Tide

Tides are the regular rising and falling of sea levels. They are caused by the gravitational pull of the Moon and the Sun. These forces make the water level change during the day. The Earth’s rotation and how water moves also help shape tides.

Tides are not the same everywhere. The shape of the ocean and the land around it influence how water moves. Scientists use special tools called tide gauges to measure water levels and understand these changes.

Other things like temperature changes, wind, and air pressure can also affect sea levels. These changes can sometimes make water levels higher, especially near coastlines.

Tides are not just in Earth’s oceans; they can also happen in the solid Earth. The solid part of our planet changes shape very slightly due to gravitational forces, but this is harder to notice than the movement of water.

Characteristics

Ocean tides are the regular rise and fall of sea levels that happen about twice each day. There are four main stages in this cycle: low tide (the lowest point), flooding (when water rises), high tide (the highest point), and ebbing (when water falls back down).

Illustration by the course of half a month

Tides can be semi-diurnal, meaning there are two high tides and two low tides each day, or diurnal, with just one cycle per day. The height of high tides and low tides can change slightly each day. Sometimes, tides reach their highest and lowest points in a pattern called spring tides, which happen when the Sun and Moon line up. Other times, during neap tides, the difference between high and low tides is smaller. These changes happen over about two weeks.

Main article: Tidal range

Tidal constituents

Further information: Theory of tides § Tidal constituents, and Long-period tides

Tidal constituents are the parts that make up the rise and fall of tides. They come from the Earth's rotation, the positions of the Moon and Sun, the Moon's height above Earth's equator, and the shape of the ocean floor. The most important part is called the principal lunar semi-diurnal constituent. It causes the regular pattern of high and low tides, usually about every 12 hours and 25 minutes.

The Moon's distance from Earth also changes tides. When the Moon is closest to Earth, tides are higher, and when it is farthest, tides are lower. Other factors, like the tilt of Earth's axis and the shape of its orbit, also help make tides vary.

History

Further information: Theory of tides § History

Brouscon's Almanach of 1546: Compass bearings of high waters in the Bay of Biscay (left) and the coast from Brittany to Dover (right).

Long ago, people had many ideas about why tides happened. Some thought the sea breathed like a living thing. Others believed special forces controlled the water.

Later, scientists learned more. Seleucus of Seleucia thought the Moon caused tides. Bede also linked tides to the Moon and its phases. Many people studied tides to understand how the Moon and Sun affect them.

Today, we know tides are mainly caused by the gravity of the Moon and the Sun, but the shape of oceans and Earth’s rotation also matter.

Physics

Main article: Theory of tides

The simplest way to understand why we have two tides each day is through what we call equilibrium theory. This idea, first suggested by Isaac Newton, helps us see how the Moon’s pull creates tides. It says three things: there is no land, water moves right away when pulled by gravity, and there is no friction between Earth and water. In this imagined world, the Moon’s gravity pulls on Earth. It pulls a little more on the side closest to the Moon and also on the far side. This makes water collect in two bulges — one near the Moon and one far from it. As Earth spins, different places move through these bulges. That is why we have two high tides and two low tides each day.

The Moon’s pull is the main reason we have tides, but the Sun also helps. Even though the Sun is much bigger, it is also much farther away, so its pull on tides is weaker. Together, the Moon and Sun create different tidal patterns, like especially high tides during a full moon or new moon. The shape of coastlines and the depth of the ocean also change how tides behave in different places.

Observation and prediction

The same tidal forcing has different results depending on many factors, including coast orientation, continental shelf margin, water body dimensions.

Tides are the regular rising and falling of sea levels. They are caused mainly by the gravitational pull of the Moon and the Sun. These forces create waves of water that move around the oceans. The time when these waves reach a place tells us when high and low tides will happen.

The shape of the ocean floor and coastlines also affects tides. Some places, like the Bay of Fundy in Canada, have very high tides because of their special shape. Other places, like Southampton in the United Kingdom, can have two high tides in one day due to how tides interact in that area.

Navigation

Tidal flows are important for navigation because they can make boats think they are in the wrong place. Tidal heights matter, especially in shallow areas where boats can’t enter during low tide.

In the past, naval officers had to calculate tides carefully. Today, we use tide charts and tidal stream atlases to see when and how fast tides move. Nautical charts show water depths and the shape of the ocean floor, helping sailors know how deep the water is at different times. Tide tables tell us the high and low water heights for each day, and we can use these to find the real water depth when sailing.

Biological aspects

Main article: Intertidal ecology

Further information: Intertidal zone

Intertidal ecology looks at the plants and animals that live between the low- and high-water marks on shores. These areas go from dry land to being underwater with the tides.

Many sea creatures have special ways to survive here. They live in bands at different heights above the water line. People sometimes take food from these areas, but taking too much can hurt them. Protecting these places helps scientists learn more about nature and keeps them safe.

Other tides

Internal tides

Main article: Internal tide

When ocean currents flow over bumpy ocean floors, they make special waves called internal waves.

Lake tides

Large lakes such as Superior and Erie can have very small tides, but these are usually hidden by other water movements. For example, Lake Michigan has tiny tides that are too small to notice.

Atmospheric tides

Main article: Atmospheric tide

Tides also happen in the air, but they are too small to notice on the ground. They become important high up in the sky.

Earth tides

Main article: Earth tide

Even the Earth moves a little because of the Moon and Sun. This movement is very small but important for some scientific tools.

Galactic tides

Main article: Galactic tide

Big galaxies pull on the stars and objects around them, affecting things far away in space.

Misnomers

Tsunamis, the big waves that follow earthquakes, are sometimes called tidal waves. This name is used because they look like tides, not because they are caused by tides. Other things that use the word tide but are not really tides include rip tide, storm tide, hurricane tide, and black or red tides. Many of these names have old meanings, talking about tide as a part of time or a flow of water.

In Culture

Tides have inspired many cultures, stories, and traditions. People use the regular rise and fall of the sea to show change, time, and life's ups and downs in myths, poems, and writings.

Long ago, before we knew the science of tides, many ancient cultures thought gods or other forces moved the ocean. Artists and writers still use tides to symbolize change, reflection, and the natural cycles of our world.