Earth means many things to the people who live on it. To a farmer, earth is rich soil. To a road builder, earth means mountains of hard rock. For a sailor, earth is water as far as the eye can see. A pilot's view of earth may include part of an ocean, a mountain, and patches of farmland. An astronaut speeding through space sees the earth's round shape and the outline of lands and oceans. Each of these different views helps describe the earth but none really tells what the earth is.
The earth is a huge sphere (ball) covered with water, rock, and soil, and surrounded by air. It is one of nine planets that travel through space around the sun. The sun is a star--one of billions of stars that make up a galaxy called the Milky Way. The Milky Way and billions of other galaxies make up the universe.
The planet earth is only a tiny part of the universe, but it is the home of human beings and many other living things. Animals and plants live almost everywhere on the earth's surface. They can live on the earth because it is just the right distance from the sun. Living things need the sun's warmth and light for life. If the earth were too close to the sun, it would be too hot for living things. If the earth were too far from the sun, it would be too cold for anything to live. Most living things--both plants and animals--also must have water to live. The earth has plenty. Water covers most of the earth's surface.
A thin layer of rock called the crust forms the earth's surface. All life on the earth is found on this crust or in the water that covers about 70 per cent of it. Beneath the crust is hot rock. A ball of metal lies at the center of the earth.
The earth is always moving. It spins like a top and also travels around the sun at the same time. We use these two motions of the earth to measure the length of days and years. One day is the time it takes the earth to spin around once. One year is the time it takes the earth to travel once around the sun. The earth, like some of the other planets, has a ball-shaped moon traveling around it. The other planets that have moons all have two or more, except Pluto, which only has one.
The study of the earth is called geology, and scientists who study the earth are geologists. This article deals with the planet earth as it is studied in geology. For information on the earth as the home of human beings, see the article on WORLD in World Book. |
