Why tides rise and fall — Earth science, 7–10 years
The Moon’s gravity pulls on Earth’s oceans, while Earth’s turning makes coastlines pass through changing bulges of water.
The ocean’s regular rise and fall
A tide is the regular rise and fall of sea level along a coast. The Moon’s gravity pulls ocean water into broad, gentle bulges. As Earth turns, a coastline moves into a bulge, called high tide, and then away from it, called low tide.
The problem of changing shorelines
People living near the sea saw water reach the shore and later retreat in a pattern that repeated. Wind and waves can change the surface for a while, but they do not explain this dependable timing. The Moon provided the missing link: its gravity helps move the oceans in a repeating rhythm.
Reading a tide table
Imagine a harbour table says high tide is at 06:00 and 18:20, while low tide is at 12:10. At 06:00, the water is near its highest level. By 12:10 it has fallen to its lowest level, then rises again until 18:20. A sailor uses these times to plan safely.
The Moon does not make waves
It is easy to call every movement of seawater a tide, especially when waves are visible at the same beach. Waves are usually made by wind and move across the surface. Tides change the sea level over much larger areas and over hours, even when the water is not wavy.
Coasts, boats and living things
Tides matter to people who launch boats, walk on mudflats or build near a shore. A boat may need enough water to pass a shallow channel at high tide. Many shore animals and plants are adapted to being covered by water at one tide and exposed to air at another.
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