How do tides work? The Moon, the Sun and spring and neap tides

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Twice a day, along most coasts, the sea creeps up the beach and then slides away again. Tides are so regular they can be predicted years ahead, yet the reason behind them is less obvious than it first seems. Why are there two high tides a day rather than one? And why are some tides much bigger than others? This guide walks through the physics without the equations.

Gravity, but with a twist

The Moon pulls on the Earth with gravity, but gravity weakens with distance. That difference is the key.

The side of the Earth facing the Moon is closest to it, so the ocean there is pulled slightly more strongly than the Earth as a whole. Water bulges towards the Moon.
The far side of the Earth is pulled least. Relative to the Earth's centre, the water there is effectively left behind, forming a second bulge on the opposite side.
What matters is the difference in pull across the Earth, called the tidal force, not the total strength of the pull.

Why two high tides a day

The Earth rotates beneath these two bulges. A point on the coast passes through one bulge, then the low water between them, then the second bulge, then low water again.

That gives two high and two low tides in most places.
The cycle is a little longer than 24 hours, because while the Earth spins, the Moon moves along in its orbit. The Earth has to turn slightly further to catch up, so high tide is commonly about 12 hours and 25 minutes after the previous one, and arrives later each day.

The Sun's part

The Sun is vastly more massive than the Moon but also enormously farther away. Because tidal force depends on the difference in pull across the Earth, distance matters a great deal, and the Sun's tidal effect works out at a little under half of the Moon's. It is still large enough to change the tides noticeably.

Spring and neap tides

How the Sun and Moon line up decides how big the tides are.

Spring tides happen when the Sun, Moon and Earth are roughly in a line, around new Moon and full Moon. The two tidal effects add together, giving higher high tides and lower low tides. The name has nothing to do with the season; it comes from the idea of the tide springing up, and they occur twice every lunar month all year round.
Neap tides happen around the first and last quarter Moon, about a week after a spring tide, when the Sun and Moon are at right angles as seen from Earth. Their effects partly cancel, so the difference between high and low water is smaller.
The Moon's distance matters too. Its orbit is not a perfect circle, and when a spring tide coincides with the Moon at its closest, tidal ranges are larger still. These are sometimes called king tides.

Why real coasts are messier

If the Earth were covered in a smooth, deep ocean, the story would end there. Real tides are shaped by geography.

Continents block the bulges, so the water sloshes around ocean basins in complicated patterns.
Some places get one high tide a day, and others get two high tides of very different heights.
Funnel-shaped bays and estuaries can amplify the tide hugely, giving some coastlines a range of many metres while enclosed seas such as the Mediterranean have very small tides.
Weather adds to it. Strong onshore winds and low air pressure can push water higher than predicted, which is part of what makes storm surges dangerous.

Practical sense at the seaside

Tide tables published for each port are the reliable guide, since local timing can differ a lot from one stretch of coast to the next. If you walk on beaches, across sandbanks or to tidal islands, check the times before you set out, note when the tide turns, and leave plenty of margin. The sea can come in faster than you can walk on flat sand. Have you seen a tide do something surprising where you live? Tell us about it in the comments.