In some parts of the world, the sea barely rises and falls with the tides. In others, the difference between high and low tide is dramatic. Along Western Australia’s Kimberley coast, for example, it can exceed ten metres – roughly the height of a three-storey building.

Ocean tides shape coastlines and marine ecosystems, affect coastal communities, and even influence the melting of Antarctic ice shelves. But why are tides so large in some places and so small in others? And as our oceans change, will the tides change too?

The pull of the Moon and Sun

The Moon and the Sun both produce tides on Earth through their gravitational pull. Although the Sun is far more massive, the Moon is much closer, so it has a stronger influence on our ocean tides.

The Moon’s gravitational pull decreases with distance. It is strongest on the side of Earth that’s closest to the Moon, and weakest on the far side. This difference stretches both the solid Earth and oceans. But because water moves more freely than rock, the effect is most visible in the ocean.

In a simplified picture, this creates two broad tidal bulges. One forms on the side facing the Moon. A second forms on the far side because the Moon pulls Earth’s centre more strongly than it pulls........

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