潮が満ちたり引いたりする理由は？月の引力で水がふくらみます

Why Do Tides Rise and Fall?

Have you ever watched the sea at the beach for a few hours?

The water slowly comes up the sand, and later it goes back down.

This rise and fall of the sea is called the tide.

Tides are caused by the pull of gravity from the moon and the sun.

The moon is the stronger one, because it is so close to Earth.

The sun is much bigger, but it is far away.

Its pull on the tides is only about half of the moon's.

The moon pulls the water on the side of Earth that faces it.

On the far side, the pull is weaker, because that side is farther from the moon.

So a second bulge of water forms there.

The two bulges stay lined up with the moon while Earth turns.

Scientists think of the tide as a very long wave.

When the top of the wave, called the crest, reaches a place, it is high tide.

When the lowest part, called the trough, arrives, it is low tide.

Most coasts have two high tides and two low tides every 24 hours and 50 minutes.

The high tides come about 12 hours and 25 minutes apart.

The difference in height between high and low tide is called the tidal range.

Next time you visit the sea, check a tide table first.

You may find that the water is farther away than you expect.