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Earth, only planet in the solar system on which the presence of living things is definitely known. It is the third planet outward from the sun, the fifth largest in the solar system. Together with its single moon, it travels around the sun at an average distance of 92,960,000 mi (149,600,100 km). The earth also spins on an axis that is tilted at 23.5° from a line perpendicular to its path around the sun. This spinning motion makes the sun appear to travel across the sky from east to west, and it also causes day and night. The tilt of the earth's axis and its movement around the sun causes the change of seasons. The moon affects the earth's path around the sun and, together with the sun, causes tides. According to the most recent estimate, the earth is 4.55 billion years old. The first signs of life on the planet are some fossilized bacteria found in rocks estimated to be 3.3 billion years old.

The earth consists of 3 main zones: the atmosphere; the hydrosphere, including all the bodies of water and ice on earth; and the lithosphere, the rocks that form the earth's crust, mantle, and core. The atmosphere provides the air for breathing, shields plants and animals from excess heat from the sun, and filters out the lethal shorter ultraviolet rays that would otherwise destroy life. The hydrosphere is also essential for life. Water and ice cover more than 70% of the earth's surface, making possible the hydrologic cycle by which water is evaporated from the oceans and precipitated as rain or snow, moistening the land and then returning to the oceans. Finally, the lithosphere makes up the land masses or continents, sea beds, and the inner mass of the planet. The earth's crust varies in thickness from between an average of 20 mi (32 km) under the continents to 5 mi (8 km) under the oceans. Beneath the crust, the distinct zone called the mantle is some 1,800 mi (2,900 km) thick. Within the mantle lies the earth's core, with a diameter of about 4,300 mi (6,900 km). Seismic data suggest that the core may consist of a solid center surrounded by a liquid outer core.

See also: Solar System.

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21st Century Webster's Family Encyclopedia21st Century Webster's Family Encyclopedia - Dream to Eijkman, Christiaan