Tides

The Moon pulls on the oceans

Twice a day the sea 'comes in' onto the shore and twice it 'goes out'. The Moon does this! Its gravity pulls the ocean water — on the side of Earth facing the Moon, the water rises. Curiously, it rises on the opposite side too (the Earth itself is pulled toward the Moon and 'leaves' the water behind). Look at the globe: the blue shell stretches toward the Moon; the white dot shows where the Moon is. In Canada the water rises 16 metres — like a 5-storey building!

Tides are caused by the uneven pull of the Moon's gravity (and the Sun's, at 45% of the strength) on different parts of Earth (a gradient): the near side is pulled harder, the far side more weakly, producing two bulges. As Earth rotates, every place passes through both bulges in 24 hours 50 minutes — two high tides and two low tides (the Moon rises 50 minutes later each day). At new and full Moon the Sun and Moon line up — spring tides (the highest); at the quarters — neap tides (the lowest). In the open ocean the range is 0.5 m, but coastlines amplify it: the Bay of Fundy (Canada) — 16 m, Mont-Saint-Michel — 14 m. Uzbekistan is landlocked, but Earth's solid crust also 'rises' by 30 cm! Tidal friction slowly brakes Earth's spin (2 ms per century) and the Moon recedes 3.8 cm a year. Tidal power stations exist (Rance, France, 1966).

Fun facts

• The Moon moves 3.8 cm farther from Earth every year — because of the tides.
• In the Bay of Fundy the water rises 16 m — 100 billion tonnes of water flow in and out daily.
• Earth's solid ground also rises and falls 30 cm a day because of the Moon — we just don't notice.