards the Moon and one away from it. As Earth turns, most coasts pass through both, giving two high and two low tides in a little over a day.
The tidal range is the height difference between high and low water, and it changes through the month. When the Sun, Moon and Earth are nearly in line, at new and full moon, their effects add together, giving higher highs and lower lows: spring tides. When the Sun and Moon are at right angles, around the quarter moons, they partly cancel and the range is smaller: neap tides. Each happens twice a month, all year round.
The name is misleading: spring tides have nothing to do with the season of spring. The word refers to the tide "springing forth".
Local geography makes a huge difference. A funnel-shaped bay or estuary can amplify the range dramatically, while some enclosed seas barely have tides at all. Because the timing is so predictable, tides have long shaped the planning of voyages, landings and even battles, in which a falling tide can strand ships in a river mouth.
Coming up in future stories
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- The River Trap That Sank Kublai Khan's Fleet 1288 · Bạch Đằng River, Vietnam
- They Tied the Tides to the Moon Long before 1788 · Arnhem Land (Yolngu country), Northern Territory, Australia