Tides: the Moon moves the oceans — Astronomy, 11–13 years
The sea does not rise and fall by chance. The Moon’s gravity, helped a little by the Sun, creates a regular pattern of high and low tides.
What tides are
A tide is the slow rise and fall of ocean water along a coast. The Moon pulls more strongly on the side of Earth nearest to it, making a bulge of water there; another bulge forms on the far side. As Earth turns, a coastline passes through these bulges, usually getting two high tides and two low tides in about a day.
Why tides happen
People first needed tides to plan fishing, travel and harbour work. The important clue was that the sea’s rhythm follows the Moon, not the weather alone. The Sun also pulls on Earth’s oceans: when the Sun, Earth and Moon line up, their pulls combine and make especially large spring tides; at a right angle, smaller neap tides occur.
A tide timetable
Suppose a harbour has high tide at 06:00. First, the Moon needs about 50 minutes more each day to return above the same place, so the next high tide is roughly 12 hours 25 minutes later. Add that time: 06:00 + 12:25 = 18:25. The following high tide is about 06:50 the next day, though coast shape and depth can shift the actual times.
The reasonable mistake
A common mistake is to say that high tide happens only on the side of Earth facing the Moon. That seems reasonable because the Moon’s pull is strongest there. But the whole Earth is pulled, while the ocean and solid ground respond slightly differently; this creates a second water bulge on the far side. Local coastlines then make the pattern less tidy.
Tides in real life
Tide tables help boats enter shallow harbours safely and help people choose times for coastal work. Some power stations guide seawater through turbines as tides flow in and out. Tides are not equally strong everywhere: a narrow bay can funnel water into a much larger rise, while another coast may show only a small change.
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