Earth & climate
Oceans & tides
Why the moon moves the sea: low tide, high tide, spring tide and the currents that carry heat around the world.
What you need first
If you spend a holiday by the sea, you experience something strange: in the morning the beach lies wide and open, a few hours later the water laps almost up to the dunes. The sea breathes in and out, twice a day. And the timekeeper for this hangs in the sky: the moon.
The blue planet
Seen from space the Earth looks blue, because oceans cover about 70 percent of its surface. They hold almost all of the planet's water: out of every 100 litres of water on Earth, about 97 litres are sea water. The oceans are huge stores of heat, they supply the water vapour for the , and they are constantly moving: through waves, through currents and through the tides.
The moon pulls the water into two bulges, one facing it and one on the far side. Because Earth rotates beneath both, high tide comes about twice a day. At full and new moon the sun pulls the same way: spring tide.
The moon pulls on the water
The moon's pulls on the water of the oceans. On the side facing the moon, the water bulges up into a tidal bulge. On the opposite side a second bulge forms, because the pull is weakest there and the water is in a sense left behind. Since the Earth rotates underneath these two bulges once a day, a coast experiences high tide about twice a day and low tide twice a day. The moon itself keeps moving on, so high tide arrives about 50 minutes later each day.
Spring tides and warm currents
The sun also pulls on the sea, just more weakly than the moon. At full moon and new moon the sun, Earth and moon line up, their forces add together, and the high tide turns out especially high. This is called a spring tide. Besides the tides, enormous currents flow through the oceans, carrying warm water from the equator to cooler regions. The Gulf Stream, for example, carries warm water as far as northern Europe and helps keep winters in western Europe milder than in regions that lie just as far from the equator, such as eastern Canada.
Exercises
0 of 6 solvedTime to try it yourself. You can't break anything, every attempt counts.
What mainly creates the tides?
How many tidal bulges exist on Earth at the same time?
About how many times a day does a coast have high tide?
Match each cause to the result it has at the sea.
When does a spring tide occur?
The oceans cover about … percent of the Earth's surface, which is why the Earth looks blue from space.
Where this leads