Spherical Earth
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The idea of a spherical Earth was espoused by Pythagoras apparently on aesthetic grounds, as he also held all other celestial bodies to be spherical.
This idea probably replaced widespread belief 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. Other speculations as to the shape of the Earth include a seven-layered ziggurat or cosmic mountain, alluded to in the Avesta and ancient Persian writings (see seven climes).
In the 4th century BC, Aristotle provided physical evidence for a spherical Earth:
- Ships receding over the horizon disappear hull-first.
- Travelers going south see southern constellations rise higher above the horizon.
- The shadow of Earth on the Moon during a lunar eclipse is round.
Earth's circumference was estimated around 240 BC by Eratosthenes, who 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 40,076 kilometers.
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. More recent measurements have measured the geoid to unprecedented accuracy, revealing mascons beneath Earth's surface.
In spite of these discoveries, the model of Earth as a sphere (to a first approximation) remains useful for many purposes. Higher-order features of Earth's geoid's shape are often represented as spherical harmonics.
See also
External links
- You say the earth is round? Prove it (http://www.straightdope.com/classics/a2_087.html) (from The Straight Dope)