Spherical 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 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.[3][4][5][6] A practical demonstration of Earth's sphericity was achieved by Ferdinand Magellan and Juan Sebastián Elcano's circumnavigation (1519–1522).[7]
The concept of a spherical Earth displaced earlier beliefs in a
).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.[9]
Cause
Earth is massive enough that the pull of
Formation
The Solar System formed from a dust cloud that was at least partially the remnant of one or more supernovas 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 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
Later shape changes and effects
Though the surface rocks of Earth have cooled enough to solidify, the
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
History of concept and measurement
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, c. 300 BC, has been interpreted as stating that the contemporary Brahmans of India believed in a spherical Earth as the center of the universe.[11] 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.
Measurement and representation
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
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 have measured the geoid to unprecedented accuracy, revealing mass concentrations beneath Earth's surface.
Evidence
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 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 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.See also
- Shapes of other astronomical bodies
- Celestial sphere
- Celestial spheres
- Earth radius
- Earth's rotation#Empirical tests
- Geographical distance
- Myth of the flat Earth
- Physical geodesy
- Terra Australis
- WGS 84
References
- ISBN 978-0-8014-0561-7.
- ISBN 9780674915473
- ^ Continuation into Roman and medieval thought: Reinhard Krüger: "Materialien und Dokumente zur mittelalterlichen Erdkugeltheorie von der Spätantike bis zur Kolumbusfahrt (1492)"
- ISBN 978-90-04-17852-6.
- ISBN 0-415-96930-1, p. 463
- S2CID 171017315.
- ^ Pigafetta, Antonio (1906). Magellan's Voyage around the World. Arthur A. Clark. [1]
- ISBN 978-3-540-06995-9.
- 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.
Works cited
- Needham, Joseph; Wang, Ling (1995) [1959]. Science and Civilization in China: Mathematics and the Sciences of the Heavens and the Earth. Vol. 3 (reprint ed.). Cambridge: Cambridge University Press. ISBN 0-521-05801-5.
Further reading
- Janice VanCleave (2002). Janice VanCleave'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.
External links
- You say the earth is round? Prove it (from The Straight Dope)
- NASA – Most Changes in Earth's Shape Are Due to Changes in Climate
- The Round Earth and Christopher Columbus, educational web site
- Top 10 Ways to Know the Earth is Not Flat, science education site