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The concept of a ''' and Hecataeus . Other speculations as to the shape of Earth include a seven-layered Ziggurat or Cosmic Mountain , alluded to in the Avesta and ancient Persian writings (see Seven Climes ). In fact, Earth is an Oblate Spheriod . EARLY DEVELOPMENT Pythagoras Pythagoras (b. 570 BC ) found harmony in the universe and sought to explain it. He reasoned that Earth and the other planets must be spheres, since the most harmonious Geometric form was a Circle . Plato Plato ( 427 BC - 347 BC ) travelled to southern Italy to study Pythagorean Mathematics . When he returned to Athens and established his school, Plato also taught his students that Earth was a sphere. If man could soar high above the clouds, Earth would resemble ''"a ball made of twelve pieces of leather, variegated, a patchwork of colours."'' Aristotle Aristotle ( 384 BC - 322 BC ) was Plato's prize student and ''"the mind of the school."'' Aristotle observed ''"there are Stars seen in Egypt and {Link without Title} Cyperus which are not seen in the northerly regions."'' Since this could only happen on a curved surface, he too believed Earth was a sphere ''"of no great size, for otherwise the effect of so slight a change of place would not be quickly apparent."'' Aristotle provided physical evidence for a spherical Earth:
The concepts of symmetry, equilibrium and cyclic repetition permeated Aristotle's work. In , as well as two cold inhospitable regions, ''"one near our upper or northern pole and the other near the ... southern pole,"'' both impenetrable and girdled with ice. Although no humans could survive in the frigid zones, inhabitants in the southern temperate regions could exist. He called these theoretical people Antipodes , literally "''feet opposite''". Eratosthenes Eratosthenes ( 276 BC - 194 BC ) estimated Earth 's circumference around 240 BC . He had heard about a place in Egypt where the Sun was directly overhead at the summer solstice and used geometry to come up with a circumference of 250,000 stades. This estimate astonishes some modern writers, as it is within 2% of the modern value of the equatorial circumference, 40,075 kilometres. The length of a 'stade' is contentious - this value uses the most generous estimate for this length. Claudius Ptolemy Claudius Ptolemy ( 90 - 168 AD ) lived in Alexandria , the centre of scholarship in the Second Century . Around 150, he produced his eight-volumne Geographia . The first part of the ''Geographia'' is a discussion of the data and of the methods he used. Like with the model of the solar system in the ''Almagest'', Ptolemy put all this information into a grand scheme. He assigned Coordinate s to all the places and geographic features he knew, in a Grid that spanned the globe. Latitude was measured from the Equator , as it is today, but Ptolemy preferred to express it as the length of the longest day rather than Degrees Of Arc (the length of the Midsummer day increases from 12h to 24h as you go from the equator to the Polar Circle ). He put the Meridian of 0 Longitude at the most western land he knew, the Canary Islands . Geographia indicated the countries of " Serica " and "Sinae" ( China ) at the extreme right, beyond the island of "Taprobane" ( Sri Lanka , oversized) and the "Aurea Chersonesus" ( Southeast Asian Peninsula ).]] Ptolemy also devised and provided instructions on how to create maps both of the whole inhabited world (''oikoumenè'') and of the Roman provinces. In the second part of the ''Geographia'' he provided the necessary Topographic lists, and captions for the maps. His ''oikoumenè'' spanned 180 degrees of longitude from the Canary islands in the Atlantic Ocean to China , and about 80 degrees of latitude from the Arctic to the East Indies and deep into Africa ; Ptolemy was well aware that he knew about only a quarter of the globe. GEODESY Geodesy aka Geodetics is the scientific discipline that deals with the measurement and representation of the 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 speaking world, geodesy is divided in Geomensuration ("Erdmessung" or "höhere Geodäsie"), which is concerned with measuring the 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 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 measurements have measured the geoid to unprecedented accuracy, revealing Mass Concentration s beneath Earth's surface. SPHERICAL MODELS mission.]] There are a few different types of spherical models in common usage, the most popular being the circumferential extremes:
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