What are tides?

• Tides are the regular rise and fall of sea levels.

• The gravitational pull of the Moon and Sun causes the tides.

• Knowledge of the tides is essential to safely enjoy the seaside.

Tides are the rise and fall of sea levels around the world.

There are usually two high tides and two low tides each day.

The size of the high and low tides change over the month. During a full moon or a new moon the difference between the high and low tide is at its greatest. This is because when the Earth, the Sun and the Moon are in a line, the gravitational pull is at its strongest.

Gravity from both the Sun and the Moon pulls water across the Earth’s surface. Just as the Earth pulls water downwards (in waterfalls, for example), the weaker gravitational pull of the Sun and Moon pulls water sideways across the Earth.

Each day the Earth rotates once on its axis. This means that each day, any place on Earth will rotate to face the Moon and then away from it.

As it does so, the gravitational force in this place increases and then decreases. When this place is over a sea or ocean, this increase in gravitational force causes the water there to move as a long, slow wave. Where this water meets the land, this wave causes a tide. This usually rises and falls twice a day.

The times of high and low tides change each day. They can be found on RNLI information boards and the Met Office beach forecast.

At any time, it can be high tide on one part of the coast of a country, and low tide on another. The height of tides on different beaches varies.

Many industries require a knowledge of the tides, including shipping, fishing and coastal engineering.

Tides can also be dangerous. It is important to know if the tide is coming in or going out. When the tide comes in (high tide) the whole beach can be covered by water. Lifeboats are often needed to rescue people who do not realise that a high tide is coming that could trap them.

Test your knowledge of tides with this quick quiz.