The Earth, our world, is the third planet from the Sun. It is one of the four terrestrial planets in our Solar System. This means most of its mass is solid. The other three are Mercury, Venus and Mars.
The Earth is home to millions of species of plants and animals, including humans. Earth is the only place in the universe where life has been confirmed to exist. The Earth formed around 4.5 billion years ago. The things that live on Earth have had a great impact on many aspects of the planet. For example, early lifeforms completely changed its air (atmosphere). The total of all life is called the biosphere.
About 71% of Earth's surface is covered in salt water oceans. Earth is the only planet where liquid water is known to exist at present.
The other 29% is made of rocky land in the shape of continents and islands. Earth interacts with other objects in the Solar System, particularly the Sun and the Moon. The Earth orbits or circles the Sun roughly once every 365.25 days. One spin of the Earth is called a day and one orbit around the Sun is called a year. This is why there are 365 days in a year. Earth has only one moon, known as the Moon.
The Earth and the other planets formed about 4.6 billion years ago. They were made of the leftover gas from the nebula that made the Sun. The Moon may have been formed after a collision between Earth and a smaller planet (sometimes called Theia). Scientists believe that parts of both planets broke off — becoming (by gravity) the Moon.
Condensing water vapour, comets and asteroids hitting the Earth made the oceans. Within a billion years (that is at about 3.6 billion years ago) the first life evolved, in the Archaean era. Some bacteria developed photosynthesis, which lets plants make food from the Sun's light and water. This released a lot of oxygen, which was first taken up by iron in solution. Eventually, free oxygen got into the atmosphere or air, making the Earth's surface suitable for aerobic life (see Great Oxygenation Event). This oxygen also formed the ozone layer which protects the Earth from bad ultraviolet radiation from the Sun. This protection made it possible for things to move from the deep ocean to the surface.
Long ago almost all land was in one place. This is called a supercontinent. The earliest known supercontinent was called Rodina. Scientists think that soon after this there was a time (the Cryogenian) when the Earth was almost entirely covered by thick ice sheets (glaciers). This is called the Snowball Earth theory.
thumb|right|A rough size comparison of Earth and the other three terrestrial planets, [[Mercury (planet)|Mercury, Venus and Mars.]]
Earth is a terrestrial planet. This means it is made up of solid rock unlike a gas giant such as Jupiter. It is the largest out of the four terrestrial planets in mass and diameter.
The Earth is composed mostly of iron (32.1%), oxygen (30.1%), silicon (15.1%), magnesium (13.9%), sulfur (2.9%), nickel (1.8%), calcium (1.5%), and aluminium (1.4%); with the remaining 1.2% consisting of trace amounts of other elements. The core region is mainly composed of iron (88.8%), with smaller amounts of nickel (5.8%), sulfur (4.5%), and less than 1% trace elements. Morgan J.W. & Anders E. 1980. Chemical composition of Earth, Venus, and Mercury. Proceedings of the National Academy of Science 77 (12): 6973–6977. [#] Full free text
A little more than 47% of the Earth's crust consists of oxygen. The more common rocks of the Earth's crust are nearly all oxides. The main oxides are silica, alumina, iron oxides, lime (calcium oxide), magnesia, potash and soda. From an analysis of all kinds of rocks, 99.22% were composed of 11 oxides, with other constituents occurring in minute quantities.This article incorporates text from a publication now in the public domain: Chisholm, Hugh (ed) 1911. "Petrology". In Encyclopædia Britannica, 11th ed. Cambridge University Press.
The Earth's shape is an oblate spheroid. This means it is basically a sphere but it bulges around the middle. The circumference of the Earth is about 40,000 kilometers; the average width of the Earth is about 12,700 km. The highest point on Earth is the peak of Mount Everest at 8,848 m above sea level. The lowest natural point is the bottom of the Mariana Trench at 10,911 meters below sea level. Because of the bulge at the middle or the equator, the farthest point from the Earth's center is the top of Mount Chimborazo in Ecuador.
thumb|right|A diagram of the inside of the Earth, showing the different layers.
Inside, the Earth is similar to the other terrestrial planets. It has an outer, solid rock layer called the crust. Everything that lives on Earth is on top of the crust. Below that is a layer of thick, semi-liquid rock called the mantle. Under that is a thin liquid layer called the outer core and then the solid iron inner core. The thickness of the crust changes. On land the average is between 30–50 kilometers thick. Under the oceans in some places it is only 6 kilometers thick. The inside of the Earth is very hot, the temperature of the outer core may be as high as .
thumb|right|A map showing the Earth's major tectonic plates.
The Earth is the only terrestrial planet with active plate tectonics. Due to plate tectonics the Earth's crust basically floats on the thick liquid rock of the mantle below. The crust is split up into parts called plates. These plates interact as they move about causing earthquakes and creating volcanoes and mountain ranges. The place where plates meet are called plate boundaries. There are three types of plate boundary: constructive, destructive and transform.
The Earth changes greatly from place to place. Over 70% of the Earth surface is covered by water. The underwater surface has many of the same features as the above sea with volcanos, mountains and trenches or canyons. The 30% not covered by water is mostly forests, deserts, plains, mountains and plateaux. Human civilisation has led to increasing urbanisation — the growth of cities.
Many things can change the surface of the Earth. Plate tectonics is main cause of change but there are others such as erosion from wind and rain, erosion by the oceans or meteorite impacts. There are three main types of rock that make up the Earth's surface:
All the water on Earth, on land or in the atmosphere, is part of the hydrosphere. About 97.5% of all water is salt water. About half the fresh water is currently ice. The oceans absorb or soak up carbon dioxide, a gas that adds to the greenhouse effect and global warming.
A planet's atmosphere is a layer of different gases surrounding it. It is kept there by gravity. The Earth's atmosphere is made of about 78% nitrogen, 21% oxygen and small amounts of other gases. This mixture is often called air. Further up there is a layer of ozone gas called the Ozone layer. Ozone absorbs ultraviolet radiation from the Sun. Ultraviolet radiation is dangerous to people, so without the Ozone layer life would not be possible. The atmosphere also protects the earth from crashes with meteors and small asteroids. This is because they burn up due to all the friction as they fly through it. It also helps to keep Earth warm. Some gases including carbon dioxide and methane act like a blanket around the Earth, they trap heat under them, keeping the Earth warm. This is called the natural greenhouse effect. When humans build factories and power plants to make electricity, combustion is involved. Combustion lets out a lot of carbon dioxide. This carbon dioxide goes up into the atmosphere and traps more heat. This is called global warming.
Hot air rises. As it rises it gets colder again and falls. This is called convection currents. When hot air meets cold air different weather effects happen. Convection currents are the cause of almost all weather on Earth. When it gets hot on the surface water evaporates and becomes steam or water vapour. This hot water vapour rises. As it rises it gets colder. When it gets cold enough it turns back into water again. This causes the clouds and rain. It is called the water cycle.
thumb|right|200px|Earth's axial tilt (or obliquity) and its relation to the [[Rotation|rotation axis and plane of orbit.]] The Earth takes about 24 hours to complete one day and about 365 days to complete a year. Actually, the Earth takes 365.24 days to revolve around the Sun. To make up this quarter day every year, an extra day is added every four years making the year 366 days. This is a leap year. The Earth is, on average, 150 million kilometers away from the Sun, and moving at an average speed of almost 30 kilometers a second (66,616 miles an hour).
The Moon orbits the Earth at an average distance of 250,000 miles. It is tidally locked to Earth, which means it always has the same side facing the Earth. It takes roughly one month to complete one orbit.
The Earth is part of the Solar System and orbits the Sun along with thousands of small objects and seven other planets. The Sun, and therefore the Solar System, are currently traveling through the Orion Arm of the Milky Way Galaxy, and will be for approximately the next 10,000 years.
| Sun • Heliosphere | Planets ☾ = moon(s) ∅ = rings | Mercury | Venus | Earth ☾ | Mars ☾ | |||||||||||||||||||||||||||||||||||||||||||
| Jupiter ☾ ∅ | Saturn ☾ ∅ | Uranus ☾ ∅ | Neptune ☾ ∅ | |||||||||||||||||||||||||||||||||||||||||||||
| Dwarf planets | Ceres | Pluto ☾ | Haumea ☾ | Makemake | ||||||||||||||||||||||||||||||||||||||||||||
| Eris ☾ | ||||||||||||||||||||||||||||||||||||||||||||||||
| Small Solar System bodies Asteroids | (minor planets) Groups and families: Vulcanoids · Near-Earth asteroids · Asteroid belt | Jupiter Trojans · Centaurs · Neptune Trojans · Asteroid moons · Meteoroids · Pallas · Juno · Vesta · Hygiea · Interamnia · Europa See also the list of asteroids. | | Trans- | Neptunians Kuiper belt – Plutinos: Orcus · Ixion – Cubewanos: Varuna · | Quaoar · Huya Scattered disc: Sedna | | Comets | Periodic comets and non-periodic comets | Damocloids · Oort cloud See also the list of solar system objects | | |||||||||||||||||||||||||||||||||||||
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