From Wikipedia, the free encyclopedia This article is about the planet. For other uses, see

According to evidence from radiometric dating and other sources, Earth was formed around four and a half billion years ago. Within its first billion years,^[36] life appeared in its oceans and began to affect its atmosphere and surface, promoting the proliferation of aerobic as well as anaerobic organisms and causing the formation of the atmosphere's ozone layer. This layer and the geomagnetic field block the most life-threatening parts of the Sun's radiation so life was able to flourish on land as well as in water.^[37] Since then, the combination of Earth's distance from the Sun, its physical properties and its geological history have allowed life to persist.

Earth's lithosphere is divided into several rigid tectonic plates that migrate across the surface over periods of many millions of years. Seventy-one percent of Earth's surface is covered with water,^[38] with the remainder consisting of continents and islands that together have many lakes and other sources of water that contribute to the hydrosphere. Earth's poles are mostly covered with ice that includes the solid ice of the Antarctic ice sheet and the sea ice of the polar ice packs. Earth's interior remains active with a solid iron inner core, a liquid outer core that generates the magnetic field, and a thick layer of relatively solid mantle.

Earth gravitationally interacts with other objects in space, especially the Sun and the Moon. During one orbit around the Sun, Earth rotates about its own axis 366.26 times, creating 365.26 solar days or one sidereal year.^{[n 6]} Earth's axis of rotation is tilted 23.4° away from the perpendicular of its orbital plane, producing seasonal variations on the planet's surface with a period of one tropical year (365.24 solar days).^[39] The Moon is Earth's only natural satellite. It began orbiting Earth about 4.53 billion years ago. The Moon's gravitational interaction with Earth stimulates ocean tides, stabilizes the axial tilt and gradually slows the planet's rotation.

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  1 Name and etymology
  
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  2 Composition and structure
  
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    2.1 Shape
    
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    2.2 Chemical composition
    
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    2.3 Internal structure
    
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    2.4 Heat
    
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    2.5 Tectonic plates
    
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    2.6 Surface
    
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    2.7 Hydrosphere
    
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    2.8 Atmosphere
    
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      2.8.1 Weather and climate
      
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      2.8.2 Upper atmosphere
      
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    2.9 Magnetic field
    
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    2.10 Magnetosphere
    
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  3 Orbit and rotation
  
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    3.1 Rotation
    
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    3.2 Orbit
    
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    3.3 Axial tilt and seasons
    
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  4 Habitability
  
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    4.1 Biosphere
    
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    4.2 Evolution of life
    
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    4.3 Natural resources and land use
    
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    4.4 Natural and environmental hazards
    
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    4.5 Human geography
    
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  5 Cultural and historical viewpoint
  
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  6 Chronology
  
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    6.1 Formation
    
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    6.2 Geological history
    
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    6.3 Predicted future
    
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  7 Moon
  
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  8 Asteroids and artificial satellites
  
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  9 Notes
  
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  10 See also
  
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  11 References
  
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  12 Further reading
  
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  13 External links
  

The modern English word Earth developed from a wide variety of Middle English forms,^[41] which derived from an Old English noun most often spelled eorðe.^[40] It has cognates in every Germanic language, and their proto-Germanic root has been reconstructed as *erþō. In its earliest appearances, eorðe was already being used to translate the many senses of Latin terra and Greek γῆ (gē): the ground,^[43] its soil,^[45] dry land,^[48] the human world,^[50] the surface of the world (including the sea),^[53] and the globe itself.^[55] As with Terra and Gaia, Earth was a personified goddess in Germanic paganism: the Angles were listed by Tacitus as among the devotees of Nerthus,^[56] and later Norse mythology included Jörð, a giantess often given as the mother of Thor.^[57]

Originally, earth was written in lowercase and, from early Middle English, its definite sense as "the globe" was expressed as the earth. By early Modern English, many nouns were capitalized and the earth became (and often remained) the Earth, particularly when referenced along with other heavenly bodies. More recently, the name is sometimes simply given as Earth, by analogy with the names of the other planets.^[40] House styles now vary: Oxford spelling recognizes the lowercase form as the most common, with the capitalized form an acceptable variant. Another convention capitalizes Earth when appearing as a name (e.g. "Earth's atmosphere") but writes it in lowercase when preceded by the (e.g. "the atmosphere of the earth"). It almost always appears in lowercase in colloquial expressions such as "what on earth are you doing?"^[58]

Composition and structure

Shape

Main article: Figure of the Earth

World map color-coded by relative height

Stratocumulus clouds over the Pacific, viewed from orbit

The shape of Earth approximates an oblate spheroid, a sphere flattened along the axis from pole to pole such that there is a bulge around the equator.^[59] This bulge results from the rotation of Earth, and causes the diameter at the equator to be 43 kilometres (27 mi) larger than the pole-to-pole diameter.^[60] Thus the point on the surface farthest from Earth's center of mass is the Chimborazo volcano in Ecuador.^[61] The average diameter of the reference spheroid is about 12,742 kilometres (7,918 mi), which is approximately 40,000 km/π, because the meter was originally defined as 1/10,000,000 of the distance from the equator to the North Pole through Paris, France.^[62]

Local topography deviates from this idealized spheroid, although on a global scale these deviations are small compared to Earth's radius: The maximum deviation of only 0.17% is at the Mariana Trench (10911 m below local sea level), whereas Mount Everest (8,848 m above local sea level) represents a deviation of 0.14%. If Earth were shrunk to the size of a cue ball, some areas of Earth such as mountain ranges and oceanic trenches would feel like small imperfections, whereas much of the planet, including the Great Plains and the Abyssal plains, would actually feel smoother than a cue ball.^[63] Due to the equatorial bulge, the surface locations farthest from Earth's center are the summits of Mount Chimborazo in Ecuador and Huascarán in Peru.^{[64][65][66][67]}