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 Greek philosophers.[1][2] In the 3rd century BC, Hellenistic astronomy established the 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 and the Middle Ages, displacing earlier beliefs in a flat Earth.[3][4][5][6] A practical demonstration of Earth's sphericity was achieved by Ferdinand Magellan and Juan Sebastián Elcano's circumnigation (1519–1522).[7]
The realization that the figure of the Earth is more accurately described as an ellipsoid dates to the 17th century, as described by Isaac Newton in Principia. In the early 19th century, the flattening of the earth ellipsoid was determined to be of the order of 1/300 (Delambre, Everest). The modern value as determined by the US DoD World Geodetic System since the 1960s is close to 1/298.25.[8] The scientific study of the shape of the Earth is known as geodesy.
Cause[edit] Main article: Equatorial bulge See also: Hydrostatic equilibrium § Planetary geologyEarth is massive enough that the pull of grity maintains its roughly spherical shape. Most of its deviation from spherical stems from the centrifugal force caused by rotation around its north-south axis. This force deforms the sphere into an oblate ellipsoid.[9]
Formation[edit] Further information: History of EarthThe Solar System formed from a dust cloud that was at least partially the remnant of one or more supernovas that produced hey elements by nucleosynthesis. Grains of matter accreted through electrostatic interaction. As they grew in mass, grity 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 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, gritationally 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 hing 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 gritational force per volume.
The liquidity of a hot, newly formed planet allows heier 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 heier elements were iron and nickel, which now form the Earth's core.
Later shape changes and effects[edit]Though the surface rocks of Earth he cooled enough to solidify, the outer core of the planet is still hot enough to remain liquid. Energy is still being released; volcanic and tectonic activity has pushed rocks into hills and mountains and blown them out of calderas. Meteors also cause impact craters and surrounding ridges. However, if the energy release from these processes halts, then they tend to 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 grity of the Sun and Moon as they move around with respect to Earth. This is what causes tides in the oceans' water, which can flow freely along the changing potential.
History of concept and measurement[edit] Further information: History of geodesy Medieval artistic representation of a spherical Earth – with compartments representing earth, air, and water (c. 1400)The Erdapfel, the oldest surviving terrestrial globe (1492/1493)The spherical shape of the Earth was known and measured by astronomers, mathematicians, and nigators from a variety of literate ancient cultures, including the Hellenic World, and Ancient India. Greek ethnographer Megasthenes, c. 300 BC, has been interpreted as stating that the contemporary Brahmins of India believed in a spherical Earth as the center of the universe.[10] The knowledge of the Greeks was inherited by Ancient Rome, and Christian and Islamic realms in the Middle Ages. Circumnigation 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 Greek philosophers.[1][11] In the 3rd century BC, Hellenistic astronomy established the 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 and the Middle Ages.[3][4][5][6] A practical demonstration of Earth's sphericity was achieved by Ferdinand Magellan and Juan Sebastián Elcano's circumnigation (1519–1522).[12]
The concept of a spherical Earth displaced earlier beliefs in a flat Earth: In early Mesopotamian mythology, the world was portrayed as a disk floating in the ocean with a hemispherical sky-dome above,[13] 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 Persian writings (see seven climes).
The realization that the figure of the Earth is more accurately described as an ellipsoid dates to the 17th century, as described by Isaac Newton in Principia. In the early 19th century, the flattening of the earth ellipsoid was determined to be of the order of 1/300 (Delambre, Everest). The modern value as determined by the US DoD World Geodetic System since the 1960s is close to 1/298.25.[14]
Measurement and representation[edit] Main article: GeodesyGeodesy, also called geodetics, is the scientific discipline that deals with the measurement and representation of Earth, its gritational field and geodynamic phenomena (polar motion, Earth tides, and crustal motion) in three-dimensional time-varying space.
Geodesy is primarily concerned with positioning and the grity field and geometrical aspects of their temporal variations, although it can also include the study of Earth's magnetic field. Especially in the German speaking world, geodesy is divided into geomensuration ("Erdmessung" or "höhere Geodäsie"), which is concerned with measuring Earth on a global scale, and 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 oblate spheroid, a specific type of ellipsoid. More recent[when?] measurements he measured the geoid to unprecedented accuracy, revealing mass concentrations beneath Earth's surface.
Evidence[edit] This section is an excerpt from Empirical evidence for the spherical shape of Earth.[edit] A sequence of pictures taken at Strait of Magellan, Chile, showing a tanker ship sailing towards the camera and rising over the horizon, demonstrating the Earth's curvature. Distance at first picture is about 18 km which would imply a theoretical horizon vertical drop of about 23 meters. Ship length is 186 m.The roughly spherical shape of Earth can be empirically evidenced by many different types of observation, ranging from ground level, flight, or orbit. The spherical shape causes a number of effects and phenomena that when combined disprove flat Earth beliefs.
These include the visibility of distant objects on Earth's surface; lunar eclipses; appearance of the Moon; observation of the sky from a certain altitude; observation of certain fixed stars from different locations; observing the Sun; surface nigation; grid distortion on a spherical surface; weather systems; grity; and modern technology. See also[edit] Celestial sphere Celestial spheres Earth ellipsoid Earth radius Earth's rotation § Empirical tests Geodesy Geographical distance Myth of the flat Earth Physical geodesy Shapes of other astronomical bodies Terra Australis Timeline of Earth estimates WGS 84 References[edit] ^ a b Dicks, D.R. (1970). Early Greek Astronomy to Aristotle. Ithaca, N.Y.: Cornell University Press. pp. 72–198. ISBN 978-0-8014-0561-7. ^ Cormack, Lesley B. (2015), "That before Columbus, geographers and other educated people knew the Earth was flat", in Numbers, Ronald L.; Kampourakis, Kostas (eds.), Newton's Apple and Other Myths about Science, Harvard University Press, pp. 16–24, ISBN 9780674915473 ^ a b Continuation into Roman and medieval thought: Reinhard Krüger: "Materialien und Dokumente zur mittelalterlichen Erdkugeltheorie von der Spätantike bis zur Kolumbusfahrt (1492)" ^ a b Jamil, Jamil (2009). "Astronomy". In Fleet, Kate; Krämer, Gudrun; Matringe, Denis; Nawas, John; Rowson, Everett (eds.). Encyclopaedia of Islam. doi:10.1163/1573-3912_ei3_COM_22652. ISBN 978-90-04-17852-6. ^ a b Direct adoption by India: 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 ^ a b Adoption by China via European science: Martzloff, Jean-Claude (1993). "Space and Time in Chinese Texts of Astronomy and of Mathematical Astronomy in the Seventeenth and Eighteenth Centuries". Chinese Science. 11 (11): 66–92. doi:10.1163/26669323-01101005. JSTOR 43290474. Archived from the original on 2021-10-26. Retrieved 2021-10-12. and Cullen, C. (1976). "A Chinese Eratosthenes of the Flat Earth: A Study of a Fragment of Cosmology in Huai Nan tzu 淮 南 子". Bulletin of the School of Oriental and African Studies, University of London. 39 (1): 106–127. doi:10.1017/S0041977X00052137. JSTOR 616189. S2CID 171017315. ^ Pigafetta, Antonio (1906). Magellan's Voyage around the World. Arthur A. Clark. [1] ^ See Figure of the Earth and Earth radius § Global radii for details. Recent measurements from satellites suggest that Earth is actually slightly pear-shaped. Hugh Thurston, Early Astronomy, (New York: Springer-Verlag), p. 119. ISBN 0-387-94107-X. ^ "Why Are Planets Round?". NASA Space Place. June 27, 2019. Retrieved 2019-08-31. ^ E. At. Schwanbeck (1877). 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. p. 101. ^ Cormack, Lesley B. (2015), "That before Columbus, geographers and other educated people knew the Earth was flat", in Numbers, Ronald L.; Kampourakis, Kostas (eds.), Newton's Apple and Other Myths about Science, Harvard University Press, pp. 16–24, ISBN 9780674915473 ^ Pigafetta, Antonio (1906). Magellan's Voyage around the World. Arthur A. Clark. [2] ^ Neugebauer, Otto E. (1975). A History of Ancient Mathematical Astronomy. Birkhäuser. p. 577. ISBN 978-3-540-06995-9. ^ See Figure of the Earth and Earth radius § Global radii for details. Recent measurements from satellites suggest that Earth is actually slightly pear-shaped. Hugh Thurston, Early Astronomy, (New York: Springer-Verlag), p. 119. ISBN 0-387-94107-X. Works cited[edit] Needham, Joseph; Wang, Ling (1995) [1959]. Science and Civilization in China: Mathematics and the Sciences of the Heens and the Earth. Vol. 3 (reprint ed.). Cambridge: Cambridge University Press. ISBN 0-521-05801-5. Further reading[edit] Janice VanClee (2002). Janice VanClee's Science Through the Ages. John Wiley & Sons. ISBN 9780471208303. Menon, CPS (2003). Early Astronomy and Cosmology: A Reconstruction of the Earliest Cosmic System, Etc. Whitegishm MT, US.: Kessinger Publishing. ISBN 9780766131040. Isaac Asimov (1972). How Did We Find Out the Earth is Round?. New York, Walker. ISBN 978-0802761217. Hannam, James (2025). The Globe: How the Earth Became Round. London: Reaktion Books. ISBN 9781836391128. External links[edit] Wikimedia Commons has media related to Spherical Earth. You say the earth is round? 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