The largemass of the Sun keeps the Earth inorbit through the force ofgravity.[31] Earth also turns around in space, so that different parts face the Sun at different times. Earth goes around the Sun once (oneyear) for every 3651⁄4 times it turns around (oneday).
Earth is the onlyplanet in the Solar System that has a large amount ofliquidwater on its surface.[32][33][34] About 71% of the surface of Earth is covered by liquid or frozen water.[35] Because of this, people sometimes call it the blue planet.[36]
Because of its water, Earth is home to millions ofspecies ofplants andanimals which need water to survive.[37][38] The things that live on Earth have changed its surface greatly. For example, earlycyanobacteria changed theair and gave itoxygen. The living part of Earth's surface is called the "biosphere".[39]
Earth turns at an angle (an "axial tilt") in relation to its path around the Sun
Earth is one of the eightplanets in the Solar System. There are also thousands of small bodies which move around the Sun. The Solar System is moving through theOrion Arm of theMilky Waygalaxy, and will be for about the next 10,000 years.[40][41]
Earth is about 150,000,000 kilometres or 93,000,000 miles away from the Sun (this distance is called an "astronomical unit" or au. It moves on itsorbit at anaverage speed of about 30 km/s (19 mi/s).[42] Earth turns around about 3651⁄4 times in the time it takes for Earth to go all the way around the Sun.[4] To make up this extra bit of a day every year, anadditional day is used every four years. This is called a "leap year".
TheMoon goes around Earth at anaverage distance of 400,000 kilometres or 250,000 miles. It islocked to Earth, so that it always has the same half facing Earth; the other half is called the "dark side of the moon". It takes about 271⁄3 days for the Moon to go all the way around Earth, but because Earth is moving around the Sun at the same time, it takes about 291⁄2 days for the Moon to go from dark to bright to dark again. This is where the word "month" came from, even though most months now have 30 or 31 days.[43]
Earth and the otherplanets formed about 4.6 billion years ago.[44] Their origin was different from that of theSun. The Sun was formed almost entirely ofhydrogen, while the planets were formed mostly from higher elements. The smaller "rocky" planets are made almost entirely of higher elements. The Sun must have moved through areas wheresupernovae had previously exploded.[45] All the planets have higher elements which are only made in supernovae.[46][47][48] Only the so-called "gas giants" have much hydrogen andhelium.
The Moon may have been formed after a collision between the early Earth and a smaller planet (sometimes calledTheia). Scientists believe that parts of both planets broke off – becoming (bygravity) the Moon.[49]
Earth's land andclimate has been very different in the past. About 3 to 3.5 billion years ago almost all land was in one place. This is called asupercontinent. The earliest known supercontinent was calledVaalbara. Much later, there many times the Earth was covered inice sheets. (For example, theCryogenian).[53] This is called theSnowball Earththeory.[53]
Overall, Earth is made ofiron (32.1%),oxygen (30.1%),silicon (15.1%),magnesium (13.9%),sulfur (2.9%),nickel (1.8%),calcium (1.5%), andaluminium (1.4%). The 1.2% left over is made of many different kinds of other chemicals. Some rare metals (not justgold andplatinum) are very valuable.Rare earth metals are often used in electronic phones and computers.[54]
Earth's shape is aspheroid: not quite asphere because it is slightlysquashed on the top and bottom. The shape is called anoblate spheroid. As Earth spins around itself,centrifugal force forces theequator out a little and pulls thepoles in a little. The equator, around the middle of Earth's surface, is about 40,075 kilometers or 24,900 miles long.[58] The reason the Earth is roughly asphere (and so are all planets and stars) isgravity.[59] Meteorites, on the other hand may be any shape because, in their case, the force of gravity is too weak to change their shape.
The highest mountain abovesea level—the well-knownMount Everest (which is 8,848 metres or 29,029 feetabove sea level)—isnot actually the one that is the farthest away from the center of the Earth. Instead, the sleepingvolcanoMount Chimborazo inEcuador is; it is only 6,263 metres or 20,548 feetabove sea level but it is almost at theequator. Because of this, Mount Chimborazo is 6,384 kilometres or 3,967 miles from the center of the Earth, whileMount Everest is closer to it (2 kilometres or 1.2 miles).[60][61][62] Similarly, the lowest point below sea level that we are conscious of is theChallenger Deep in theMariana Trench in thePacific Ocean. It is about 10,971 metres or 35,994 feetbelow sea level,[63] but, again, there are probably places at the bottom of theArctic Ocean that are nearer to the center of the Earth.
The inside of Earth is very different from the outside. Almost all of Earth's liquid water is in theseas or close to the surface. The surface also has a lot ofoxygen, which comes from plants. Small and simple kinds of life can live far under the surface, but animals and plants only live on the surface or in the seas. The rocks on the surface of Earth (Earth's crust) are well known. They are thicker where there is land, between 30 to 50 km or 19 to 31 mi thick. Under theseas they are sometimes only 6 km or 3.7 mi thick.[64]
There are three groups of rocks that make up most of the Earth's crust. Some rock is made when the hot liquid rock comes from inside the earth (igneous rocks); another type of rock is made whensediment is laid down, usually under the sea (sedimentary rocks); and a third kind of rock is made when the other two are changed by very hightemperature orpressure (metamorphic rocks).
Below the crust is hot and almost-liquid rock which is always moving around (theEarth's mantle). Then, there is a thin liquid layer of heated rock (theouter core). This is very hot: 7,000 °C or 13,000 °F or 7,300 K.[65] The middle of the inside of the Earth would be liquid as well but all the pressure of the rock above it makes it a solid. This solid middle part (theinner core) is almost alliron. It is what makes the Earthmagnetic.
TheEarth's crust is solid but made ofparts which move very slowly.[66] The thin skin of hard rock on the outside of the Earth rests on hot liquid material below it in the deepermantle.[67] This liquid material moves because it gets heat from the hot center of the Earth. The slow movement of the plates is a factor inearthquakes,volcanoes and large groups of mountains on the Earth.
All places on Earth are made of, or are on top of, rocks. The outside of the Earth is usually not uncovered rock. Over 70% of the Earth is covered byseas full ofsalty water.[72] This salty water makes up about 971⁄2% of all Earth's water. The drinkablefresh water is mostly in the form ofice. There is only a small amount (less than 3%) of fresh water inrivers and under the ground for people to drink.[73] Gravity stops the water from going away intoouter space. Also, much of the land on Earth is covered with plants, or with what is left from earlier living things. Places with very little rain are dry wastes calleddeserts. Deserts usually have few living things, but life is able to grow very quickly when these wastes have rainfall. Places with large amounts of rain may berain forests. Lately, people have changed theenvironment of the Earth a great deal. As population has increased, so has farming. Farming is done on what were once natural forests and grassland.[74][75]
All around the Earth is the of air (theatmosphere). The mass of the Earth holds thegasses in the air down and does not let them go into outer space. The air is mostly made ofnitrogen (about 78%) andoxygen (about 21%) and there are a few other gasses as well.[76] Living things need both the air and water.
The air, which animals and plants use to live, is only the first level of the air around the Earth (thetroposphere). The day to day changes in this level of air are calledweather; the larger differences between distant places, and from year to year, are called theclimate.Rain andstorms come about because this part of the air gets colder as it goes up.Cold air becomes thicker and falls, and warm air becomes thinner and goes up.[77] The turning Earth also moves the air as well and air moves north and south because the middle of the Earth generally gets more power from the Sun and is warmer than the north and south points. Air over warm waterevaporates but, because cold air is not able to take in as much water, it starts to makeclouds andrain as it gets colder. The way water moves around in a circle like this is called thewater cycle.[77]
Above this first level, there are four other levels. The air gets colder as it goes up in the first level; in the second level (thestratosphere), the air gets warmer as it goes up. This level has a special kind ofoxygen calledozone. Theozone in this air keeps living things safe fromdamaging rays from the Sun. The power from these rays is what makes this level warmer and warmer. The middle level (themesosphere) gets colder and colder with height; the fourth level (thethermosphere) gets warmer and warmer; and the last level (theexosphere) is almost outer space and has very little air at all. It reaches about half the way to the Moon. The three outer levels have a lot ofelectric power moving through them; this is called theionosphere and is important forradio and other electric waves in the air.
Even though air seems very light, the weight of all of the air above the outside of the Earth (air pressure) is important. Generally, fromsea level to the top ofthe outer level of the air, a space of air onecm2 across has a mass of about 1.03kg and a space of air onesq in across has a weight of about 14.7lb. Because of friction in the air, small meteorites generally burn up long before they get to the Earth.
The air also keeps the Earth warm, specially the half turned away from the Sun. Some gasses – especiallymethane andcarbon dioxide –work like a blanket to keep things warm.[78]In the past, the Earth has been much warmer and much colder than it is now. Since people have adapted to the heat we have now, we do not want the Earth to be too much warmer or colder. Most of the ways people createelectric power use burning kinds ofcarbon – especiallycoal,oil, andnatural gas. Burning these fuels creates morecarbon dioxide which causes more warming. Adiscussion is going on now about what people should do aboutthe Earth's latest warming, which has gone on for about 150 years. So far, this warming has been acceptable: plants have grown better. The weather has generally been better thanwhen it was colder.
About eightbillion people live on Earth. They live in about 200 different lands calledcountries. Some, for example,Russia, are large with many large cities. Others, for example,Vatican City, are small. The seven countries with the most people areIndia,China, theUnited States,Indonesia,Pakistan,Brazil andNigeria. About 90% of people live in thenorthern hemisphere of the world, which has most of the land. Human beings originally came fromAfrica. Now, 70% of all people do not live in Africa but inEurope andAsia.[79]
People change the Earth in many ways. They have been able to grow plants for food and clothes for about ten thousand years. When there was enough food, they were able to build towns and cities. Near these places, men and women were able to change rivers,bring water to farms, and stopfloods (rising water) from coming over their land. People found useful animals andbred them so they were easier to keep.[80]
There is wide agreement that the long-term future of Earth is tied to the future of theSun.[81] As time passes, the Sun will get hotter, and that will eventually make the Earth a planet without life.
↑All astronomical quantities vary, both in time (secularly) and frequency (periodically). The quantities given are the values at the instantJ2000.0 of the secular variation, ignoring all periodic variations.
↑2.02.1aphelion =a × (1 +e); perihelion =a × (1 –e), wherea is the semi-major axis ande is the eccentricity. The difference between Earth's perihelion and aphelion is 5 million kilometers.
↑3.03.1Simon, J.L.; Bretagnon, P.; Chapront, J.; Chapront-Touzé, M.; Francou, G.; Laskar, J. (February 1994). "Numerical expressions for precession formulae and mean elements for the Moon and planets".Astronomy and Astrophysics.282 (2):663–83.Bibcode:1994A&A...282..663S.
↑4.04.14.2Staff (7 August 2007)."Useful Constants". International Earth Rotation and Reference Systems Service. Retrieved23 September 2008.
↑"UCS Satellite Database".Nuclear Weapons & Global Security. Union of Concerned Scientists. 10 August 2018. Retrieved27 September 2018.
↑As of 4 January 2018, the United States Strategic Command tracked a total of 18,835 artificial objects, mostly debris. See:Anz-Meador, Phillip; Shoots, Debi, eds. (February 2018)."Satellite Box Score"(PDF).Orbital Debris Quarterly News.22 (1): 12. Retrieved18 April 2018.
↑International Earth Rotation and Reference Systems Service (IERS) Working Group (2004)."General Definitions and Numerical Standards"(PDF). In McCarthy, Dennis D.; Petit, Gérard (eds.).IERS Conventions (2003(PDF). Dennis D. McCarthy, Gérard Petit, IERS Convertions Centre. Frankfurt am Main: Verlag des Bundesamts für Kartographie und Geodäsie. p. 12.ISBN978-3-89888-884-4. Retrieved29 April 2016.
↑Earth'scircumference is almost exactly 40,000 km because the metre was calibrated on this measurement—more specifically, 1/10-millionth of the distance between the poles and the equator.
↑Staff (24 July 2008)."World".The World Factbook. Central Intelligence Agency. Archived fromthe original on 5 January 2010. Retrieved5 August 2008.
↑Due to natural fluctuations, ambiguities surroundingice shelves, and mapping conventions forvertical datums, exact values for land and ocean coverage are not meaningful. Based on data from theVector Map andGlobal LandcoverArchived 2015-03-26 at theWayback Machine datasets, extreme values for coverage of lakes and streams are 0.6% and 1.0% of Earth's surface. The ice shields ofAntarctica andGreenland are counted as land, even though much of the rock that supports them lies below sea level.
↑Luzum, Brian; Capitaine, Nicole; Fienga, Agnès; Folkner, William; Fukushima, Toshio; et al. (August 2011). "The IAU 2009 system of astronomical constants: The report of the IAU working group on numerical standards for Fundamental Astronomy".Celestial Mechanics and Dynamical Astronomy.110 (4):293–304.Bibcode:2011CeMDA.110..293L.doi:10.1007/s10569-011-9352-4.ISSN0923-2958.S2CID122755461.
↑The international system of units (SI)(PDF) (2008 ed.). United States Department of Commerce, National Institute of Standards and Technology Special Publication 330. p. 52. Archived fromthe original(PDF) on 3 June 2016. Retrieved13 June 2019.
↑Sun's motion and in general the motion of stars in theMilky Way is known fromGaia data release #2.
↑Burbidge, E.M; Burbidge G.R.; Fowler W.A.; Hoyle F. 1957. Synthesis of the elements in stars. Reviews of Modern Physics.29 (4): 547–650.[2]
↑Clayton D. 2003.Handbook of isotopes in the cosmos. Cambridge University Press.ISBN 978-0-521-82381-4
↑ Kasen D; Metzger B; Barnes J; Quataert E; Ramirez-Ruiz, E. 2017. Origin of the heavy elements in binary neutron-star mergers from a gravitational-wave event.Nature.551 (7678): 80–84.
↑Ghosh, Pallab 2017. Earliest evidence of life on Earth 'found'. BBC News Science & Environment. [2]
↑Sridharan R; Ahmed S.M; Pratim Das, Tirtha; Sreelatha P.; Pradeepkumar P.; Naik, Neha; Supriya, Gogulapati 2010. Direct evidence for water (H2O) in the sunlit lunar ambience from CHACE on MIP of Chandrayaan I". Planetary and Space Science.58 (6): 947–950. Bibcode:2010P&SS...58..947S. doi:10.1016/j.pss.2010.02.013.
↑53.053.1"The Snowball Earth".Paul F. Hoffman and Daniel P. Schrag. Harvard University. 8 August 1999. Archived fromthe original on 10 September 2009. Retrieved28 July 2009.
↑Rare Earths explained. Jordy Lee Calderon, Milken Institute Review.[3]
↑Morgan J.W. & Anders E. 1980. Chemical composition of Earth, Venus, and Mercury.Proceedings of the National Academy of Science77 (12): 6973–6977. [https://www.ncbi.nlm.nih.gov/pmc/articles/PMC350422/?tool=pmcentrez Chemical composition of Earth, Venus, and Mercury. Full free text
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↑Taiz, Lincoln 2013. Agriculture, plant physiology, and human population growth: past, present, and future.Theoretical and Experimental Plant Physiology25: p167–181.
↑Zulkarnaen, Diny, and Marianito R. Rodrigo 2020. Modelling human carrying capacity as a function of food availability.The ANZIAM Journal62.3: p318–333.
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