Spherical Earth orEarth's curvature refers to theapproximation of thefigure of the Earth as asphere. The earliest documented mention of the concept dates from around the 5th century BC, when it appears in the writings ofGreek philosophers.[1][2] In the 3rd century BC,Hellenistic astronomy established theroughly spherical shape of Earth as a physical fact and calculated theEarth's circumference. This knowledge was gradually adopted throughout theOld World duringLate Antiquity and theMiddle Ages, displacing earlier beliefs in aflat Earth.[3][4][5][6] A practical demonstration of Earth'ssphericity was achieved byFerdinand Magellan andJuan Sebastián Elcano'scircumnavigation (1519–1522).[7]
The realization that thefigure of the Earth is more accurately described as anellipsoid dates to the 17th century, as described byIsaac Newton inPrincipia. 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 theUS DoDWorld Geodetic System since the 1960s is close to 1/298.25.[8] The scientific study of the shape of the Earth is known asgeodesy.
Earth is massive enough that the pull ofgravity maintains its roughly spherical shape. Most of its deviation from spherical stems from thecentrifugal force caused byrotation around its north-south axis. This force deforms the sphere into anoblate ellipsoid.[9]
TheSolar System formed from a dust cloud that was at least partially the remnant of one or moresupernovas that produced heavy elements bynucleosynthesis. Grains of matter accreted through electrostatic interaction. As they grew in mass, gravity took over in gathering yet more mass, releasing thepotential energy of their collisions and in-falling asheat. Theprotoplanetary disk also had a greater proportion of radioactive elements than Earth today because, over time, those elementsdecayed. Their decay heated the early Earth even further, and continue to contribute toEarth'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 anellipsoid, 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 asplanetary differentiation. This event is known as theiron catastrophe; the most abundant heavier elements wereiron andnickel, which now form theEarth's core.
Though the surface rocks of Earth have cooled enough to solidify, theouter core of the planet is still hot enough to remain liquid. Energy is still being released;volcanic andtectonic activity has pushed rocks into hills and mountains and blown them out ofcalderas.Meteors also causeimpact craters and surrounding ridges. However, if the energy release from these processes halts, then they tend toerode away over time and return toward the lowest potential-energy curve of the ellipsoid.Weather powered bysolar 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 causestides in theoceans' water, which can flow freely along the changing potential.
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 ethnographerMegasthenes,c. 300 BC, has been interpreted as stating that the contemporaryBrahmins 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.Circumnavigation of the world in theAge 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 ofGreek philosophers.[1][11] In the 3rd century BC,Hellenistic astronomy established theroughly spherical shape of Earth as a physical fact and calculated theEarth's circumference. This knowledge was gradually adopted throughout theOld World duringLate Antiquity and theMiddle Ages.[3][4][5][6] A practical demonstration of Earth'ssphericity was achieved byFerdinand Magellan andJuan Sebastián Elcano'scircumnavigation (1519–1522).[12]
The concept of a spherical Earth displaced earlier beliefs in aflat Earth: In earlyMesopotamian mythology, the world was portrayed as a disk floating in the ocean with a hemispherical sky-dome above,[13] and this forms the premise forearly world maps like those ofAnaximander andHecataeus of Miletus. Other speculations on the shape of Earth include a seven-layeredziggurat orcosmic mountain, alluded to in theAvesta and ancientPersian writings (seeseven climes).
The realization that thefigure of the Earth is more accurately described as anellipsoid dates to the 17th century, as described byIsaac Newton inPrincipia. 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 theUS DoDWorld Geodetic System since the 1960s is close to 1/298.25.[14]
Geodesy, also called geodetics, is the scientific discipline that deals with the measurement and representation of Earth, itsgravitational field and geodynamic phenomena (polar motion, Earthtides, 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'smagnetic field. Especially in theGerman speaking world, geodesy is divided intogeomensuration ("Erdmessung" or "höhere Geodäsie"), which is concerned with measuring Earth on a global scale, andsurveying ("Ingenieurgeodäsie"), which is concerned with measuring parts of the surface.
Earth's shape can be thought of in at least two ways:
As the science ofgeodesy measured Earth more accurately, the shape of the geoid was first found not to be a perfect sphere but to approximate anoblate spheroid, a specific type ofellipsoid. More recent[when?] measurements have measured the geoid to unprecedented accuracy, revealingmass concentrations beneath Earth's surface.
The roughly spherical shape of Earth can beempirically evidenced by many different types ofobservation, ranging from ground level, flight, or orbit. The spherical shape causes a number of effects and phenomena that when combined disproveflat 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 navigation; grid distortion on a spherical surface; weather systems; gravity; and modern technology.
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