Ocean Science · Explainer

How Tides Work

Twice a day, the sea quietly rises and falls along most of the world's coasts. Here's the real reason why — the Moon, the Sun, and a gentle tug-of-war with gravity.

01 What a tide actually is

A tide is the slow, regular rise and fall of the sea surface, repeating on a rhythm of hours rather than the quick up-and-down of wind waves. Watch a harbour wall through a day and you'll see the waterline climb to a high tide, then slide back down to a low tide, and do it again — like the ocean breathing.

The whole ocean is being gently reshaped. It isn't the water being "sucked up" from one spot; it's the entire sea bulging slightly in some places and thinning in others, while the solid Earth turns beneath it. To see why, we have to look up — at the Moon.

  • ~2high tides most coasts see each day
  • 24h 50mlength of a tidal day
  • Moonthe main driver of the tides

02 The Moon's uneven pull: two bulges

Gravity gets weaker with distance. The Moon pulls on every part of the Earth, but it pulls hardest on the side facing it, a little less on the Earth's center, and weakest on the far side. Tides come from those differences in pull — what scientists call the tidal or differential force — not from the Moon's raw gravity alone.

Compare each part of the Earth to the planet's center. On the near side the Moon's pull is stronger than average, so the oceans are drawn toward the Moon, forming a bulge. On the far side the pull is weaker than average, so relative to the center the water is effectively left behind, forming a second bulge pointing away from the Moon. The result is two bulges on opposite sides of the planet.

The Earth with two tidal bulges and the Moon to the right A blue Earth is stretched into an oval by two ocean bulges — one facing the Moon on the right, one on the opposite far side on the left. The Moon sits to the right. Small arrows show the Moon's pull being strongest on the near side and weakest on the far side. Near-side bulge strongest pull → Far-side bulge ← weakest pull Earth Moon Vertical scale of the bulges is exaggerated for clarity
Two tidal bulges. The ocean is stretched into a slight oval — one bulge toward the Moon, one directly opposite. Bulge height is drawn far larger than in reality (real ocean tides are typically under a metre in the open sea).
Common myth, corrected: the far-side bulge is not water being "flung outward by centrifugal force." It appears because the Moon pulls that far-side water less than it pulls the Earth's center — so, relative to the Earth as a whole, the water lags behind and heaps up away from the Moon.

03 Why two highs and two lows a day

The two bulges stay lined up with the Moon while the Earth rotates through them. As your stretch of coastline is carried around, it passes through a bulge (high tide), then a thin spot a quarter-turn later (low tide), then the other bulge (high tide again), then the other thin spot (low tide) — roughly two highs and two lows in a day.

Why not exactly every 12 hours? Because the Moon is also moving. In the time the Earth spins once, the Moon has travelled a little farther along its orbit, so the Earth must turn an extra ~50 minutes to "catch up" and point at the Moon again. That's the tidal day of about 24 hours and 50 minutes — which is why high tide slides a little later each day.

Four positions of a coastline as the Earth rotates through the two bulges The Earth in the middle with two ocean bulges pointing left and right toward the Moon. A marked coastline is shown at four points around the Earth: high tide when it is in a bulge, low tide a quarter turn away. Earth's spin High High Low Low Earth → to Moon bulge
One rotation, two highs. A single coastline (the dot) is carried around the spinning Earth, passing through both bulges and both thin spots — high, low, high, low — over roughly a day.

04 The Sun's role: spring & neap tides

The Sun is vastly more massive than the Moon, but it's also far away — and because tides depend on the difference in gravity across the Earth, distance matters enormously. The upshot: the Sun's tidal effect is real but only about half as strong as the Moon's. It doesn't make its own separate tides; instead it reinforces or partly cancels the Moon's.

Spring tides — Sun and Moon pulling together

At new moon and full moon, the Sun, Earth, and Moon line up. Their tidal effects add together, so high tides are higher and low tides are lower than usual. These are spring tides — the name comes from the sea "springing up," nothing to do with the season, and they happen about twice a month.

Neap tides — pulling at right angles

At the first- and third-quarter moons, the Sun and Moon sit at right angles as seen from Earth. The Sun's pull partly cancels the Moon's, so the difference between high and low tide is at its smallest. These are neap tides.

Spring tide alignment The Sun, Earth and Moon in a straight line, so their tidal bulges add up to a large stretched oval. Sun Earth Moon Spring tide pulls add up
Spring tide. Alignment at new & full moon — the biggest tidal range.
Neap tide alignment The Moon at a right angle to the Sun, so the Sun's smaller bulge pulls crosswise and partly cancels the Moon's, giving a rounder, smaller tidal range. Sun Earth Moon Neap tide pulls compete
Neap tide. Right-angle at quarter moons — the smallest tidal range.
Sun Moon Earth Ocean bulge

05 Why real coasts differ

The two-bulge picture is the engine, but the tide you actually measure depends heavily on local geography. Ocean basins, continents, and the sea floor all get in the way, so the neat oval never sweeps cleanly around a smooth water-world.

Coastline and basin shape funnel and amplify the tide. The Bay of Fundy in Canada is famous for tidal ranges over 15 metres because its length and depth let the water slosh in near-resonance with the tidal rhythm, while many enclosed seas — like much of the Mediterranean — barely have a noticeable tide at all.

Patterns differ too. Many coasts have semidiurnal tides (two nearly equal highs and lows a day); some have diurnal tides (just one high and one low); and others are mixed, with two unequal highs and lows. Which one you get depends on how the tide resonates in that particular ocean basin.