, also known as “the Earth” and “the World” and sometimes referred to as the “Blue Planet”, the “Blue Marble“, Terra or “Gaia“, is the third-most distant planet from the Sun, the densest planet in the Solar System, the largest of the Solar System’s four terrestrial planets and the only celestial body known to accommodate life.
The modern English noun earthdeveloped from Middle Englisherthe (recorded in 1137), itself from Old English eorthe (dating from before 725), deriving from Proto-Germanic *erthō. Earth has cognates in all other Germanic languages, including Dutchaarde, German Erde, and Swedish, Norwegian, and Danish jord. The Earth is personified as a goddess in Germanic paganism (appearing as Jörð in Norse mythology, mother of the god Thor).
Earth is a terrestrial planet, meaning that it is a rocky body, rather than a gas giantlike Jupiter. It is the largest of the four terrestrial planets in size and mass. Of these four planets, Earth also has the highest density, the highest surface gravity, the strongest magnetic field, and fastest rotation, and is probably the only one with active plate tectonics.
A planet that can sustain life is termed habitable, even if life did not originate there. The Earth provides liquid water—an environment where complex organic molecules can assemble and interact, and sufficient energy to sustain metabolism. The distance of the Earth from the Sun, as well as its orbital eccentricity, rate of rotation, axial tilt, geological history, sustaining atmosphere and protective magnetic field all contribute to the current climatic conditions at the surface.
It is home to millions of species, including a global population of humans, that are supported and nourished by its biosphere and minerals. The human population is grouped into around two-hundred independent sovereign states that interact, among other means, through diplomacy, conflict, travel, trade and media.
According to evidence from sources such as radiometric dating, Earth was formed around four and a half billion years ago. Within its first billion years, life appearedin 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 Earth’s magnetic 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. Since then, Earth’s position in the Solar System, its physical properties and its geological history have allowed life to persist.
Earth’s lithosphere is divided into several rigid segments, or tectonic plates, that migrate across the surface over periods of many millions of years. Over 70% percent of Earth’s surface is covered with water, with the remainder consisting of continents and islands which together have many lakes and other sources of water that contribute to the hydrosphere. Earth’s poles are mostly covered with ice that is the solid ice of the Antarctic ice sheet and the sea ice that is the polar ice packs. The planet’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, the Earth rotates about its own axis 366.26 times, creating 365.26 solar days, or one sidereal year.[n 6] The 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). The Moon is Earth’s only natural satellite. It began orbiting the Earth about 4.53 billion years ago (bya). The Moon’s gravitational interaction with Earth stimulates ocean tides, stabilizes the axial tilt, and gradually slows the planet’s rotation.
The standard astronomical symbol of the Earth consists of a cross circumscribed by a circle.
Unlike the rest of the planets in the Solar System, humankind did not begin to view the Earth as a moving object in orbit around the Sun until the 16th century. Earth has often been personified as a deity, in particular a goddess. In many cultures a mother goddess is also portrayed as a fertility deity. Creation myths in many religions recall a story involving the creation of the Earth by a supernatural deity or deities. A variety of religious groups, often associated with fundamentalist branches of Protestantism or Islam, assert that their interpretations of these creation myths in sacred texts are literal truth and should be considered alongside or replace conventional scientific accounts of the formation of the Earth and the origin and development of life. Such assertions are opposed by the scientific community and by other religious groups. A prominent example is the creation–evolution controversy.
In the past, there were varying levels of belief in a flat Earth, but this was displaced by spherical Earth, a concept that has been credited to Pythagoras (6th century BC). Human cultures have developed many views of the planet, including its personification as a planetary deity, its shape as flat, its position as the center of the universe, and in the modern Gaia Principle, as a single, self-regulating organism in its own right.