What Causes Tides?

Tides — Elementary

The short answer

The Moon causes high tides and low tides.

The Moon pulls on Earth. That pull is gravity.

The pull makes a force. It is called the tidal force.

Two bulges of water

The tidal force makes Earth and its water bulge out.

One bulge is on the side closest to the Moon.

One bulge is on the side farthest from the Moon.

These bulges of water are high tides.

Earth turns through the bulges

Earth turns, or rotates.

Your part of Earth passes through both bulges each day.

When you are in a bulge, you have a high tide.

When you are not in a bulge, you have a low tide.

Most days, most coasts have two high tides and two low tides.

Why is there a bulge on the far side?

The Moon pulls hardest on the side that faces it.

The Moon pulls weakest on the side farthest away.

The pull in the middle is about the average pull.

Scientists subtract the average pull from the pull at each place.

What is left is the tidal force.

It stretches Earth and its water into two bulges.

Real Earth is not all ocean

This tells us about the tidal force. It is not the whole story of real tides.

Real Earth is not covered in one even layer of water.

There are seven continents. The land gets in the way.

So the water cannot follow the Moon’s pull in a perfect way.

In some places, high tide and low tide are not very different.

In other places, the difference is drastic. That means very big.

Does the Sun affect tides?

Yes. The Sun causes tides, too.

The Sun’s tides are somewhat smaller than the Moon’s tides.

Sometimes Earth, the Moon, and the Sun line up.

That happens at a full moon or a new moon.

Then the Moon’s tides and the Sun’s tides help each other.

The tides are more extreme. They are called spring tides.

At other times, the Moon’s tides and the Sun’s tides act against each other.

Then the tides are unusually small. They are called neap tides.

There is a new moon or a full moon about every two weeks.

So big spring tides come about every two weeks.

One more tide fact

One full tide cycle takes 24 hours and 50 minutes.

Some places have only one high tide and one low tide in a cycle.

Gravity note: the pull itself is taught on this program’s “What Is Gravity? — For Kids” page.

Moon phases note: the phases themselves are taught on this program’s built moon-phases page.

Tides — Middle

The short answer

High and low tides are caused by the Moon. The Moon’s gravitational pull generates something called the tidal force.

The tidal force causes Earth — and its water — to bulge out on the side closest to the Moon and the side farthest from the Moon. These bulges of water are high tides.

Two bulges, and Earth rotating through them

As Earth rotates, your region of Earth passes through both of these bulges each day.

When you are in one of the bulges, you experience a high tide. When you are not in one of the bulges, you experience a low tide.

This cycle of two high tides and two low tides occurs most days on most of the coastlines of the world.

The second bulge: the tidal force is a subtraction

The ocean also bulges out on the side of Earth opposite the Moon, and that can seem weird: if the Moon’s gravity is pulling the oceans toward it, how can the ocean also bulge on the side away from the Moon?

The anchor’s answer is that the tidal force is a differential force — it comes from differences in gravity over Earth’s surface.

On the side of Earth directly facing the Moon, the Moon’s gravitational pull is the strongest, and the water on that side is pulled strongly in the direction of the Moon.

On the side of Earth farthest from the Moon, the Moon’s gravitational pull is at its weakest. At the centre of Earth is approximately the average of the Moon’s gravitational pull on the whole planet.

To get the tidal force — the force that causes the tides — we subtract this average gravitational pull on Earth from the gravitational pull at each location on Earth:

Tidal force = the Moon’s gravitational pull in a specific location on Earth — the Moon’s average gravitational pull over the whole Earth.

The result of the tidal force is a stretching and squashing of Earth. This is what causes the two tidal bulges.

Those two bulges explain why in one day there are two high tides and two low tides, as Earth’s surface rotates through each of the bulges once a day.

An explanation of the tidal force — not the actual tides

The anchor is careful about one caveat: this is just an explanation of the tidal force, not the actual tides.

In real life, Earth is not a global ocean, covered in an even layer of water. There are seven continents, and that land gets in the way.

The continents prevent the water from perfectly following the Moon’s pull.

That is why in some places, the difference between high and low tide is not very big, and in other places, the difference is drastic.

Does anything else affect tides? The Sun

The Sun causes tides just like the Moon does, although they are somewhat smaller.

When Earth, the Moon, and the Sun line up — which happens at times of full moon or new moon — the lunar and solar tides reinforce each other, leading to more extreme tides, called spring tides.

When lunar and solar tides act against each other, the result is unusually small tides, called neap tides.

There is a new moon or a full moon about every two weeks, so that is how often we see large spring tides. When the gravitational pull of the Sun and Moon are combined, you get more extreme high and low tides — high and low tides that happen about every two weeks.

More tidal facts

One cycle of tides actually takes 24 hours and 50 minutes.

Some places have only one high tide and one low tide in a cycle — one cycle being 24 hours and 50 minutes.

Gravity note: the pull itself is taught on this program’s “What Is Gravity? — For Kids” page.

Moon phases note: the phases themselves are taught on this program’s built moon-phases page.