|Alternative names||Blue Planet, Terra, Planet Earth, World|
|Semi-major axis||149,597,887.5 km
|Orbital period||365.256366 days
|Average orbital speed||29.783 km/s
to Invariable plane
|Mean radius||6,378.1 km (equatorial), 6,356.8 km (polar)|
|Surface area||510,072,000 km²|
|Volume||1.08321 × 10¹² km³|
|Mass||5.9736 × 1024 kg|
|Mean density||5.515 g/cm³|
|Equatorial surface gravity||0.99732 g|
|Escape velocity||11.186 km/s|
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.
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.
What it is made of [change]
Chemical composition [change]
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.
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.
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.
Internal structure [change]
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 7,000 °C (12,630 °F).
Tectonic plates [change]
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.
- At a divergent plate boundary two plates move away from each other, and hot magma (liquid rock) is pushed upwards through the cracks. These kinds of boundaries make ocean rifts, undersea volcanoes or ridges. Example: Mid-ocean ridges; Great Rift Valley.
- At a convergent (destructive) plate boundary, two plates move towards each other. This forms islands, volcanoes and high mountain ranges. Examples: the Himalayas; the Andes; the Japanese island arc.
- At a transform plate boundary, two plates move parallel to each other. As they move the grind against each other. This kind of plate boundary causes earthquakes. Example:San Andreas Fault.
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:
- Igneous rock is made when magma or lava from the mantle reaches the surface and cools. As it gets colder it turns into rock or solidifies.
- Sedimentary rock is made from sediment, like sand or small bits of other rock, that has been crushed and packed tightly together.
- Metamorphic rock which is made when either of the other two types are changed by high or low temperatures and pressures.
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.
Weather, climate, and water cycle [change]
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.
Orbit and rotation [change]
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.
Related pages [change]
- Blue Planet is used in movies, such as Blue Planet and The Blue Planet, in the Life issue The Incredible Year '68 with the Earthrise photograph and lines from poet James Dickey Behold/The blue planet steeped in its dream/Of reality  pp. 7–8 , and in the title of the European Space Agency bulletin report Exploring the water cycle of the 'Blue Planet' 
- By International Astronomical Union convention, the term "Terra" is used for naming extensive land masses, rather than for the planet Earth. Cf. Blue, Jennifer (July 5, 2007). "Descriptor Terms (Feature Types)". Gazetteer of Planetary Nomenclature. USGS. http://planetarynames.wr.usgs.gov/jsp/append5.jsp. Retrieved 2007-07-05.
- Williams, Dr. David R.. "Earth Fact Sheet". NASA. http://nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html. Retrieved 2011-02-03.
- Standish, E. Myles; Williams, James C. "Orbital Ephemerides of the Sun, Moon, and Planets" (PDF). International Astronomical Union Commission 4: (Ephemerides). http://iau-comm4.jpl.nasa.gov/XSChap8.pdf. Retrieved 2010-04-03. See table 8.10.2. Calculation based upon 1 AU = 149,597,870,700(3) m.
- Allen, Clabon Walter; Cox, Arthur N. (2000). Allen's Astrophysical Quantities. Springer. p. 294. ISBN 0387987460. http://books.google.com/?id=w8PK2XFLLH8C&pg=PA294.
- "How many species are there on Earth". Harvard University. http://adsabs.harvard.edu/abs/1988Sci...241.1441M. Retrieved 2009-07-28.
- Purves, William Kirkwood et al (2001). Life, the science of biology. Macmillan. p. 455. ISBN 0716738732.
- "The age of the Earth in the twentieth century- a problem (mostly) solved". Geological Society, London, Special Publications. http://sp.lyellcollection.org/cgi/content/abstract/190/1/205. Retrieved 2009-07-28.
- "History of earth". Microsoft. http://ca.encarta.msn.com/encyclopedia_761569459/Earth_(planet).html. Retrieved 2009-07-28.[dead link]
- "Origins of life on Earth". Space.com. http://www.space.com/searchforlife/life_origins_001205.html. Retrieved 2009-07-28.
- "Rover reveals Mars was once wet enough for life". Microsoft. http://www.msnbc.msn.com/id/4202901/. Retrieved 2009-07-28.
- "Origin of the Moon in a giant impact near the end of the Earth's formation". Nature.com. http://www.nature.com/nature/journal/v412/n6848/abs/412708a0.html. Retrieved 2009-07-28.
- "Earth life appeared on land 1.5 billion years earlier than previously thought". SpaceRef.com. http://www.spaceref.com/news/viewnews.html?id=258. Retrieved 2009-07-03.
- "The Snowball Earth". Paul F. Hoffman and Daniel P. Schrag. Harvard University. http://www.eps.harvard.edu/people/faculty/hoffman/snowball_paper.html. Retrieved 2009-07-28.[dead link]
- 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
- 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.
- Senne, Joseph H. (2000). "Did Edmund Hillary climb the wrong mountain?". Professional Surveyor 20 (5): 16–21.
- Sharp, David (2005-03-05). "Chimborazo and the old kilogram". The Lancet 365 (9462): 831–832. doi:10.1016/S0140-6736(05)71021-7.
- "Tall tales about highest peaks". Australian Broadcasting Corporation. http://www.abc.net.au/science/k2/moments/s1086384.htm. Retrieved 2008-12-29.
- Toshiro Tanimoto. "Crustal Surface of the Earth". American Geophysical Union. http://www.agu.org/reference/gephys/15_tanimoto.pdf. Retrieved 2009-08-02.
- D. Alfé; M. J. Gillan, L. Vočadlo, J. Brodholt, G. D. Price (2002-04-25). "The ab initio simulation of the Earth’s core". D. Alfé. http://chianti.geol.ucl.ac.uk/~dario/pubblicazioni/PTRSA2002.pdf. Retrieved 2009-08-02.
- Tackley, Paul J. (2000-06-16). "Mantle convection and plate tectonics: towards an integrated physical and chemical theory". Science 288 (5473): 2002–2007. doi:10.1126/science.288.5473.2002. PMID 10856206.
- "The Crust". Oregon State University. http://volcano.oregonstate.edu/education/vwlessons/lessons/Earths_layers/Earths_layers4.html. Retrieved 2009-07-03.
- Seyfert, Carl K. (1987). The Encyclopedia of Structural Geology and Plate Tectonics. ISBN 9780442281250.
- Oreskes (2003). Plate Tectonics : An Insider's History of the Modern Theory of the Earth. Westview Press. ISBN 9780813341329.
- "Plate Tectonics: plate boundaries". platetectonics.com. http://www.platetectonics.com/book/page_5.asp. Retrieved 12 June 2010.
- "Understanding plate motions". USGS. http://pubs.usgs.gov/gip/dynamic/understanding.html. Retrieved 12 June 2010.
- "CHAPTER 8: Introduction to the Hydrosphere". Physical Geography. http://www.physicalgeography.net/fundamentals/8a.html. Retrieved 2009-08-06.
- "WORLD WATER RESOURCES AND THEIR USES". UNESCO. http://espejo.unesco.org.uy/. Retrieved 2009-08-06.[dead link]
- "NASA - Earth's atmosphere". NASA. http://www.nasa.gov/audience/forstudents/9-12/features/912_liftoff_atm.html. Retrieved 2009-08-06.
- "Fundamentals of physical geography - the greenhouse effect". Physical Geography. http://www.physicalgeography.net/fundamentals/7h.html. Retrieved 2009-08-06.
- "What causes weather?". NASA. http://www.nasa.gov/worldbook/weather_worldbook.html. Retrieved 2009-08-06.
- Staff (2007-08-07). "Useful Constants". International Earth Rotation and Reference Systems Service. http://hpiers.obspm.fr/eop-pc/models/constants.html. Retrieved 2008-09-23.
- "NASA- an Earth fact sheet". NASA. http://nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html. Retrieved 2009-08-06.
- "Earth's location in the Milky Way". NASA. http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/030827a.html. Retrieved 2009-08-06.
Other websites [change]
|Wikimedia Commons has media related to: Earth|
|The Simple English Wiktionary has a definition for: Earth and earth.|
- "Solar Views". Calvin J. Hamilton. http://www.solarviews.com/eng/earth.htm. Retrieved 06-08-2009.
- "NASA – Earth". Nasa. http://www.earth.nasa.gov/science/questions.html. Retrieved 06-08-2009.
☾ = moon(s) ∅ = rings
|Mercury||Venus||Earth ☾||Mars ☾|
|Jupiter ☾ ∅||Saturn ☾ ∅||Uranus ☾ ∅||Neptune ☾ ∅|
|Dwarf planets||Ceres||Pluto ☾||Haumea ☾||Makemake|
|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.|
|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|