World Book at NASA for Students
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Earth is the planet we live on. Earth is a huge sphere, or ball, that goes around the sun in a circle. It is covered with water, rock, and soil and surrounded by air. Animals and plants live almost everywhere on Earth's surface. They can live on Earth because it is just the right distance from the sun. Living things need the sun's warmth and light. But if Earth were closer to the sun, it would be too hot for living things. If Earth were farther from the sun, it would be too cold for anything to live. Also, Earth has plenty of water. Most living things need water.
Image to right: Seen from space, Earth is a blue ball covered with white clouds. This picture was taken by a satellite far out in space. Credit: NASA
Earth in space
Earth is one of the nine planets that travel through space around the sun. Earth is always moving. It spins like a top, and at the same time it travels around the sun. Human beings use these movements to measure the length of days and years on Earth. One day is the time it takes Earth to spin around once. One year is the time it takes Earth to travel once around the sun. Earth has a ball-shaped moon that travels around it the same way the Earth travels around the sun.
Outside Earth
Earth is not perfectly round. It is a little bit flattened at the poles. The North Pole is at Earth's top, and the South Pole is at its bottom. Halfway between the poles is an imaginary circle around Earth's center. This circle is called the equator. The equator cuts Earth into two halves called the Northern Hemisphere and the Southern Hemisphere.
The air that surrounds Earth is called the atmosphere. An invisible gas called nitrogen makes up most of the atmosphere. Oxygen, the gas that people and animals need to breathe, makes up most of the rest. About 1,000 miles (1,600 kilometers) from Earth, the atmosphere fades away into the emptiness of space.
Earth's surface is about 70 percent water. Almost all of this water is in Earth's oceans. These oceans form one great body of water often called the world ocean or global ocean. Land makes up about 30 percent of Earth. The largest pieces of land are called continents. There are seven continents. They are Asia, Africa, Antarctica, Australia, Europe, North America, and South America.
Earth is covered with a rocky "skin" called the crust. The crust is about 5 miles (8 kilometers) thick under the oceans, and about 25 miles (40 kilometers) thick under the continents. The crust is made up of about 30 huge pieces, called plates, that fit together like a jigsaw puzzle.
Inside Earth
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Underneath the crust, Earth has three layers of hot rock and metal. These layers are the mantle, the outer core, and the inner core. The mantle is a layer of hot, melted rock below the crust. It is about 1,800 miles (2,900 kilometers) thick. The outer core lies beneath the mantle. It is made mostly of melted iron, and it is even hotter than the mantle. The inner core is the hottest part of Earth. Scientists believe that the inner core is a ball of solid iron.
Image to left: Earth is made up of four layers -- the crust, the mantle, the outer core and the inner core. Credit: World Book illustration by Raymond Perlman
How Earth began
Scientists believe that Earth is at least 4.5 billion years old. They think Earth began as a dry ball of rock covered by a cloud of gas. The rock began to heat up, and the inside of Earth melted. Heavy materials, like iron, sank to Earth's middle and formed the core. The lighter rocks moved to the top and formed the first crust.
The heating of the inside of Earth caused other chemicals to rise to the surface. Some of these chemicals formed water. Others formed the gases that make up the atmosphere. Over millions of years, the water slowly collected in low places on the crust and formed the oceans.
How Earth changes
Earth's surface is always changing. Some of these changes happen suddenly. For example, earthquakes break hillsides apart and cause cracks in the ground. Floods wash away soil and form new lakes and rivers. Volcanoes erupt, or explode, burying huge areas in ash and rock.
Other changes happen slowly. For example, water and wind are part of a process called erosion. Erosion takes place when parts of the Earth's surface are worn down and their materials are moved somewhere else. Over millions of years, water and wind can make great changes. For example, erosion caused by flowing water carved the Grand Canyon in North America out of hard rock.
Other slow changes happen because Earth's plates are always moving. The plates glide along slowly on a layer of very hot rock. In some places, the plates bump into each other. This creates mountains. In other places, the plates move apart. This causes new crust to form. These changes take millions of years.
The drifting plates moved Earth's continents to where they are today. Scientists who study Earth believe that hundreds of millions of years ago there was only one huge continent, which scientists called Pangaea. There was only one ocean, called Panthalassa. About 200 million years ago, the plates that formed Pangaea began to drift apart. Pangaea split into two continents called Gondwanaland and Laurasia. Slowly, Gondwanaland broke apart. It formed the continents of Africa, Antarctica, Australia, and South America. Laurasia split into Eurasia and North America. The movement of the continents to where they are now took place over millions of years.
How to cite this article: To cite this article, World Book recommends the following format: "Earth." The World Book Student Discovery Encyclopedia. Chicago: World Book, Inc., 2005.
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