Why Doesn't the Ocean Fall Off the Earth?

Imagine looking at a globe.

You can see enormous blue oceans covering most of Earth's surface. There's the Pacific Ocean, the Atlantic Ocean, the Indian Ocean and all the other seas stretching around our planet.

But here's a strange question.

If Earth is a giant ball floating in space, why doesn't all that ocean water fall off the bottom?

After all, when you pour water from a glass, it falls towards the ground. When rain falls from clouds, it travels downwards. And when you tip a bucket upside down, the water spills out.

So why doesn't the same thing happen to Earth's oceans?

The answer involves one of the most important forces in the universe: gravity.

What Is Gravity?

Gravity is the force that attracts objects with mass towards one another.

Everything with mass has gravity, including you, your chair, a tiny pebble and even a grain of sand.

But Earth is enormous, with a mass of about 5.97 sextillion tonnes. That's a 5 followed by more than 20 digits!

All that mass creates a powerful gravitational attraction.

Earth's gravity pulls objects towards the planet's centre.

It's what keeps your feet on the ground, makes raindrops fall and stops the oceans from floating away into space.

But there's something important to understand.

Gravity doesn't pull everything towards the bottom of Earth. It pulls everything towards Earth's centre.

And that makes all the difference.

There Is No Bottom of Earth!

When you look at a globe, Australia might appear near the bottom, while Europe and North America are closer to the top.

But that's only because of the way we usually draw maps and display globes.

In space, Earth has no natural top or bottom.

Imagine standing on a beach in Australia. Gravity pulls you towards Earth's centre, keeping your feet on the sand.

Now imagine someone standing on a beach on the opposite side of the planet.

Gravity pulls that person towards Earth's centre too.

You're both standing upright from your own point of view, even though your feet point in different directions when seen from space.

The same thing happens to ocean water.

Everywhere on Earth, gravity pulls water towards the centre of the planet.

That's why the oceans can wrap around a spherical world without spilling into space.

Imagine Thousands of Tiny Arrows

Here's a way to picture it.

Imagine drawing a giant circle to represent Earth.

Now draw little arrows all around the outside of the circle.

At the top, the arrows point downwards.

At the bottom, the arrows point upwards.

On the left, they point right.

On the right, they point left.

At first, the arrows seem to be pointing in completely different directions.

But look more closely.

Every arrow points towards the centre of the circle.

That's how gravity works around Earth.

What we call "down" depends on where we're standing.

Down in Australia points in a different direction through space from down in Canada, but both directions lead towards Earth's centre.

Why Doesn't the Ocean Water Float Away?

Water is made of molecules, and those molecules have mass.

Because they have mass, gravity attracts them towards Earth.

The oceans are also held against the planet by this gravitational force.

Without gravity, water wouldn't stay neatly spread across Earth's surface.

But gravity doesn't mean the ocean has to be perfectly still.

Water can move sideways across the planet while still being held close to its surface.

That's why we can have waves, currents, waterfalls and flowing rivers without the oceans escaping into space.

Why Is the Ocean Surface Curved?

If you stand on a beach and look across the water, the ocean might appear completely flat.

But Earth is round, so the ocean's surface follows the planet's overall curved shape.

On a calm ocean, water settles into a shape determined mainly by gravity and Earth's rotation.

Scientists call an idealised version of this shape the geoid.

It isn't a perfect sphere because Earth's gravity varies slightly from place to place, and the planet itself isn't perfectly round.

In fact, Earth bulges a little around its equator because it rotates.

The oceans follow this slightly uneven gravitational shape, even though we usually can't notice the curvature when looking across a small stretch of water.

But What About Tides?

If Earth's gravity holds the oceans in place, why does the sea level rise and fall?

That's where the Moon comes in.

The Moon has gravity too, and its gravitational pull affects Earth's oceans.

Because the Moon pulls more strongly on the side of Earth closest to it than on the far side, it creates differences in gravitational forces across our planet.

These differences help produce two broad tidal bulges in the oceans.

As Earth rotates, coastlines move through the changing tidal pattern, causing the familiar rise and fall of sea levels.

The Sun also affects tides.

But even during the highest tide, Earth's gravity remains strong enough to keep the oceans attached to our planet.

Could the Ocean Ever Escape into Space?

Under normal conditions, Earth's oceans aren't going anywhere.

To escape Earth's gravity completely, an object near the surface needs an enormous amount of speed.

This is called escape velocity.

For Earth, it's about 11.2 kilometres per second, or more than 40,000 kilometres per hour, when air resistance is ignored.

Ocean water doesn't suddenly reach anything close to that speed.

Even enormous waves and powerful storms move water far too slowly to send the oceans flying into space.

Small amounts of water can reach the upper atmosphere as water vapour and eventually contribute to the loss of hydrogen into space, but this is nothing like an ocean spilling off the planet.

What Would Happen If Earth Had No Gravity?

Now imagine something impossible but interesting.

What if Earth's gravity suddenly disappeared?

The oceans would no longer be held against the planet.

Water, air and anything else not attached to the ground would begin moving according to their existing motion and other forces.

Earth's atmosphere would no longer remain bound to the planet, and the oceans would eventually disperse rather than staying wrapped around its surface.

Fortunately, gravity cannot simply switch off like a light.

As long as Earth has mass, it creates gravity.

The Ocean's Invisible Anchor

The next time you look at a globe, imagine all the ocean water covering its surface.

It doesn't matter whether the water is at the top, bottom or sides of the globe.

Gravity is pulling it towards Earth's centre from every direction.

And because there's no universal up or down in space, there's no special bottom where the water could simply spill away.

So why doesn't the ocean fall off the Earth?

Because gravity pulls the oceans towards the centre of our planet, keeping water wrapped around Earth's curved surface.

The oceans don't need giant walls or invisible containers to hold them in place.

They have something much more powerful.

The gravitational pull of an entire planet.

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