Earth (or the Earth) is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's fourterrestrial planets. It is sometimes referred to as the world, the Blue Planet, or by its Latin name, Terra.

Earth formed approximately 4.54 billion years ago by accretion from the solar nebula, andlife appeared on its surface within one billion years. The planet is home to millions ofspecies, including humans. Earth's biosphere has significantly altered the atmosphereand other abiotic conditions on the planet, enabling the proliferation of aerobic organismsas well as the formation of the ozone layer which, together with Earth's magnetic field, blocks harmful solar radiation, permitting life on land. The physical properties of the Earth, as well as its geological history and orbit, have allowed life to persist during this period. The planet is expected to continue supporting life for another 500 million to2.3 billion years.

Earth's crust is divided into several rigid segments, or tectonic plates, that migrate across the surface over periods of many millions of years. About 71% of the surface is covered by salt water oceans, with the remainder consisting of continents and islands which together have many lakes and other sources of water that contribute to the hydrosphere. Earth'spoles are mostly covered with solid ice (Antarctic ice sheet) or sea ice (Arctic ice cap).The planet's interior remains active, with a thick layer of relatively solid mantle, a liquidouter core that generates a magnetic field, and a solid iron inner core.

Earth interacts with other objects in space, especially the Sun and the Moon. At present, Earth orbits the Sun once every 366.26 times it rotates about its own axis, which is equal to 365.26 solar days, or one sidereal year. 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). 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. Between approximately 3.8 billion and 4.1 billion years ago, numerous asteroid impacts during the Late Heavy Bombardment caused significant changes to the greater surface environment.

Both the mineral resources of the planet and the products of the biosphere contribute resources that are used to support a global human population. These inhabitants are grouped into about 200 independent sovereign states (193 United Nations recognized sovereign states), which interact through diplomacy, travel, trade, and military action.Human cultures have developed many views of the planet, including personification as adeity, a belief in a flat Earth or in the Earth as the center of the universe, and a modern perspective of the world as an integrated environment that requires stewardship.

The modern English noun earth developed from Middle English erthe (recorded in 1137), itself from Old English eorthe (dating from before 725), ultimately deriving from Proto-Germanic *erthō. Earth has cognates in all other Germanic languages, including Dutch aarde, GermanErde, 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).

In general English usage, the name earth can be capitalized or spelled in lowercase interchangeably, either when used absolutely or prefixed with "the" (i.e. "Earth", "the Earth", "earth" or "the earth"). Many deliberately spell the name of the planet with a capital, both as "Earth" or "the Earth". This is to distinguish it as a proper noun, distinct from the senses of the term as a count noun or verb (e.g. referring to soil, the ground, earthing in the electrical sense, etc.). Oxford spelling recognizes the lowercase form as the most common, with the capitalized form as a variant of it. Another convention that is very common is to spell the name with a capital when occurring absolutely (e.g. Earth's atmosphere) and lowercase when preceded by "the" (e.g. the atmosphere of the earth). The term almost exclusively exists in lowercase when appearing in common phrases, even without "the" preceding it (e.g. it does not cost the earth; what on earth are you doing?).

(source: wikipedia)

The history of the Earth

The history of the Earth describes the most important events and fundamental stages in the development of the planet Earth from its formation4.54 billion years ago to the present day. Nearly all branches of natural science have contributed to the understanding of the main events of the Earth's past. The age of Earth is approximately one-third of the age of the universe. An immense amount of biological and geological change has occurred in that time span.

Earth formed around 4.54 billion years ago by accretion from the solar nebula. Volcanic outgassing likely created the primordial atmosphere, but it contained almost no oxygen and would have been toxic to humans and most modern life. Much of the Earth was molten because of extreme volcanism and frequent collisions with other bodies. One very large collisionis thought to have been responsible for tilting the Earth at an angle and forming the Moon. Over time, such cosmic bombardments ceased, allowing our planet to cool and form a solid crust. Water that was brought here bycomets and asteroids condensed into clouds and the oceans took shape. Earth was finally hospitable to life, and the earliest forms that arose enriched the atmosphere with oxygen. Life on Earth remained small and microscopic for at least one billion years. About 580 million years ago, complex multicellular life arose, and during the Cambrian period it experienced a rapid diversification into most major phyla. Around six million years ago, the primate lineage that would lead to chimpanzees (our closest living relatives) diverged from the lineage that would lead tomodern humans.

Biological and geological change has been constantly occurring on our planet since the time of its formation. Organisms continuously evolve, taking on new forms or going extinct in response to an ever-changing planet. The process of plate tectonics has played a major role in the shaping of Earth's oceans and continents, as well as the life they harbor. The biosphere, in turn, has had a significant effect on the atmosphere and other abiotic conditions on the planet, such as the formation of the ozone layer, the proliferation of oxygen, and the creation of soil. Though humans are unable to perceive it due to their relatively brief life spans, this change is ongoing and will continue for the next few billion years.
(source: wikipedia)

The atmosphere of Earth

The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention (greenhouse effect), and reducing temperature extremes between day and night (the diurnal temperature variation).

Atmospheric stratification describes the structure of the atmosphere, dividing it into distinct layers, each with specific characteristics such as temperature or composition. The atmosphere has a mass of about 5×10¹⁸ kg, three quarters of which is within about 11 km (6.8 mi; 36,000 ft) of the surface. The atmosphere becomes thinner and thinner with increasing altitude, with no definite boundary between the atmosphere and outer space. An altitude of 120 km (75 mi) is where atmospheric effects become noticeable during atmospheric reentry of spacecraft. The Karmen line, at 100 km (62 mi), also is often regarded as the boundary between atmosphere and outer space.

Air is the name given to atmosphere used in breathing and photosynthesis. Dry air contains roughly (by volume) 78.09% nitrogen, 20.95% oxygen, 0.93% argon, 0.039% carbon dioxide, and small amounts of other gases. Air also contains a variable amount of water vapor, on average around 1%. While air content and atmospheric pressure varies at different layers, air suitable for the survival of terrestrial plants and terrestrial animals is currently only known to be found in Earth's troposphere and artificial atmospheres.

(source: wikipedia)