The outside of the earth

We live on the surface of the earth. The surface, or crust of the earth, is made of rock. In some places, it is covered with soil. In many places, it is covered with water. All around it is air.

Some of us live on huge pieces of land called continents. A continent is like a great platform of rock that sticks up a little higer than the rest of the rocky crust.

Some of us live on smaller pieces of land called islands. An island is the top of an underwater mountain or part of a continent that has become separated from the rest of the continent.

The continents and islands where we live are surrounded by water. Water covers nearly three-fourths of the crust of the earth. Most of this water lies in enormous pits that are like great bowls in the rocky crust. These huge "bowls" of water are the oceans. Water also lies in smaller pits in the continents and islands. These inland pits of water are lakes and ponds. Water also flows in rivers and streams, from the high parts of the land down to the lakes and seas.

All around the surface of the earth is a layer of air. This layer of air is hundreds of miles (kilometers) high. The air is thickest next to the land and water. It gets thinner the higher it goes. Where the air comes to an end, outer space begins.

So that's our earth - a big, spinning, moving ball of rock and hot metal, with a thin coating of soil, water, and air on the outside.

What's inside the earth ?

Could you dig a hole to the other side of the earth?
No, you couldn't. The center of the earth is about 4,000 miles (6,400 kilometers) beneath your feet. So it's almost 8,000 miles (13,000 kilometers) to the other side of the earth. You could't dig that far. And for most of that distance, the earth is either solid rock or metal so hot that it's melted! You certainly couldn't dig through that !

When the outside of the earth cooled, it became a kind of shell of rock. We call this the crust. The oceans and the continents lie on top of the crust. Beneath the oceans, the crust is about five miles (8 kilometers) thick. Beneath the land, it is about twenty miles (32 kilometer) thick.

Under the crust there is another layer of rock called the mantle. The mantle is made of a diffrerent kind of rock than the crust. The deeper the mantle goes, the hotter it gets. It is about 1,800 miles (2,900 kilometers) thick. At its bottom, it is hot enough to melt iron.

Beneath the mantle is a layer of melted metal - metal so hot that it's like thick syrup ! This layer is called the outer core. Scientists think the outer core is made of iron and nickel and is about 1,400 miles (2,250 kilometers) thick.

In the center of the earth is the inner core. It's a ball of hot, solid, squeezed-together metal about 1,600 miles (2,570 kilometers) thick.

That's what's inside the earth.

How was the earth born ?

Why is the earth shaped like a ball ? Why isn't it shaped like a block ? Or why isn't it round and flat, like a pie ? Why does it spin ? And why does it whirl around and around the sun ?

Most scientists think that the answers to these questions are part of the story of how the earth was born. They think it is a story that began billions of years ago, with a giant, spinning cloud in space.

There are many such clouds in space right now. They are made up of chemicals in the form of gas and dust. Scientists have found that some of these clouds seem to be turning into new stars. And scientists think that, about five billion years ago, our sun and its planets were formed from just such a cloud.

The cloud was hundreds of millions of miles (kilometers) wide ! The force of gravitation pulled it into the shape of a huge, flat wheel. And it spun around and around, just like a wheel.

Gravitation slowly pulled the cloud together. Most of the gas collected in the middle of the cloud. It formed a lump that got bigger and bigger. As the lump grew, its gravity became stronger. Then it pulled more and more gas into itself. Because gravity pulls equally from all directions, the lump was squeezed into the shape of a round ball. This was the only shape it could take.

After a long while, there was a gigantic ball of gas in the middle of the wheel-shaped cloud. This was the beginning of the sun.

Farther out, in the rest of the cloud, other balls were forming as gravity pulled bits of dust and gas together. After a while, most of the cloud was used up. There were only balls of gas and dust, whirling around the sun. They were spinning and moving around the sun because the cloud they were made from had been spinning. These balls were the beginning of the earth and the other planets and moons. So that is why the earth is a spinning ball that whirls around the sun.

But if the earth was once a spinning ball of dust and gas, how did it become a ball of rock and metal ?

As earth's gravity pulled more dust and gas in, everything was squeezed together - tighter and tighter and tighter ! This made the ball grow hotter and hotter ! It became so hot that the bits of dust in it, which were mostly rock and metal dust, melted together. The earth became a glowing ball.

But the outside of the earth could'n't stay hot. It began to cool. And when melted rock and metal cools, it gets hard. So the earth became a ball of hard rock and metal, as it is today.

But the earth still hasn't cooled off. The middle is still fiercely hot, and part of the inside is still melted.

There are many other beliefs about how the earth was born. The book of Genesis in the Bible says that God created the earth and all living things. But no one actually knows how it happened

Around and around the sun

The Earth doesn't just spin-it also moves through space.

Right this very moment, the earth is rushing through space at tremendous speed-more than 66,000 miles straight line, though. It's whirling around and around the sun, in a kind of stretched-out circle. This circle the earth makes around the sun is called an orbit.

What makes the earth keep moving around and around the sun ? Why doesn't it just fly off into space ?

Everything in space pulls at everything else. This pull is called gravitation. The bigger a thing is, the stronger its pull. The sun is more than a million times bigger than the earth, so it tugs hard at the earth. It is this strong tug that keeps the earth in orbit. If you fasten some string to a ball, you can whirl the ball around and around, to show the way the earth goes around the sun. The string is like the pull of the sun's gravity-it holds onto the ball. Even though the ball is moving, it can only move in a circle.

The time it takes the earth to go all the way around the sun is a little more than 365 days. This is what we call a year

The spinning world

You may think our world, the earth, is standing still. But it isn't. The earth is spinning around and around, like a huge top

Push a stick through a ball of clay and then twirl the stick. The clay ball will turn around like a wheel. That's how the earth spins - around a kind of imaginary axle that runs through its middle. This imaginary axle is called the axis. One end of the axis is the North Pole and the other end is the South Pole.

Why can't we feel the earth turn ? Because we're so tiny and it's so big. But we know itu does turn because that's what gives us our day and night. In the morning, when the sky is bright, we know that our part of the earth is turned toward the sun. In the evening, when the sky grows dark, we know that we've turned away from the sun.

The time it takes the earth to make one complete turn is just a little less that 24 hours-one full day and night.

An island in space

We live on an island. An island in space ! An island is a piece of land with water all around it. The Earth is a giant ball of rock and metal with space all around it

A ball of rock and metal such as the earth is called a planet. Planets are the "prisoners" of stars. They circle around and around the stars they belong to. Earth is one of nine planets that circle around and around our star, the sun.

The sun is an enormous ball of hot, glowing gas. To us, the earth seems big, but it's small compared to the sun. The sun is bigger than a million earths !

The earth has one near neighbor, the moon. The moon is a ball of rock, like earth, but much smaller. It circles around earth just as earth circles around the sun

The moon is thousands of miles (kilometers) away from the earth. The sun and the other planets are millions of miles away. And the stars that you see twinkling in the sky at night are trillions of miles (kilometers) away. So the earth is really a tiny island in the great, black emptiness of space. But for us, it's the most important place of all. It's our home