What Causes Tides? The Moon’s Invisible Pull on the Oceans

Take a morning walk on an ocean beach and you might see the water pulled back hundreds of metres — then, a few hours later, the same beach completely underwater. This regular breathing of the sea is called the tide, and behind this vast movement of water lies the invisible pull of the Moon, about 384,000 kilometres away.

How does the Moon pull on the oceans?

Every object in the universe attracts every other through gravity, and that pull weakens quickly with distance. The Moon pulls on Earth and everything on it. But because Earth is a large body, about 12,700 kilometres across, the Moon’s pull is not the same everywhere:

• the side facing the Moon is about 6,400 kilometres closer and is pulled hardest;
• Earth’s centre is pulled with medium strength;
• the far side is pulled most weakly.

What matters is not the pull itself but this difference, which scientists call the tidal force.

Why are there two high tides?

A bulge on the side facing the Moon is easy to understand. But there is also a bulge on the opposite side of Earth — the most puzzling part of tides.

Think of it this way:

• water on the near side is pulled more strongly than Earth’s centre and shifts towards the Moon;
• Earth’s centre is pulled more strongly than water on the far side, so Earth itself is effectively pulled towards the Moon, leaving the far-side water behind.

The result is that the oceans stretch into a slightly elongated shape along the line pointing to the Moon: two bulges, one facing the Moon and one facing directly away, with two troughs between them.

Two high tides, two low tides

Earth spins on its axis roughly every 24 hours, carrying coasts through the bulges and troughs in turn:

• passing under a bulge, the sea rises (high tide);
• passing under a trough, the sea falls (low tide).

But the Moon does not stay put — it orbits Earth in the same direction. So it takes not 24 hours but about 24 hours and 50 minutes for a coast to return to the same position relative to the Moon. As a result, most coasts have high tides about 12 hours and 25 minutes apart, and tide times shift about 50 minutes later each day.

The Sun’s role: spring and neap tides

The Sun is far more massive than the Moon but much further away. Because tidal force weakens with distance even faster than gravity does, the Sun’s tidal effect is only about half the Moon’s. Even so, it noticeably changes how big tides are.

• Spring tides: at full Moon and new Moon, the Sun, Earth and Moon line up and their tidal forces add together. High tides are higher and low tides lower. The name has nothing to do with the season; it refers to the water “springing up”.
• Neap tides: when the Moon is at first or last quarter, the Sun and Moon form a right angle with Earth. Their forces partly cancel and the tidal range is at its smallest.

This cycle repeats about every two weeks, in step with the Moon’s phases — which we explain in Why does the Moon change shape?.

Why isn’t it the same everywhere?

In theory, the tidal range in the open ocean is only about half a metre. Along coasts, though, it varies enormously, depending on the shape of the coastline and the depth of the seabed:

Funnel-shaped bays squeeze and amplify the incoming tide. In the Bay of Fundy, the natural sloshing period of the water is very close to the tidal cycle, creating resonance — like pushing a swing at just the right moment — and an extraordinary range.

Enclosed seas, by contrast, have tiny tides. The Mediterranean connects to the Atlantic only through the Strait of Gibraltar, about 14 kilometres wide, so the ocean’s tidal wave barely gets in. The Black Sea and the Baltic are even more cut off. In such seas, wind and air pressure often change the sea level more than the tide does.

Surprising effects of tides

Tides do more than move the sea:

• The land moves too: the Moon’s pull also flexes Earth’s solid crust. Wherever you are, the ground rises and falls by a few tens of centimetres each day without you noticing.
• Earth is slowing down: friction between the tides and the seabed very slowly brakes Earth’s rotation. Days lengthen by about 2 milliseconds per century; in the age of the dinosaurs, a day was noticeably shorter than 24 hours.
• The Moon is moving away: the same interaction transfers energy to the Moon, pushing it into an orbit about 3.8 centimetres further away each year — measured by bouncing lasers off mirrors left on the Moon by Apollo astronauts.
• Habitats: tidal flats, regularly flooded and exposed by the tide, are home to countless creatures. The Wadden Sea along the coasts of the Netherlands, Germany and Denmark is one of the most famous examples.

Energy from the tides

Tides are extremely regular and can be predicted precisely years in advance, making them a reliable renewable energy source. France’s La Rance power station has generated electricity from tidal currents since 1966, and South Korea’s Sihwa Lake station is one of the largest in the world. Such plants, however, can only be built on a few coasts with very large tidal ranges.

In short

Tides arise because the gravity of the Moon and Sun pulls on Earth’s near and far sides with different strengths. That difference creates two bulges in the oceans, and as Earth rotates, coasts experience two high tides and two low tides a day. Spring tides come when the Sun and Moon line up, neap tides when they form a right angle. The shape of the coast largely determines the range — which is why it reaches 16 metres in Canada but only a few centimetres in the Black Sea.