Earth is the third planet from the Sun. Earth is the largest of the terrestrial planets in the Solar System in diameter, mass and density. It is also referred to as the Earth, Planet Earth, the World, and Terra.
Home to millions of species,including humans, Earth is the only place in the universe where life is known to exist. Scientific evidence indicates that the planet formed 4.54 billion years ago, and life appeared on its surface within a billion years. Since then, Earth’s biosphere has significantly altered the atmosphere and other abiotic conditions on the planet, enabling the proliferation of aerobic organisms as well as the formation of the ozone layer which, together with Earth’s magnetic field, blocks harmful radiation, permitting life on land.
Earth’s outer surface is divided into several rigid segments, or tectonic plates, that gradually migrate across the surface over periods of many millions of years. About 71% of the surface is covered with salt-water oceans, the remainder consisting of continents and islands; liquid water, necessary for all known life, is not known to exist on any other planet’s surface. Earth’s interior remains active, with a thick layer of relatively solid mantle, a liquid outer core that generates a magnetic field, and a solid iron inner core.
Earth interacts with other objects in outer space, including the Sun and the Moon. At present, Earth orbits the Sun once for every roughly 366.26 times it rotates about its axis. This length of time is a sidereal year, which is equal to 365.26 solar days.The Earth’s axis of rotation is tilted 23.4° away from the perpendicular to its orbital plane,producing seasonal variations on the planet’s surface with a period of one tropical year (365.24 solar days). Earth’s only known natural satellite, the Moon, which began orbiting it about 4.53 billion years ago, provides ocean tides, stabilizes the axial tilt and gradually slows the planet’s rotation. A cometary bombardment during the early history of the planet played a role in the formation of the oceans.Later, asteroid impacts caused significant changes to the surface environment.
Cultural viewpoint
The first photograph ever taken of an “Earthrise,” on Apollo 8.
Etymology
The name Earth originates from the 8th century Anglo-Saxon word erda, which means ground or soil. In Old English the word became eorthe, then erthe in Middle English. Earth was first used as the name of the sphere of the Earth around 1400.It is the only planet whose name in English is not derived from Greco-Roman mythology.
The standard astronomical symbol of the Earth consists of a cross circumscribed by a circle. This symbol is known as the wheel cross, sun cross, Odin’s cross or Woden’s cross. Although it has been used in various cultures for different purposes, it came to represent the compass points, earth and the land. Another version of the symbol is a cross on top of a circle; a stylized globus cruciger that was also used as an early astronomical symbol for the planet Earth.
Religious beliefs
Earth has often been personified as a deity, in particular a goddess. In many cultures the mother goddess, also called the Mother Earth, is also portrayed as a fertility deity. See also Graha. To the Aztec, Earth was called Tonantzin—”our mother”; to the Incas, Earth was called Pachamama—”mother earth”. The Chinese Earth goddess Hou-T’u is similar to Gaia, the Greek goddess personifying the Earth. To Hindus it is called Bhuma Devi, the Goddess of Earth. In Norse mythology, the Earth goddess Jord was the mother of Thor and the daughter of Annar. Ancient Egyptian mythology is different from that of other cultures because Earth is male, Geb, and sky is female, Nut.
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 Protestant or Islam, assert that their interpretations of the accounts of creation 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 other religious groups. A prominent example is the creation-evolution controversy.
Exploration and mapping
In the ancient past there were varying levels of belief in a flat Earth, with the Mesopotamian culture portraying the world as a flat disk afloat in an ocean. The spherical form of the Earth was suggested by early Greek philosophers; a belief espoused by Pythagoras. By the Middle Ages—as evidenced by thinkers such as Thomas Aquinas—European belief in a spherical Earth was widespread. Prior to circumnavigation of the planet and the introduction of space flight, belief in a spherical Earth was based on observations of the secondary effects of the Earth’s shape and parallels drawn with the shape of other planets
Cartography, the study and practice of map making, and vicariously geography, have historically been the disciplines devoted to depicting the Earth. Surveying, the determination of locations and distances, to a lesser extent navigation, the determination of position and direction, have developed alongside cartography and geography, providing and suitably quantifying the requisite information.
Modern perspective
The technological developments of the latter half of the 20th century are widely considered to have altered the public’s perception of the Earth. Before space flight, the popular image of Earth was of a green world. Science fiction artist Frank R. Paul provided perhaps the first image of a cloudless blue planet (with sharply defined land masses) on the back cover of the July 1940 issue of Amazing Stories, a common depiction for several decades thereafter.
Earth and Moon from Mars, imaged by Mars Global Surveyor. From space, the Earth can be seen to go through phases similar to the phases of the Moon.
Earth was first photographed from space by Explorer 6 in 1959.Yuri Gagarin became the first human to view Earth from space in 1961. The crew of the Apollo 8 was the first to view an Earth-rise from lunar orbit in 1968. In 1972 the crew of the Apollo 17 produced the famous “Blue Marble” photograph of the planet Earth from cislunar space (see top of page). This became an iconic image of the planet as a marble of cloud-swirled blue ocean broken by green-brown continents. NASA archivist Mike Gentry has speculated that “The Blue Marble” is the most widely distributed image in human history. A photo taken of a distant Earth by Voyager 1 in 1990 inspired Carl Sagan to describe the planet as a “Pale Blue Dot.”
Since the 1960s, Earth has also been described as a massive “Spaceship Earth,” with a life support system that requires maintenance, or, in the Gaia hypothesis, as having a biosphere that forms one large organism.
Over the past two centuries a growing environmental movement has emerged that is concerned about humankind’s effects on the Earth. The key issues of this socio-political movement are the conservation of natural resources, elimination of pollution, and the usage of land. Environmentalists advocate sustainable management of resources and stewardship of the environment through changes in public policy and individual behavior. Of particular concern is the large-scale exploitation of non-renewable resources. Changes sought by the environmental movements are sometimes in conflict with commercial interests due to the additional costs associated with managing the environmental impact of those interests.
