How Tides Work: The Moon, the Sun and Ocean Water

Walk along a coastline at different times of day and the water line moves. This rise and fall, known as the tide, is one of the most regular natural patterns on Earth. It is caused mainly by the gravity of the Moon, with a smaller contribution from the Sun.

Key takeaways

• Tides are caused by differences in gravitational pull of the Moon and Sun across the Earth.
• Most coasts see two high tides and two low tides in roughly a day, but patterns vary by place.
• Spring tides, with the biggest range, happen near new and full Moon.
• Neap tides, with the smallest range, happen near the quarter Moons.
• Coastline shape and sea floor strongly affect how big tides are locally.

Gravity and the tidal bulge

The Moon pulls on every part of Earth, but it pulls harder on the side nearest to it and more weakly on the far side. These differences create a stretching effect on the oceans, producing a bulge of water facing the Moon. A second bulge forms on the opposite side of Earth. As the planet spins, a coastline passes through each bulge about once a day, giving two high tides.

Why the cycle is a bit longer than a day

While Earth rotates, the Moon is also moving along its orbit. It takes a little more than 24 hours for a given spot to line up with the Moon again, about 24 hours and 50 minutes. That is why high tide happens roughly 50 minutes later each day.

The Sun's part

The Sun is far more massive than the Moon but much farther away, so its tidal effect is smaller, about half as strong. When the Sun, Earth and Moon line up at new Moon and full Moon, the effects combine and produce larger tides, called spring tides. The name has nothing to do with the season. When the Sun and Moon pull at right angles at the quarter phases, the effects partly cancel, giving smaller neap tides.

Why local tides differ

The simple picture of two bulges is only a starting point. Continents block the movement of water, and ocean basins, bays and river mouths shape the flow. Some places, such as certain narrow bays, funnel the water and create very large tidal ranges, while some regions have only one high tide a day or very small tides. Local tide tables, produced from long records, are used to predict what will happen at a specific harbour.

Why tides matter

Tides affect navigation, because ships may need enough water to enter harbours. They shape coastal habitats such as tidal flats and rock pools, where animals have adapted to being covered and uncovered. Tidal currents are also studied as a source of renewable energy. For anyone walking on a beach or estuary, checking tide times is a basic safety step, as incoming water can cut off routes surprisingly quickly.

How tide tables are made

Predicting tides is a mix of astronomy and local measurement. For each port, scientists record water levels over a long period and use a method called harmonic analysis to break the pattern into many components, each linked to the motions of the Moon and Sun. Once the constants for a location are known, tides can be predicted far into the future. The lunar day, the time between one moonrise and the next, lasts about 24 hours and 50 minutes, which is why high tide comes later each day. Tide tables also have to account for weather, since strong winds and low pressure can raise or lower sea levels.

Extreme tides

Tidal range varies enormously between places. In the open ocean, tides are small, but in some narrow, funnel-shaped bays, the water piles up. The Bay of Fundy in Canada has among the largest tides in the world, with a difference between high and low water that can exceed sixteen metres. At Mont-Saint-Michel in France, the tide rushes in across wide flats, and the Severn estuary in Britain has a tidal bore, a wave that travels upriver on large tides. Other places, such as the Mediterranean, have tiny tides.

Tides and living things

The intertidal zone, the strip between high and low tide, is a tough habitat. Animals such as barnacles, mussels and limpets cope with being underwater and then exposed to air and sun, while crabs and fish move with the water. Rock pools hold marine life at low tide and are fun places to explore, though visitors should take care not to disturb animals. Mudflats feed vast numbers of migrating birds, and mangroves and salt marshes depend on tidal flooding. Some species time their spawning to spring tides.

Harnessing tidal power

Tides are predictable, so they offer a reliable source of renewable energy. The La Rance power station in France, opened in 1966, uses a barrage across an estuary, and a larger barrage operates at Sihwa Lake in South Korea. Other designs use underwater turbines, like wind turbines in the sea, placed in fast tidal streams. Tidal power is expensive to build and can affect habitats and sediment, so projects need careful assessment, and it supplies only a small share of world electricity.

Tides beyond the Earth

Tidal forces shape other worlds. The Moon is tidally locked to Earth, so it always shows the same face, and tides raised by Earth heat the interior of Jupiter's moon Io, driving its volcanoes. Tides may also keep subsurface oceans liquid on moons such as Europa and Enceladus. On Earth, tidal friction is slowing the planet's rotation, lengthening the day by about two milliseconds per century, while the Moon moves slowly away by a few centimetres a year.

Frequently asked questions

Do lakes have tides?

Very large lakes have tiny tides, but they are usually too small to notice.

Is a tide the same as a wave?

No. Waves are mostly driven by wind. Tides are slow, regular changes in water level caused by gravity.

Why are there two high tides a day?

There is a bulge of water on the side facing the Moon and another on the far side, so a coastline passes through two bulges as Earth rotates.

What is a king tide?

King tide is a popular term for an especially high tide, usually occurring a few times a year when the Sun and Moon line up with the Earth and the Moon is closest to Earth. These tides can cause coastal flooding, especially when combined with storms and rising sea levels.

What causes a tidal bore?

A tidal bore forms when a rising tide enters a narrow, shallow river or estuary and pushes upstream as a wave or series of waves. It needs a large tidal range and a funnel-shaped channel, and bores occur in places such as the Severn in Britain and the Qiantang in China.

Are tides affected by the weather?

Tides themselves are driven by astronomy, but weather can change the actual water level. Strong onshore winds and low air pressure can create a storm surge that adds to the tide, and high pressure can slightly lower the level.

Sources and further reading

• Tide (Wikipedia)
• NOAA National Ocean Service