Earth is the third planet fromthe Sun and the only astronomical object known to harbor life. Whilelarge volumes of water can be found throughout the Solar System, onlyEarth sustains liquid surface water. About 71% of Earth's surface ismade up of the ocean, dwarfing Earth's polar ice, lakes, and rivers.The remaining 29% of Earth's surface is land, consisting ofcontinents and islands. Earth's surface layer is formed of severalslowly moving tectonic plates, interacting to produce mountainranges, volcanoes, and earthquakes. Earth's liquid outer coregenerates the magnetic field that shapes Earth's magnetosphere,deflecting destructive solar winds.
Earth's atmosphere consists mostly ofnitrogen and oxygen. More solar energy is received by tropicalregions than polar regions and is redistributed by atmospheric andocean circulation. Water vapor is widely present in the atmosphereand forms clouds that cover most of the planet. Greenhouse gases inthe atmosphere like carbon dioxide (CO2) trap a part of the energyfrom the Sun close to the surface. A region's climate is governed bylatitude, but also by elevation and proximity to moderating oceans.Severe weather, such as tropical cyclones, thunderstorms, andheatwaves, occurs in most areas and greatly impacts life.
Earth is an ellipsoid with acircumference of about 40,000 km. It is the densest planet in theSolar System. Of the four rocky planets, it is the largest and mostmassive. Earth is about eight light minutes away from the Sun andorbits it, taking a year (about 365.25 days) to complete onerevolution. Earth rotates around its own axis in slightly less than aday (in about 23 hours and 56 minutes). Earth's axis of rotation istilted with respect to the perpendicular to its orbital plane aroundthe Sun, producing seasons. Earth is orbited by one permanent naturalsatellite, the Moon, which orbits Earth at 380,000 km (1.3 lightseconds) and is roughly a quarter as wide as Earth. The Moon alwaysfaces the Earth with the same side through tidal locking and causestides, stabilizes Earth's axis, and gradually slows its rotation.
Earth formed over 4.5 billion yearsago. During the first billion years of Earth's history, the oceanformed and then life developed within it. Life spread globally andbegan to affect Earth's atmosphere and surface, leading to Earth'sGreat Oxidation Event two billion years ago. Humans emerged 300,000years ago, and have reached a population of almost 8 billion today.Humans depend on Earth's biosphere and natural resources for theirsurvival, but have increasingly impacted Earth's environment. Today,humanity's impact on Earth's climate, soils, waters, and ecosystemsis unsustainable, threatening people's lives and causing widespreadextinction of other life.
Etymology
The modern English word Earthdeveloped, via Middle English, from an Old English noun most oftenspelled eorðe. It has cognates in every Germanic language, and theirancestral root has been reconstructed as erþō. In its earliestattestation, the word eorðe was already being used to translate themany senses of Latin terra and Greek γῆ gē: the ground, its soil,dry land, the human world, the surface of the world (including thesea), and the globe itself. As with Roman Terra/Tellūs and GreekGaia, Earth may have been a personified goddess in Germanic paganism:late Norse mythology included Jörð ('Earth'), a giantess oftengiven as the mother of Thor.
Historically, earth has been written inlowercase. From early Middle English, its definite sense as "theglobe" was expressed as the earth. By Early Modern English,many nouns were capitalized, and the earth was also written theEarth, particularly when referenced along with other heavenly bodies.More recently, the name is sometimes simply given as Earth, byanalogy with the names of the other planets, though earth and formswith the remain common. House styles now vary: Oxford spellingrecognizes the lowercase form as the most common, with thecapitalized form an acceptable variant. Another conventioncapitalizes "Earth" when appearing as a name (forexample, "Earth's atmosphere") but writes it inlowercase when preceded by the (for example, "the atmosphere ofthe earth"). It almost always appears in lowercase in colloquialexpressions such as "what on earth are you doing?"
