Earth

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Earth, also referred to as the World, Blue Planet, or Terra, is the third planet from the Sun, and the largest of the four terrestrial planets in the Solar System. Earth is a small planet in the vastness of space. It is one of nine planets that travel through space around the sun.

The planet Earth is only a tiny part of the universe, but it is the home of human beings and, in fact, all known life in the universe. Animals, plants, and other organisms live almost everywhere on Earth's surface. They can live on Earth because it is just the right distance from the sun. Most living things need the sun's warmth and light for life. If Earth were too close to the sun, it would be too hot for living things. If Earth were too far from the sun, it would be too cold for anything to live. Living things also must have water to live. Earth has plenty. Water covers most of Earth's surface.

Earth's internal structure is similar to that of its planetary neighbors, but it is unique in the solar system in that it has abundant liquid water at its surface, an oxygen-rich atmosphere, and is known to support life. Earth's surface is in a state of constant dynamic change as a result of processes occurring within its interior and in its oceans and atmosphere.

Earth is surrounded by the atmosphere, a layer of gases many hundreds of miles thick. Through the effects of gravity, the atmosphere is densest at Earth's surface and rapidly thins with altitude. With increasing altitude, there are also changes in temperature and a progressive drop in atmospheric pressure.

From space, the flatter areas of Earth's land surface (apart from the areas dominated by ice) appear either dark green or various shades of yellow-brown. The green areas are forests and major grasslands, which comprise a major component of Earth's biosphere. The yellow-brown areas are mainly deserts, which have been created over long periods by various weathering and erosional processes

Most of Earth's tectonic features are associated with plate boundaries. At constructive (or divergent) boundaries, plates move apart and new crust is added. Examples are mid-ocean ridges and the East African Rift. At destructive (or convergent) boundaries, two plates push against each other, producing a range of features, depending on the nature of the crust on each plate. Many plate boundaries are associated with an increased frequency of volcanism, earthquakes, or both.

Some of the most striking features of Earth's surface are large bodies and flows of liquid water, such as oceans, seas, lakes, and rivers. In addition to these, there are landforms caused by the erosional or depositional power of liquid water, which include gorges, river valleys, and coastal features ranging from beaches to eroded headlands. Ice, too, has had a major impact on Earth's appearance. Ice-formed features include existing bodies of ice, such as glaciers and ice-sheets, and landforms, such as U-shaped valleys, sculpted by the movement of past glaciers.