Why ocean tides rise and fall — Astronomy, 7–10
The Moon’s pull helps move the oceans, making sea level rise and fall in a repeating pattern.
Tides are changing sea levels
A tide is the regular rise and fall of sea level along a coast. The Moon’s gravity pulls ocean water, and as Earth turns, different coastlines move into and out of the raised water areas.
Why did people need to understand tides?
Fishers, sailors and people living by the sea needed to know when water would cover rocks, reveal a beach or make a harbour deep enough. Watching the Moon and recording sea levels showed that tides follow a pattern, not a random trick of the waves.
Reading a simple tide table
A harbour table says: high tide at 08:00 and low tide at 14:00. Step 1: At 10:00, two hours after high tide, the water is going down. Step 2: At 14:00 it reaches low tide. Step 3: After that, it starts rising again.
Tides are not just big waves
It is reasonable to confuse tides with waves because both move seawater and can be seen at a beach. Waves are usually made by wind and repeat quickly; tides change the overall sea level over several hours because of the Moon’s pull and, to a lesser extent, the Sun’s.
Tides at a working coast
Harbours use tide times to plan when boats can enter or leave safely. People who collect shellfish or walk across tidal flats also check them, because a path covered at high tide can be dry at low tide and covered again later.
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