What causes the tides?

The sea breathes in and out

Spend a whole day at the beach and you will notice the sea move. At high tide, the water reaches far up the sand. At low tide, it pulls back and uncovers rock pools, mudflats and sandbanks. On most coasts this happens twice a day: two high tides and two low tides.

The times are not the same every day, though. High tide comes about 50 minutes later each day. That clue points straight to the cause, because the Moon also rises about 50 minutes later each day. Tides are driven mainly by the Moon's gravity.

A bulge on each side of Earth

Gravity is the pull that every object has on every other object. The Moon's gravity pulls on the whole Earth, including its oceans. The water on the side of Earth facing the Moon is pulled the hardest, so it heaps up into a bulge.

There is a second bulge too, on the side of Earth facing away from the Moon. That water is pulled a little less strongly than the rest of the planet, so it gets left behind and bulges outwards. So Earth has two bulges of water: one towards the Moon and one away from it.

As Earth spins, your beach passes through both bulges, and each time it does you get a high tide. In between, you get low tides. Because the Moon moves along its orbit in the same direction as Earth spins, Earth needs about 24 hours and 50 minutes to catch up to the Moon again. That is why high tides come roughly 12 hours and 25 minutes apart.

The Sun joins in: spring and neap tides

The Sun is enormously bigger than the Moon, but it is about 390 times further away. Distance matters a lot for tides, so the Sun's tide-making pull is only about half as strong as the Moon's. It still makes a difference.

At new moon and full moon, the Sun, Moon and Earth line up. Their pulls work together, so high tides are extra high and low tides are extra low. These are called spring tides, and people sometimes call the biggest ones king tides. The name has nothing to do with the season of spring. It comes from the idea of the tide springing up.

About a week later, when the Moon is at first or last quarter, the Sun and Moon pull at right angles to each other. They partly cancel out, so the difference between high and low tide is smaller. These are neap tides. Every lunar month, about 29.5 days, brings two sets of spring tides and two sets of neap tides.

Australia's giant tides

The tidal range, the difference between high and low tide, is very different around Australia. The Bureau of Meteorology says it averages less than a metre in south-west Australia, but more than 8 metres at spring tides in the north-west, and even more than 11 metres during king tides.

Of all Australia's major ports, Derby, on King Sound in the Kimberley, has the biggest tides. In 2018, its biggest predicted tide rose from 0.36 metres at low tide to 11.78 metres at high tide, a difference of more than 11 metres. That is taller than a three-storey building. Only a few places in the world, such as the Bay of Fundy in Canada, have bigger tides.

Why so big? Off north-west Australia lies a very wide, shallow continental shelf that stretches hundreds of kilometres out to sea. As the tide moves in over it, it grows taller, a bit like a wave rising as it reaches a beach. Funnel-shaped bays such as King Sound squeeze the water even higher.

A waterfall that runs sideways

Just over 100 kilometres north of Derby is Talbot Bay, home of the Horizontal Falls. Here, the tide pushes huge amounts of seawater through two narrow gaps in the orange sandstone cliffs. The water piles up on one side faster than it can squeeze through, so one side can be up to 4 metres higher than the other. The result looks like a waterfall lying on its side.

The Traditional Owners, the Dambeemangarddee (Dambimangari) people, call the falls Garaan-ngaddim. Their cultural law says people should not travel through the falls while the tide is rushing through.

Tides can be powerful and can rise fast, so it is always wise to check tide times before exploring rock pools, sandbars or mudflats. The Bureau of Meteorology publishes tide predictions for places all around Australia.

📚 Words to know

Tide

The regular rise and fall of the sea, caused mainly by the Moon's gravity.

Tidal range

The difference in height between high tide and low tide.

Spring tide

An extra-big tide that happens near new moon and full moon, when the Sun and Moon line up.

Neap tide

A smaller tide that happens when the Sun and Moon pull at right angles to each other.

Continental shelf

The shallow edge of a continent that lies under the sea before the ocean floor drops away.

🧪 Try this: keep a tide diary

1. With an adult, find the tide predictions for the place on the coast nearest you. The Bureau of Meteorology website has tide predictions for many places around Australia.
2. Write down the time of the first high tide each day for a week. How much later is it each day? Is it close to 50 minutes?
3. Look up the Moon's phase for each day too. Which days have the biggest difference between high and low tide? Are they near a new moon or a full moon?
4. If you live near a beach, go with an adult at low tide and push a stick into the sand at the water's edge. Come back an hour or two later. How far has the water moved? Stay away from rocks and fast-moving water.