What Are Tides?

Tides are the regular rising and falling of ocean water levels that happen every day along coastlines around the world. If you have ever spent a day at the beach, you may have noticed the water creeping higher up the sand and then pulling back again. This back-and-forth movement is caused mainly by the gravitational pull of the Moon on Earth’s oceans. The Sun also plays a smaller role in creating tides, but the Moon’s effect is about twice as strong. Tides follow a predictable pattern that people have observed and relied on for thousands of years.

How the Moon Creates Tides

The Moon’s gravity pulls on everything on Earth, but it pulls most strongly on the water that is closest to it. This pull creates a bulge of water on the side of Earth facing the Moon, which we experience as a high tide. At the same time, a second bulge forms on the opposite side of Earth because the planet itself is being pulled slightly toward the Moon, leaving the far-side water behind. The areas between the two bulges experience low tides, where the water level drops. As Earth rotates on its axis, different places pass through these bulges, giving most coastlines two high tides and two low tides each day.

High Tide and Low Tide

During high tide, the ocean water rises and moves farther up the shore, covering more of the beach. During low tide, the water pulls back and exposes sand, rocks, and tide pools that were previously underwater. The difference in water level between high and low tide can range from less than a foot in some places to more than 40 feet in others. The Bay of Fundy in Canada has the largest tidal range in the world, with water levels changing by up to 53 feet. Most coastal areas experience two high tides and two low tides roughly every 24 hours and 50 minutes.

Spring Tides and Neap Tides

About twice a month, the Sun, Moon, and Earth line up in a row during full moons and new moons. When this happens, the Sun’s gravity adds to the Moon’s pull, creating extra-high high tides and extra-low low tides called spring tides. The name “spring” does not refer to the season but comes from an old word meaning to “spring up” or surge. About a week after each spring tide, the Sun and Moon are at right angles to each other, and their gravitational pulls partially cancel out. These weaker tides are called neap tides, and the difference between high and low water is smaller than usual.

The Sun’s Role in Tides

Although the Moon is the main cause of tides, the Sun also has a significant gravitational effect on Earth’s oceans. The Sun is much more massive than the Moon, but it is also much farther away, so its tide-producing force is only about 46 percent as strong as the Moon’s. When the Sun and Moon work together during spring tides, the combined pull creates the most dramatic tidal changes. When they work against each other during neap tides, the tides are gentler. The interaction between the Sun’s and Moon’s gravity creates a complex but predictable pattern of tidal changes throughout each month.

Tidal Zones and Wildlife

The area between the high tide line and the low tide line is called the intertidal zone, and it is one of the most challenging habitats on Earth. Animals and plants that live here must survive being underwater during high tide and exposed to air during low tide. Tide pools form in rocky areas when the water recedes, creating small ponds full of sea stars, anemones, crabs, and small fish. Creatures like barnacles and mussels attach firmly to rocks so they are not swept away by the changing water. The intertidal zone is a great place to explore and observe how living things adapt to constantly changing conditions.

How People Use Tides

For centuries, people have used their knowledge of tides for fishing, sailing, and building. Sailors must understand tides to navigate harbors safely, since shallow areas that are passable at high tide may be dangerously shallow at low tide. Fishers use tidal patterns to know when and where fish will be feeding. In some places, people have built tidal power stations that use the movement of tidal water to generate clean electricity. The Rance Tidal Power Station in France, built in 1966, was one of the first large-scale tidal power plants in the world.

Tides Beyond Earth

Earth is not the only place in our solar system where tides play an important role. Jupiter’s moon Io experiences powerful tidal forces from Jupiter’s enormous gravity, which squeeze and stretch the small moon so much that its interior stays hot enough to produce active volcanoes. Europa, another moon of Jupiter, may have a liquid ocean beneath its icy surface, kept warm partly by tidal heating. Saturn’s moon Enceladus also shows signs of tidal heating, shooting geysers of water ice into space. Studying tides on other worlds helps scientists search for places where life might exist beyond Earth.