Earth

## The Earth

Earth as seen from outer space; the image, known as The Blue Marble, was taken by the crew of Apollo 17 on 7 December 1972

## Designations

### Alternative names

The world, the globe, Sol III, Terra, Tellus, Gaia, Mother Earth

## Orbital characteristics

### Inclination

155° to the Sun's equator;1.57869°[4] to invariable plane;0.00005° to J2000 ecliptic

## Physical characteristics

### Circumference

40075.017 km(24901.461 mi), equatorial40007.86 km(24859.73 mi), meridional

### Surface area

Total: 510072000 km2(196940000 sq mi)Land: 148940000 km2(57510000 sq mi) – 29.2%

Water: 361132000 km2(139434000 sq mi) – 70.8%

## Atmosphere

### Composition by volume

78.08% nitrogen (N2; dry air)20.95% oxygen (O2)≤1% water vapor (climate variable)0.9340% argon0.0415% carbon dioxide0.00182% neon0.00052% helium0.00017% methane0.00011% krypton0.00006% hydrogen

*"Planet Earth" redirects here. For other uses, see Earth (disambiguation) and Planet Earth (disambiguation).*

**Earth** is the third planet from the Sun and the only astronomical object known to harbor life. This is enabled by Earth being a water world, the only one in the Solar System sustaining liquid surface water. Almost all of Earth's water is contained in its global ocean, covering 70.8% of Earth's crust. The remaining 29.2% of Earth's crust is land, most of which is located in the form of continental landmasses within one hemisphere, Earth's land hemisphere. Most of Earth's land is somewhat humid and covered by vegetation, while large sheets of ice at Earth's polar deserts retain more water than Earth's groundwater, lakes, rivers and atmospheric water combined. Earth's crust consists of slowly moving tectonic plates, which interact to produce mountain ranges, volcanoes, and earthquakes. Earth has a liquid outer core that generates a magnetosphere capable of deflecting most of the destructive solar winds and cosmic radiation.

Earth has a dynamic atmosphere, which sustains Earth's surface conditions and protects it from most meteoroids and UV-light at entry. It has a composition of primarily nitrogen and oxygen. Water vapor is widely present in the atmosphere, forming clouds that cover most of the planet. The water vapor acts as a greenhouse gas and, together with other greenhouse gases in the atmosphere, particularly carbon dioxide (CO2), creates the conditions for both liquid surface water and water vapor to persist via the capturing of energy from the Sun's light. This process maintains the current average surface temperature of 14.76 °C, at which water is liquid under atmospheric pressure. Differences in the amount of captured energy between geographic regions (as with the equatorial region receiving more sunlight than the polar regions) drive atmospheric and ocean currents, producing a global climate system with different climate regions, and a range of weather phenomena such as precipitation, allowing components such as nitrogen to cycle.

Earth is rounded into an ellipsoid with a circumference of about 40,000 km. It is the densest planet in the Solar System. Of the four rocky planets, it is the largest and most massive. Earth is about eight light-minutes away from the Sun and orbits it, taking a year (about 365.25 days) to complete one revolution. Earth rotates around its own axis in slightly less than a day (in about 23 hours and 56 minutes). Earth's axis of rotation is tilted with respect to the perpendicular to its orbital plane around the Sun, producing seasons. Earth is orbited by one permanent natural satellite, the Moon, which orbits Earth at 384,400 km (1.28 light seconds) and is roughly a quarter as wide as Earth. The Moon's gravity helps stabilize Earth's axis, and also causes tides which gradually slow Earth's rotation. As a result of tidal locking, the same side of the Moon always faces Earth.

Earth, like most other bodies in the Solar System, formed 4.5 billion years ago from gas in the early Solar System. During the first billion years of Earth's history, the ocean formed and then life developed within it. Life spread globally and has been altering Earth's atmosphere and surface, leading to the Great Oxidation Event two billion years ago. Humans emerged 300,000 years ago in Africa and have spread across every continent on Earth with the exception of Antarctica. Humans depend on Earth's biosphere and natural resources for their survival, but have increasingly impacted the planet's environment. Humanity's current impact on Earth's climate and biosphere is unsustainable, threatening the livelihood of humans and many other forms of life, and causing widespread extinctions.

## Etymology

The Modern English word *Earth* developed, via Middle English, from an Old English noun most often spelled *eorðe*. It has cognates in every Germanic language, and their ancestral root has been reconstructed as \**erþō*. In its earliest attestation, the word *eorðe* was used to translate the many senses of Latin *terra* and Greek γῆ *gē*: the ground, its soil, dry land, the human world, the surface of the world (including the sea), and the globe itself. As with Roman Terra/Tellūs and Greek Gaia, Earth may have been a personified goddess in Germanic paganism: late Norse mythology included Jörð ("Earth"), a giantess often given as the mother of Thor.

Historically, "earth" has been written in lowercase. Beginning with the use of Early Middle English, its definite sense as "the globe" was expressed as "the earth". By the era of Early Modern English, capitalization of nouns began to prevail, and *the earth* was also written *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, though "earth" and forms with "the earth" remain common. 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, such as a description of the "Earth's atmosphere", but employs the lowercase when it is preceded by "the", such as "the atmosphere of the earth"). It almost always appears in lowercase in colloquial expressions such as "what on earth are you doing?"

The name *Terra* /ˈtɛrə/ occasionally is used in scientific writing and especially in science fiction to distinguish humanity's inhabited planet from others, while in poetry *Tellus* /ˈtɛləs/ has been used to denote personification of the Earth. *Terra* is also the name of the planet in some Romance languages, languages that evolved from Latin, like Italian and Portuguese, while in other Romance languages the word gave rise to names with slightly altered spellings, like the Spanish *Tierra* and the French *Terre*. The Latinate form *Gæa* or *Gaea* (English: /ˈdʒiː.ə/) of the Greek poetic name *Gaia* (Γαῖα; Ancient Greek: [ɡâi̯.a] or [ɡâj.ja]) is rare, though the alternative spelling *Gaia* has become common due to the Gaia hypothesis, in which case its pronunciation is /ˈɡaɪ.ə/ rather than the more classical English /ˈɡeɪ.ə/.

There are a number of adjectives for the planet Earth. The word "earthly" is derived from "Earth". From the Latin *Terra* comes *terran* /ˈtɛrən/, *terrestrial* /təˈrɛstriəl/, and (via French) *terrene* /təˈriːn/, and from the Latin *Tellus* comes *tellurian* /tɛˈlʊəriən/ and *telluric*.