How tides work — Earth science, 14–17 years
Tides are regular changes in sea level caused mainly by the Moon’s gravity, with help from the Sun. Earth’s rotation and the shape of coastlines turn this force into the local pattern people observe.
A moving ocean level
The Moon pulls on Earth, but it pulls more strongly on the side facing it than on the far side. This difference stretches the oceans into two broad bulges. As Earth rotates through these bulges, many coasts experience high tide and low tide in a repeating pattern.
The problem it solved
People needed to predict when water would cover rocks, harbours or mudflats. A simple idea that the Moon merely lifts water directly underneath it cannot explain two tidal bulges or different local timings. The useful model considers differential gravity, Earth’s rotation and basin shape.
Predicting a spring tide
At new Moon, the Sun, Moon and Earth are roughly aligned. Step 1: the Moon creates a tidal pull. Step 2: the Sun pulls in nearly the same direction, so their effects reinforce each other. Step 3: the high tides become higher and the low tides lower: this is a spring tide, not a seasonal spring.
The water-bulge mix-up
A reasonable mistake is to picture one water bulge only on the Moon-facing side. Gravity feels strongest there, so that picture seems natural. But tides depend on differences in gravity across Earth, and the far side also forms a bulge because the solid Earth is pulled more strongly towards the Moon than the distant water.
Where it is used
Tide predictions guide ship departures, bridge and harbour design, coastal flood warnings and the timing of some beach surveys. Tidal energy can generate electricity in suitable places, but local ecosystems and sediment movement must be considered. A tide table is local, not universal.
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