Tides — Earth science, 4–6 years
The sea level rises and falls in a repeating pattern called tides. The Moon’s gravity pulls on Earth’s oceans, while Earth’s turning helps make the pattern move around the coast.
The sea comes in and goes out
At a coast, the water may cover more sand at one time and less sand several hours later. When the sea is higher, it is high tide; when it is lower, it is low tide. This rise and fall repeats, although the exact time and height change from place to place.
Why do tides happen?
The Moon pulls on Earth with gravity, and it pulls especially on the oceans because water can move. As Earth turns, different coastlines pass through places where the ocean is more raised and places where it is lower. The Sun also pulls on the oceans, so its position changes the tide pattern too.
Watching a rock pool
At 9 o’clock, the tide is low, so a rock pool and its crabs are easy to see. The tide table says high tide will be at 3 o’clock, six hours later. By then, more seawater covers the sand and reaches the rocks. The water has not vanished; it has moved with the tide.
Tides are not waves
A child may call every change at the beach a wave, because waves are the movement seen most clearly. A wave rises and breaks again and again in a short time, often because of wind. A tide changes the sea level over hours and can make the shoreline move far in or out.
Planning by the tide
People who fish, sail or explore rock pools check tide times before going out. A path that is dry at low tide may be covered by water later. Knowing the tide helps boats enter harbours, helps rescuers work safely and helps beach visitors avoid being cut off from the shore.
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