Stand on a beach for a few hours and you will notice that the sea does not stay still. The water slowly creeps up the sand, covering rocks and filling pools, then slowly retreats, revealing seaweed, shells and wet sand. In most places, this happens twice a day. In some bays, the water rises by more than 40 feet (12 meters) between low and high tide; in other places, it changes by only a few inches. The cause of this daily rhythm lies far above the waves, in the gravitational pull of the Moon and the Sun. This explainer describes how tides work and why they differ so much from place to place.
Gravity and the Moon
Every object with mass attracts every other object through gravity. The Moon pulls on Earth, and Earth pulls on the Moon. Because the Moon is relatively close, its gravitational pull is the main cause of ocean tides, even though the Sun is far more massive.
The key to tides is not the strength of the Moon's gravity itself but the difference in its pull across Earth. The side of Earth facing the Moon is about 4,000 miles (6,400 kilometers) closer to the Moon than Earth's center, and the far side is about 4,000 miles farther away. Since gravity weakens with distance, the Moon pulls slightly harder on the near side of Earth, slightly less on the center and even less on the far side.
These differences in pull are called tidal forces. They stretch Earth very slightly along the line toward the Moon.
Why there are two high tides
The stretching effect creates two bulges of water on opposite sides of Earth:
- On the side facing the Moon, the water is pulled toward the Moon more strongly than Earth's center, forming a bulge.
- On the opposite side, Earth's center is pulled toward the Moon more strongly than the water there, so the water is effectively "left behind," forming a second bulge.
Between the bulges, the water level is lower. As Earth rotates once a day beneath these bulges, most coastlines pass through two high tides and two low tides.
A more complete explanation considers that Earth and the Moon both orbit around their common center of mass, which lies inside Earth about 2,900 miles (4,700 kilometers) from its center. But the basic idea is the same: differences in the Moon's pull across Earth create two bulges.
Why tides come later each day
High tides do not happen at the same time each day. They occur about 50 minutes later every day. The reason is that while Earth rotates, the Moon also moves along its orbit in the same direction. After one full rotation of Earth, a given point on the coast must rotate a little further to catch up with the Moon. A "lunar day," the time between one moonrise and the next, lasts about 24 hours and 50 minutes. As a result, in places with two tides a day, high tides arrive about 12 hours and 25 minutes apart.
The Sun's role: spring and neap tides
The Sun also creates tides, but because it is so far away, its tidal effect is only about 46 percent as strong as the Moon's. The Sun's tides combine with the Moon's tides in a monthly cycle:
- Spring tides: At new moon and full moon, the Sun, Moon and Earth are roughly in a line. Their tidal forces add together, producing 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.
- Neap tides: At first and third quarter moon, the Sun and Moon are at right angles relative to Earth. Their tidal forces partly cancel, producing a smaller difference between high and low tide.
Spring and neap tides each occur about twice a month. The difference between high and low water is called the tidal range, and it can vary greatly between spring and neap tides.
Tides are also somewhat stronger when the Moon is closest to Earth in its elliptical orbit, at perigee, and when Earth is closest to the Sun in early January. When perigee coincides with a spring tide, especially high tides called perigean spring tides, or popularly "king tides," can occur.
Why tides differ so much from place to place
If Earth were completely covered by deep ocean with no continents, tides would follow the simple pattern of two bulges. But the real world is far more complicated. Continents block the movement of water, ocean basins have different shapes and depths, and Earth's rotation deflects moving water through the Coriolis effect. As a result, tides behave very differently around the world.
Tidal patterns
There are three main types of daily tidal patterns:
| Pattern | Description | Example regions |
|---|---|---|
| Semidiurnal | two high and two low tides of similar height each day | much of the Atlantic coast of North America and Europe |
| Mixed | two high and two low tides of different heights each day | much of the Pacific coast of North America |
| Diurnal | one high and one low tide each day | parts of the Gulf of Mexico and Southeast Asia |
Tidal ranges
The shape of coastlines and the seafloor has a huge effect on the tidal range:
- Funnel-shaped bays concentrate tidal water. The Bay of Fundy in Canada has the largest tidal range in the world, more than 50 feet (16 meters) in some places, because its shape and length cause the water to resonate with the tidal rhythm, like water sloshing in a bathtub.
- Enclosed seas such as the Mediterranean and the Baltic have very small tides, often only a few inches, because their narrow openings limit how much water can flow in and out.
- Open ocean tides are relatively small, typically about 2 to 3 feet (less than a meter).
Amphidromic points
In ocean basins, the tide rotates around points where the tidal range is nearly zero, called amphidromic points. The tide wave travels around these points like water swirling around a drain, once for each tidal cycle. The farther a coast is from an amphidromic point, the larger its tidal range tends to be.
Tides in rivers: tidal bores
In some rivers, the incoming tide forms a wave that travels upstream, called a tidal bore. Famous examples include the Qiantang River in China, where the bore can be several meters high, and the Severn River in England. Surfers sometimes ride these waves for miles.
Tides and life
Tides shape coastal ecosystems. The area between high and low tide, called the intertidal zone, is home to creatures that can survive both underwater and exposed to air, such as barnacles, mussels, sea stars, crabs and seaweed. Tide pools are miniature ecosystems full of life. Many birds feed on mudflats exposed at low tide. Salt marshes and mangroves depend on tidal flooding.
Tides also matter to people. They affect shipping, fishing and coastal flooding. Tide tables, based on long-term observations and mathematical predictions, tell sailors and beachgoers when to expect high and low water. Some coastal regions use tidal energy, harnessing the flow of water to generate electricity.
Tides on land and in the Moon
Tidal forces affect not only oceans but also the solid Earth. The ground itself rises and falls by up to about 1 foot (30 centimeters) twice a day, an effect called Earth tides. We do not notice it because everything around us moves together.
Tides also affect the Moon. Earth's much stronger tidal forces on the Moon long ago slowed its rotation until it became tidally locked, always showing the same face to Earth.
The Moon is slowly drifting away
Tides gradually transfer energy between Earth and the Moon. Friction between the tidal bulges and the rotating Earth slowly slows Earth's rotation, making days longer by about 1.7 milliseconds per century. At the same time, the Moon gains energy and moves slowly away from Earth, by about 1.5 inches (3.8 centimeters) per year. Scientists have measured this precisely by bouncing lasers off reflectors left on the Moon by the Apollo astronauts.
Questions readers ask
Why are there two high tides a day? Differences in the Moon's gravitational pull across Earth create two bulges of water, one facing the Moon and one on the opposite side. As Earth rotates, coastlines pass through both bulges.
What are spring tides? Higher-than-average tides that occur when the Sun, Moon and Earth are aligned, around new and full moon.
Why do some places have very large tides? The shape of the coastline and seafloor can funnel and amplify tidal water, as in the Bay of Fundy.
Does the Sun cause tides? Yes, but its tidal effect is less than half as strong as the Moon's because it is much farther away.
Do lakes have tides? Very large lakes, like the Great Lakes, have tiny tides of a few centimeters, but they are usually masked by wind and air pressure changes.
In short
Tides are caused mainly by differences in the Moon's gravitational pull across Earth, which create two bulges of water on opposite sides of the planet. As Earth rotates, most coasts experience two high and two low tides roughly every 24 hours and 50 minutes. The Sun adds its own weaker tides, creating stronger spring tides and weaker neap tides each month. Coastlines, seafloors and Earth's rotation make tides vary enormously from place to place, from barely noticeable ripples to towering ranges in funnel-shaped bays. This daily rhythm shapes coastlines, ecosystems and human life along the sea.