Tide

Tides are the regular rising and falling of sea levels caused by the gravitational pull of the Moon and the Sun. These forces create a difference in water level that changes throughout the day. The Earth’s rotation and the way water moves around the planet also play important roles in shaping tides.

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

Although tides are mainly driven by the Moon and Sun, other things like temperature changes in the water, wind, and air pressure can also affect sea levels. These changes can sometimes cause higher water levels, especially near coastlines.

Tides are not just a feature of Earth’s oceans; they can also happen in solid Earth itself. The solid part of our planet changes shape very slightly due to gravitational forces, though this effect is much 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).

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

See also: Earth tide § Tidal constituents

Tidal constituents are the different 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. There are many tidal constituents, but the most important one is called the principal lunar semi-diurnal constituent. It causes the regular pattern of high and low tides that we see, 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. These changes help create the different heights we see in tides from day to day. Other factors, like the tilt of Earth's axis and the shape of its orbit, also play a part in making tides vary.

History

Further information: Theory of tides § History

Long ago, people thought tides were caused by myths or spirits. Stories from India and East Asia described the sea as breathing like a living thing. Others believed supernatural forces controlled the rising and falling of the water.

Later, scientists began to understand tides better. Seleucus of Seleucia around 150 BC thought the Moon caused tides. Bede in 725 linked tides to the Moon and its phases, noticing how tides changed over time. Over the centuries, many smart people studied tides, trying to figure out 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 play big roles.

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 picture how the Moon’s pull creates tides. It assumes three things: there is no land, water responds to gravity instantly, and there is no friction between Earth and water. In this imagined world, the Moon’s gravity pulls on Earth, creating a small extra pull on the side closest to the Moon and a similar pull on the far side. This causes water to pile up in two bulges — one near the Moon and one far from it. As Earth spins, different places move through these bulges, which is why we experience two high tides and two low tides each day.

The Moon’s pull is the main reason we have tides, but the Sun also plays a part. Even though the Sun is much bigger, it is also much farther away, so its tidal pull is weaker. Together, the Moon and Sun’s pulls 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

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

The shape of the ocean floor and coastlines also affects when and how high tides occur. 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 very important for navigation because they can cause big mistakes in where boats think they are. Tidal heights matter too, especially in places where rivers and harbors have shallow areas that boats can’t enter during low tide.

In the past, naval officers needed to know how to calculate tides well. Today, we use tide charts and tidal stream atlases to see when and how fast tides move. Nautical charts show water depths and shapes of the ocean floor, which help sailors know how deep the water really is at different times. Tide tables tell us the high and low water heights for each day, and we can use these to figure out the real water depth when we’re sailing.

Biological aspects

Main article: Intertidal ecology

Further information: Intertidal zone

Intertidal ecology studies the ecosystems found between the low- and high-water marks on shores. These areas change from being dry land to being underwater with the tides. Scientists look at how the plants and animals in these zones interact with each other and their changing environment.

Many sea creatures have special ways to survive in this tough place, where they can dry out or get washed away. They live in bands at different heights above the water line. People sometimes take food from these areas, but taking too much can harm 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 move over uneven ocean floors, they create 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 itself 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 large waves that happen after earthquakes, are sometimes called tidal waves. This name is used because they look like tides, not because they are caused by tides. Other phenomena that use the word tide but are not actually related to tides include rip tide, storm tide, hurricane tide, and black or red tides. Many of these terms have historical meanings, referring to tide as a portion of time or a flow of water.

In Culture

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

In the past, before we understood the science behind tides, many ancient cultures believed that gods or other forces controlled the ocean's movements. Artists and writers continue to use tides as symbols for transformation, reflection, and the natural cycles that shape our world.