How Do Tides Work?

If you stand on a beach in Brittany, France, and watch the ocean retreat and return twice each day, you are witnessing one of the most predictable phenomena in nature. The water rises and falls with clockwork regularity, not because of wind or waves, but because of forces you cannot see.

Tides are the response of Earth’s oceans to gravity from the Moon and Sun, combined with the spinning of the Earth-Moon system. In this story, the Moon is the main actor, the Sun plays a supporting role, and Earth’s rotation sets the pace.

The short answer

Tides are the rise and fall of sea levels caused by the gravitational pull of the Moon and Sun on Earth’s oceans. The Moon’s gravity pulls water toward it, creating a bulge on both the near side and far side of Earth. As Earth rotates beneath this bulge, coastal areas experience two high tides and two low tides each day. The Sun also exerts gravitational pull, amplifying or reducing the Moon’s effect depending on their relative positions.

The full picture

The force behind tides: differential gravity

Your first guess might be that the Moon pulls water toward itself, creating a bulge on one side of Earth. This is partially correct, but the full picture is more interesting. There is also a bulge on the opposite side of Earth, which seems counterintuitive until you understand differential gravity.

Gravity gets weaker with distance. The Moon pulls hardest on the water closest to it, less hard on the water in the middle, and least hard on the water farthest from it. Meanwhile, Earth itself is pulled toward the Moon, but the water on the opposite side is not pulled as hard. The net result is that water on both sides gets pulled outward, creating two bulges.

This is called differential gravity, or the tidal force. The same effect occurs on a smaller scale with the Sun, which is far more massive but also much farther away. The Sun’s tidal influence is about 46% of the Moon’s, as noted by NASA’s ocean tides overview.

The twice-daily rhythm

Earth rotates beneath the tidal bulge once every 24 hours, but the tidal bulge itself drifts slightly ahead because the Moon is orbiting Earth. This means the time between consecutive high tides is about 12 hours and 25 minutes, not exactly 12 hours.

If you are on a coastline facing the open ocean, you will typically experience two high tides and two low tides each day. This is called a semidiurnal tide. Some locations, particularly in parts of the Gulf of Mexico and the Caribbean, have only one high and one low per day, called a diurnal tide. Most coastal areas fall between these patterns, with mixed semidiurnal tides showing two high tides of different heights each day.

The height of tides varies dramatically by location. The Bay of Fundy in Canada has the highest tides in the world, with a range of about 16 meters between high and low tide. This is due to the bay’s shape, which funnels and amplifies the tidal wave. Other locations, like the Mediterranean Sea, have tidal ranges of less than half a meter because they are nearly enclosed.

Spring tides and neap tides: the monthly cycle

The tidal range is not constant throughout the month. Every two weeks, the Sun and Moon align in one of two ways.

During the new moon and full moon, the Sun, Moon, and Earth are aligned. The Sun’s gravity adds to the Moon’s gravity, pulling the ocean water to its highest reach. These are spring tides, from the Germanic word for jump, not the season. Spring tides have the highest high tides and the lowest low tides.

During the first quarter and third quarter moons, the Sun and Moon are at right angles as seen from Earth. The Sun’s gravity partially cancels the Moon’s, reducing the tidal range. These are neap tides, with moderate highs and higher-than-average lows.

The spring-neap cycle repeats every 14.77 days, the time between consecutive alignments of the three bodies. This creates a predictable rhythm that sailors, fishermen, and coastal communities have relied on for millennia.

Beyond the ocean: earth tides and internal tides

The solid Earth also experiences tides, called earth tides, flexing by about 30 centimeters at the surface. This is measurable with precise instruments but does not cause earthquakes or noticeable movement. The crust rises and falls in the same twice-daily rhythm as the ocean.

Inside the ocean, there are internal tides that travel along density boundaries between water layers of different temperature and salinity, as described by NOAA’s tides and currents documentation. These internal waves are invisible from the surface but can be hundreds of meters tall and travel for thousands of kilometers.

What this means in real life

Tides affect coastal life in practical ways.

Fishing tidal cycles determine when certain species are catchable. Many fish feed more actively during the turning of the tide, when water movement is highest. Recreational anglers often plan trips around specific tide stages.

Shipping relies on tide tables to navigate shallow ports. A ship that sails on a high tide may run aground on the same channel at low tide. Major ports like Rotterdam and Antwerp publish detailed tide schedules for vessel planning.

Marine ecosystems are shaped by tides. Intertidal zones, the areas between high and low tide, are their own ecosystems with species adapted to survive both immersion and exposure. The twice-daily flooding and drying shapes everything from snail behavior to algal growth.

Erosion and sediment deposition are tide-driven. Coastal wetlands accumulate sediment during high tides and are reshaped by the energy of outgoing tides. The rhythmic rise and fall distributes nutrients and shapes tidal flats.

Why it matters

Tides are one of the most predictable phenomena on Earth. Unlike weather or earthquakes, tides can be calculated centuries in advance with high accuracy. This predictability is built into coastal life around the world.

The tidal cycle also connects to broader Earth systems. The transfer of energy from the Moon to Earth’s rotation is gradually slowing Earth’s spin and lengthening the day, while pushing the Moon slightly farther away. The mutual gravitational dance that creates tides is slowly reshaping the Earth-Moon system.

For coastal communities, understanding tides is not optional. It is essential for navigation, fishing, managing wetlands, and planning coastal development. The same force that makes the water rise and fall on your local beach links to the fundamental physics of how the Earth and Moon interact.

Common misconceptions

Tides are caused by the Moon pulling water away from Earth. Not quite. The differential gravity creates bulges on both sides, which is why there are two tides per day, not one.

Tides only happen in the ocean. False. The solid Earth experiences earth tides, and the atmosphere also has atmospheric tides, though these are much smaller.

Spring tides occur in spring. The name springs from the old Germanic word meaning to leap or jump, not from the season. Spring tides happen every two weeks, regardless of the time of year.

Key terms

High tide: The highest water level of the tidal cycle, when the tidal bulge is aligned with your location.

Low tide: The lowest water level, when your location is perpendicular to the tidal bulge.

Spring tide: The highest high tides and lowest low tides, occurring during new moon and full moon when the Sun and Moon are aligned.

Neap tide: The smallest tidal range, occurring during first and third quarter moons when the Sun and Moon are at right angles.

Tidal range: The vertical difference between high tide and low tide at a given location.

Differential gravity: The gravitational force that varies across Earth because of distance, creating the tidal bulge on both near and far sides.