abstract
| - The concept of a spherical Earth dates back to around the 6th century BC, in ancient Indian philosophy, and in ancient Mediterranean thought. It remained a matter of philosophical speculation until the 3rd century BC when Hellenistic astronomy established the spherical shape of the earth as a physical given. A practical demonstration of Earth's sphericity was achieved by Ferdinand Magellan and Juan Sebastián Elcano's expedition's circumnavigation (1519−1521). The concept of a spherical Earth displaced earlier beliefs in a flat Earth: In early Mesopotamian thought, the world was portrayed as a flat disk floating in the ocean, and this forms the premise for early Greek 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 actual figure of the Earth, however, is not spherical. As determined by modern instruments, a sphere approximates the Earth's shape to within one part in 300. An oblate ellipsoid shape with a flattening of 1/300 matches even more precisely. Recent measurements from satellites suggest that the Earth is, in fact, slightly pear-shaped.
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