Tides are long, slow waves caused mainly by the Moon’s gravity, with help from the Sun.
Two bulges, not one
The Moon pulls hardest on the ocean facing it and weakest on the ocean on the far side. The difference stretches Earth’s oceans into two bulges: one towards the Moon and one directly away from it. As Earth rotates through both bulges, most coasts see two high tides and two low tides roughly every 24 hours and 50 minutes. The extra 50 minutes is because the Moon moves along its orbit while Earth turns.
Spring and neap tides
- Spring tides (nothing to do with the season) happen near new and full moon, when Sun and Moon line up. High tides are higher and low tides lower.
- Neap tides happen near the quarter moons, when Sun and Moon pull at right angles. The tidal range is smaller.
Why local tides vary so much
Coastline shape and sea-floor depth amplify or dampen the tide. The Bay of Fundy in Canada has some of the largest tidal ranges in the world, over 15 metres, while parts of the Mediterranean see only a few centimetres.
About the writer: Karim.A
Karim writes Tidibook’s science explainers: how the natural world works, in short and simple language.
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