The numbers
- How often
- 6hoursRoughly: a quarter of a day
- How high it rises
- 66ft(20 m)33 to 66 ft (10 to 20 m)In the bays where it is strongest
- Who is in charge
- The Moon, more than the SunBecause it is much closer
How it happens, step by step
The Moon pulls
The Moon's gravity pulls on everything on the Earth, but the water is free to move and the ground is not.
The sea bulges
On the side facing the Moon the water rises and forms a bulge thousands of miles wide.
And the far side
On the far side the Moon pulls the planet more than the distant water, which lags behind and bulges there too.
The Earth turns
The planet spins under those two bulges, which stay pointed toward the Moon and away from it.
Twice a day
Every place passes through both bulges in a day, and so it sees two high tides and two low tides.
To get an idea
The bulges of the sea are two, one toward the Moon and one on the opposite side.
In many places the tide rises and falls twice a day, about every six hours.
In some bays the difference between high and low tide reaches thirty to sixty-five feet.
Fun facts
These are not in the book: there was no room on paper.
In the bay of Mont Saint-Michel the difference between high and low tide reaches about sixty-five feet.
Tides also affect large lakes, not only the oceans and the open seas.
When the Sun and the Moon line up, the tides become stronger than usual.
The friction of the tides slows the Earth's spin very slightly, making the days longer.
The same forces heat the inside of some moons, and that is why Io has volcanoes.
In the Mediterranean the tides are very weak, because it is an almost closed sea.
Test yourself
The first question is the one printed under the QR code, with the answer the book did not give. The other four only exist here.
Question 1 of 5★ is the one from your book
Who rules the tides?
Question 2 of 5
How many bulges of water form?
Question 3 of 5
Why does the sea bulge on the far side too?
Question 4 of 5
How many times a day does the tide usually rise?
Question 5 of 5
How high can the sea rise?
Riddles
I have two of them, facing each other on opposite sides of the world: one looks at my mistress and the other turns its back on her. The planet spins and you pass through them. What am I?
In some bays I rise as high as a six-story building, in some closed seas you hardly notice me. I can be predicted years in advance. What am I?
Try it yourself
Look for a tide table for the nearest coast: tides are predicted years ahead, because they depend on positions that can be calculated.
Hold a rope tight between two people and pull from one side: the person closer to you moves more. It is the same reason the Moon beats the Sun.
The words
- gravity
- The force with which every body pulls all the others toward itself.
Sources: Wikidata (Q1050303), Wikipedia (Marea).