Tides: the sea’s regular rise and fall — Earth science, 11–13 years
Tides are long, regular changes in sea level, caused mainly by the Moon’s gravity and partly by the Sun’s. Coastlines do not all experience the same height or timing.
What a tide is
As Earth turns, coastlines pass through places where the ocean is slightly raised by gravitational effects. This produces high tide and, about six hours later in many places, low tide. The pattern is not simply the sea “sloshing” because of wind: it follows the positions of the Moon, the Sun and Earth, together with the shape of each coast.
Why tides follow the Moon
Coastal people needed to know when water would cover rocks, harbours or paths, so they watched repeating sea levels. The Moon is far smaller than Earth, but it is close enough for its gravity to pull ocean water noticeably; the Sun also pulls, though less strongly at this distance. When their effects line up, the difference between high and low tide becomes larger.
Finding a tide pattern
Suppose a harbour has high tide at 08:00 on Monday. A common pattern puts the next low tide roughly six hours later, at about 14:00, and the next high tide roughly six hours after that, near 20:00. The exact times vary because coastlines and ocean basins change the pattern, so a real harbour uses a local tide table rather than this estimate alone.
The trap: one tide every day
It is reasonable to expect one high tide each day because Earth spins once in about 24 hours. The Moon, however, moves along its orbit while Earth turns, so the same coastline meets the Moon’s position a little later each day. Many coasts therefore have about two high tides and two low tides in roughly 24 hours and 50 minutes, though local shapes can change this.
Tides beyond the classroom
Boats use tide information to avoid entering a shallow harbour at the wrong time. People collecting shellfish, walking across tidal flats or planning coastal work also check the forecast, because water can return faster than a person expects. Some power stations use moving tidal water, but only where the coastline and environmental effects make that sensible.
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