Editing Spherical Earth

{{Short description|Approximation of the figure of Earth as a sphere}}
{{Redirect|Round world|other uses|The World is Round (disambiguation){{!}}The World is Round}}{{Distinguish|Scleroderma (fungus){{!}}Earth ball}}

[[File:Earth_in_True_Color_from_Himawari_9.png|thumb|Image from space: The spherical surface of [[earth|planet Earth]]]]
'''Spherical Earth''' or '''Earth's curvature''' refers to the [[approximation]] of the [[figure of the Earth]] as a [[sphere]]. The earliest documented mention of the concept dates from around the 5th century BC, when it appears in the writings of [[Ancient Greek philosophy|Greek philosophers]].<ref name="Dicks" /><ref>{{citation|last=Cormack|first=Lesley B.|author-link=Lesley Cormack|article=That before Columbus, geographers and other educated people knew the Earth was flat|editor-last1=Numbers|editor-first1=Ronald L.|editor-last2=Kampourakis|editor-first2=Kostas|title=Newton's Apple and Other Myths about Science|pages=16–24|publisher=Harvard University Press|date=2015|url=https://books.google.com/books?id=pWouCwAAQBAJ|isbn=9780674915473}}</ref> In the 3rd century&nbsp;BC, [[History of geodesy#Hellenic world|Hellenistic astronomy]] established the [[figure of the Earth|roughly spherical shape of Earth]] as a physical fact and calculated the [[Earth's circumference]]. This knowledge was gradually adopted throughout the [[Old World]] during [[late antiquity|Late Antiquity]] and the [[Middle Ages]], displacing earlier beliefs in a [[flat earth]].<ref name="Krüger: Roman and medieval continuation" /><ref name="Encyclopaedia of Islam: Astronomy" /><ref name="Pingree: Indian adoption" /><ref name="Martzloff, Cullen: Chinese adoption">Adoption by China via European science: {{cite journal |last1=Martzloff |first1=Jean-Claude |title=Space and Time in Chinese Texts of Astronomy and of Mathematical Astronomy in the Seventeenth and Eighteenth Centuries |journal=Chinese Science |date=1993 |volume=11 |issue=11 |pages=66–92 |doi=10.1163/26669323-01101005 |jstor=43290474 |url=http://www.eastm.org/index.php/journal/article/view/526/457 |archive-date=2021-10-26 |archive-url=https://web.archive.org/web/20211026212951/http://www.eastm.org/index.php/journal/article/view/526/457 |url-status=dead |access-date=2021-10-12}} and {{cite journal |last1=Cullen |first1=C. |title=A Chinese Eratosthenes of the Flat Earth: A Study of a Fragment of Cosmology in Huai Nan tzu 淮 南 子 |journal=Bulletin of the School of Oriental and African Studies, University of London |date=1976 |volume=39 |issue=1 |pages=106–127 |jstor=616189 |doi=10.1017/S0041977X00052137 |s2cid=171017315}}</ref> A practical demonstration of Earth's [[sphericity]] was achieved by [[Ferdinand Magellan]] and [[Juan Sebastián Elcano]]'s [[circumnavigation]] (1519–1522).<ref>Pigafetta, Antonio (1906). Magellan's Voyage around the World. Arthur A. Clark. [https://archive.org/details/primerviajeentor00piga]</ref>

The realization that the [[figure of the Earth]] is more accurately described as an [[Earth ellipsoid|ellipsoid]] dates to the 17th century, as described by [[Isaac Newton]] in ''[[Philosophiæ Naturalis Principia Mathematica|Principia]]''. In the early 19th century, the flattening of the earth ellipsoid was determined to be of the order of 1/300 ([[Jean Baptiste Joseph Delambre|Delambre]], [[George Everest|Everest]]). The modern value as determined by the [[United States Department of Defense|US DoD]] [[World Geodetic System]] since the 1960s is close to 1/298.25.<ref>See [[Figure of the Earth]] and {{slink|Earth radius#Global radii}} for details. Recent measurements from [[satellite]]s suggest that Earth is actually slightly [[pear]]-shaped. Hugh Thurston, ''[https://www.google.com/books/edition/Early_Astronomy/5SPeevvl4oEC?hl=en&gbpv=1&pg=PA119 Early Astronomy]'', (New York: Springer-Verlag), p. 119. {{ISBN|0-387-94107-X}}.</ref> The scientific study of the shape of the Earth is known as [[geodesy]].

==Cause==
{{main|Equatorial bulge}}
{{see also|Hydrostatic equilibrium#Planetary geology}}

Earth is massive enough that the pull of [[gravity]] maintains its roughly spherical shape. Most of its deviation from spherical stems from the [[centrifugal force]] caused by [[Earth's rotation|rotation]] around its north-south axis. This force deforms the sphere into an [[oblate ellipsoid]].<ref>{{cite web |url=https://spaceplace.nasa.gov/planets-round/en/ |date=June 27, 2019 |title=Why Are Planets Round? |website=NASA Space Place |access-date=2019-08-31}}</ref>

===Formation===
{{Further|History of Earth}}

The [[formation and evolution of the Solar System|Solar System formed]] from a dust cloud that was at least partially the remnant of one or more [[supernova]]s that produced heavy elements by [[nucleosynthesis]]. Grains of matter accreted through electrostatic interaction. As they grew in mass, gravity took over in gathering yet more mass, releasing the [[potential energy]] of their collisions and in-falling as [[heat]]. The [[protoplanetary disk]] also had a greater proportion of radioactive elements than Earth today because, over time, those elements [[Radioactive decay|decayed]]. Their decay heated the early Earth even further, and continue to contribute to [[Earth's internal heat budget]]. The early Earth was thus mostly liquid.

A sphere is the only stable shape for a non-rotating, gravitationally self-attracting liquid. The outward acceleration caused by Earth's rotation is greater at the equator than at the poles (where is it zero), so the sphere gets deformed into an [[ellipsoid]], which represents the shape having the lowest potential energy for a rotating, fluid body. This ellipsoid is slightly fatter around the equator than a perfect sphere would be. Earth's shape is also slightly lumpy because it is composed of different materials of different densities that exert slightly different amounts of gravitational force per volume.

The liquidity of a hot, newly formed planet allows heavier elements to sink down to the middle and forces lighter elements closer to the surface, a process known as [[planetary differentiation]]. This event is known as the [[iron catastrophe]]; the most abundant heavier elements were [[iron]] and [[nickel]], which now form the [[Earth's core]].

===Later shape changes and effects===
Though the surface rocks of Earth have cooled enough to solidify, the [[outer core]] of the planet is still hot enough to remain liquid. Energy is still being released; [[volcano|volcanic]] and [[tectonic]] activity has pushed rocks into hills and mountains and blown them out of [[caldera]]s. [[Meteor]]s also cause [[impact crater]]s and surrounding ridges. However, if the energy release from these processes halts, then they tend to [[Erosion|erode]] away over time and return toward the lowest potential-energy curve of the ellipsoid. [[Weather]] powered by [[solar energy]] can also move water, rock, and soil to make Earth slightly out of round.

Earth undulates as the shape of its lowest potential energy changes daily due to the gravity of the Sun and Moon as they move around with respect to Earth. This is what causes [[tides]] in the [[ocean]]s' water, which can flow freely along the changing potential.

==History of concept and measurement{{anchor|History}}==
{{see|History of geodesy}}
[[File:John Gower world Vox Clamantis detail.jpg|thumb|[[Middle Ages|Medieval]] artistic representation of a spherical Earth{{snd}}with compartments representing [[Soil|earth]], [[air]], and [[water]] ({{circa|1400}})]][[File:Behaims Erdapfel.jpg|thumb|upright|The ''[[Erdapfel]]'', the oldest surviving terrestrial globe (1492/1493)]]
The spherical shape of the Earth was known and measured by astronomers, mathematicians, and navigators from a variety of literate ancient cultures, including the Hellenic World, and Ancient India. Greek ethnographer [[Megasthenes]], {{circa|300&nbsp;BC}}, has been interpreted as stating that the contemporary Brahmans of India believed in a spherical Earth as the center of the universe.<ref>{{cite book|title=Ancient India as described by Megasthenês and Arrian; being a translation of the fragments of the Indika of Megasthenês collected by Dr. Schwanbeck, and of the first part of the Indika of Arrian|url=https://archive.org/details/b29352290|author=E. At. Schwanbeck|date=1877|page=[https://archive.org/details/b29352290/page/101 101]}}</ref> The knowledge of the Greeks was inherited by Ancient Rome, and Christian and Islamic realms in the Middle Ages.  [[Circumnavigation]] of the world in the [[Age of Discovery]] provided direct evidence. Improvements in transportation and other technologies refined estimations of the size of the Earth, and helped spread knowledge of it.

The earliest documented mention of the concept dates from around the 5th century BC, when it appears in the writings of [[Ancient Greek philosophy|Greek philosophers]].<ref name="Dicks">{{cite book |last=Dicks |first=D.R. |url=https://archive.org/details/earlygreekastron0000dick |title=Early Greek Astronomy to Aristotle |date=1970 |publisher=Cornell University Press |isbn=978-0-8014-0561-7 |location=Ithaca, N.Y. |pages=72–198}}</ref><ref>{{citation |last=Cormack |first=Lesley B. |title=Newton's Apple and Other Myths about Science |date=2015 |pages=16–24 |editor-last1=Numbers |editor-first1=Ronald L. |editor-last2=Kampourakis |editor-first2=Kostas |url=https://books.google.com/books?id=pWouCwAAQBAJ |article=That before Columbus, geographers and other educated people knew the Earth was flat |publisher=Harvard University Press |isbn=9780674915473 |author-link=Lesley Cormack}}</ref> In the 3rd century&nbsp;BC, [[History of geodesy#Hellenic world|Hellenistic astronomy]] established the [[figure of the Earth|roughly spherical shape of Earth]] as a physical fact and calculated the [[Earth's circumference]]. This knowledge was gradually adopted throughout the [[Old World]] during [[late antiquity|Late Antiquity]] and the [[Middle Ages]].<ref name="Krüger: Roman and medieval continuation">Continuation into Roman and medieval thought: Reinhard Krüger: "[http://www.uni-stuttgart.de/lettres/krueger/forschungsvorhaben_erdkugeltheorie_biblio.html Materialien und Dokumente zur mittelalterlichen Erdkugeltheorie von der Spätantike bis zur Kolumbusfahrt (1492)]"</ref><ref name="Encyclopaedia of Islam: Astronomy">{{cite book |last1=Jamil |first1=Jamil |url=https://archive.org/details/encyclopaediaofi0000unse_u8d8 |title=Encyclopaedia of Islam |year=2009 |isbn=978-90-04-17852-6 |editor1-last=Fleet |editor1-first=Kate |chapter=Astronomy |doi=10.1163/1573-3912_ei3_COM_22652 |editor2-last=Krämer |editor2-first=Gudrun |editor3-last=Matringe |editor3-first=Denis |editor4-last=Nawas |editor4-first=John |editor5-last=Rowson |editor5-first=Everett |url-access=registration}}</ref><ref name="Pingree: Indian adoption">Direct adoption by India: [[David Pingree|D. Pingree]]: "History of Mathematical Astronomy in India", ''Dictionary of Scientific Biography'', Vol. 15 (1978), pp. 533–633 (554f.); Glick, Thomas F., Livesey, Steven John, Wallis, Faith (eds.): "Medieval Science, Technology, and Medicine: An Encyclopedia", Routledge, New York 2005, {{ISBN|0-415-96930-1}}, p. 463</ref><ref name="Martzloff, Cullen: Chinese adoption" /> A practical demonstration of Earth's [[sphericity]] was achieved by [[Ferdinand Magellan]] and [[Juan Sebastián Elcano]]'s [[circumnavigation]] (1519–1522).<ref>Pigafetta, Antonio (1906). Magellan's Voyage around the World. Arthur A. Clark. [https://archive.org/details/primerviajeentor00piga]</ref>

The concept of a spherical Earth displaced earlier beliefs in a [[flat Earth]]: In early [[Mesopotamian myths|Mesopotamian mythology]], the world was portrayed as a disk floating in the ocean with a hemispherical sky-dome above,<ref name="Neugebauer">{{cite book |last=Neugebauer |first=Otto E. |author-link=Otto E. Neugebauer |title=A History of Ancient Mathematical Astronomy |date=1975 |publisher=Birkhäuser |isbn=978-3-540-06995-9 |page=577}}</ref> and this forms the premise for [[early world maps]] like those of [[Anaximander]] and [[Hecataeus of Miletus]]. Other speculations on the shape of Earth include a seven-layered [[ziggurat]] or [[cosmic mountain]], alluded to in the [[Avesta]] and ancient [[Achaemenid Empire|Persian]] writings (see [[clime|seven climes]]).

The realization that the [[figure of the Earth]] is more accurately described as an [[Earth ellipsoid|ellipsoid]] dates to the 17th century, as described by [[Isaac Newton]] in ''[[Philosophiæ Naturalis Principia Mathematica|Principia]]''. In the early 19th century, the flattening of the earth ellipsoid was determined to be of the order of 1/300 ([[Jean Baptiste Joseph Delambre|Delambre]], [[George Everest|Everest]]). The modern value as determined by the [[United States Department of Defense|US DoD]] [[World Geodetic System]] since the 1960s is close to 1/298.25.<ref>See [[Figure of the Earth]] and {{slink|Earth radius#Global radii}} for details. Recent measurements from [[Satellite|satellites]] suggest that Earth is actually slightly [[pear]]-shaped. Hugh Thurston, ''[https://books.google.com/books?id=5SPeevvl4oEC&pg=PA119 Early Astronomy]'', (New York: Springer-Verlag), p. 119. {{ISBN|0-387-94107-X}}.</ref>

==Measurement and representation==
{{Main|Geodesy}}

[[Geodesy]], also called geodetics, is the scientific discipline that deals with the measurement and representation of Earth, its [[gravitation]]al field and geodynamic phenomena ([[polar motion]], Earth [[tide]]s, and crustal motion) in three-dimensional time-varying space.

Geodesy is primarily concerned with positioning and the gravity field and geometrical aspects of their temporal variations, although it can also include the study of Earth's [[magnetic field]]. Especially in the [[German language|German]] speaking world, geodesy is divided into [[geomensuration]] ("Erdmessung" or "höhere Geodäsie"), which is concerned with measuring Earth on a global scale, and [[Geophysical survey|surveying]] ("Ingenieurgeodäsie"), which is concerned with measuring parts of the surface.

Earth's shape can be thought of in at least two ways:
* as the shape of the [[geoid]], the mean sea level of the world ocean; or
* as the shape of Earth's land surface as it rises above and falls below the sea.

As the science of [[geodesy]] measured Earth more accurately, the shape of the geoid was first found not to be a perfect sphere but to approximate an [[spheroid|oblate spheroid]], a specific type of [[ellipsoid]]. More recent{{when|date=November 2021}} measurements have measured the geoid to unprecedented accuracy, revealing [[Mass concentration (astronomy)|mass concentration]]s beneath Earth's surface.

==Evidence==
{{excerpt|Empirical evidence for the spherical shape of Earth}}

==See also==
*[[Astronomical body#Shapes|Shapes of other astronomical bodies]]
*[[Celestial sphere]]
*[[Celestial spheres]]
*[[Earth radius]]
*{{section link|Earth's rotation|Empirical tests}}
*[[Geographical distance]]
*[[Myth of the flat Earth]]
*[[Physical geodesy]]
*[[Terra Australis]]
*[[WGS 84]]

==References==
{{Reflist|30em}}

===Works cited===
{{refbegin}}
*{{cite book|last1=Needham|first1=Joseph|last2=Wang|first2=Ling|title=Science and Civilization in China: Mathematics and the Sciences of the Heavens and the Earth|date=1995|orig-date=1959|volume=3|edition=reprint|location=Cambridge|publisher=Cambridge University Press|isbn=0-521-05801-5|ref={{harvid|Needham|1959}}}}
{{refend}}

==Further reading==
*{{cite book|title=Janice VanCleave's Science Through the Ages|author=Janice VanCleave|publisher=John Wiley & Sons|date=2002|url=https://books.google.com/books?id=RFp4KeYt9s8C|isbn=9780471208303}}
*{{cite book|title=Early Astronomy and Cosmology: A Reconstruction of the Earliest Cosmic System, Etc.|author=Menon, CPS|location=Whitegishm MT, US.|publisher=Kessinger Publishing|url=https://books.google.com/books?id=qnz2vGVaPjsC|isbn=9780766131040|date=2003}}
*{{cite book|title=How Did We Find Out the Earth is Round?|author=Isaac Asimov|date=1972|publisher=New York, Walker|isbn=978-0802761217|url-access=registration|url=https://archive.org/details/earthisround00asim}}

==External links==
{{Commons|Spherical Earth}}
*[http://www.straightdope.com/classics/a2_087.html You say the earth is round? Prove it] (from [[The Straight Dope]])
*[http://www.nasa.gov/centers/goddard/earthandsun/earthshape.html NASA – Most Changes in Earth's Shape Are Due to Changes in Climate]
*[http://www.phy6.org/stargaze/Scolumb.htm The Round Earth and Christopher Columbus], educational web site
*[http://www.smarterthanthat.com/astronomy/top-10-ways-to-know-the-earth-is-not-flat Top 10 Ways to Know the Earth is Not Flat], science education site

{{Greek astronomy}}

[[Category:Ancient astronomy]]
[[Category:Ancient Greek astronomy]]
[[Category:Cartography]]
[[Category:Earth]]
[[Category:Early scientific cosmologies]]
[[Category:Geodesy]]
[[Category:Spheres]]