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Posidonius calculated the Earth's circumference by reference to the position of the star Canopus.As explained by Cleomedes, Posidonius observed Canopus on but never above the horizon at Rhodes, while at Alexandria he saw it ascend as far as 7 + 1 ⁄ 2 degrees above the horizon (the meridian arc between the latitude of the two locales is actually 5 degrees 14 minutes).
Taking the Earth as spherical, the Earth's circumference would be fifty times the distance between Alexandria and Syene, that is 250,000 stadia. Since 1 Egyptian stadium is equal to 157.5 metres, the result is 39,375 km, which is 1.4% less than the real number, 39,941 km.
Both men's figures for the Earth's circumference were uncannily accurate, aided in each case by mutually compensating errors in measurement. However, the version of Posidonius' calculation popularised by Strabo was revised by correcting the distance between Rhodes and Alexandria to 3,750 stadia, resulting in a circumference of 180,000 stadia ...
He was the first to calculate the Earth's circumference (within 0.5 percent accuracy). [28] His great achievement in the field of cartography was the use of a new technique of charting with meridians , his imaginary north–south lines, and parallels , his imaginary west–east lines. [ 29 ]
The second book contains his calculation of the circumference of the Earth. This is where, according to Pliny, "The world was grasped." Here Eratosthenes described his famous story of the well in Syene, wherein at noon each summer solstice, the Sun's rays shone straight down into the city-center well. [ 22 ]
The size of the Earth was an important question to the Ancient Greeks. Eratosthenes calculated the Earth's circumference with great precision. [14] Since the distance from the Atlantic to India was roughly known, this raised the important question of what was in the vast region east of Asia and to the west of Europe.
Using geometry, the two altitudes along with the distance between the two locations allows for a calculation of Earth's size. Using observations of the star Canopus at Rhodes (in Greece) and Alexandria (in Egypt) and the distance between them, the Ancient Greek philosopher Posidonius used this technique to calculate the circumference of the ...
The Babylonians were the first civilization known to possess a functional theory of the planets. [9] The oldest surviving planetary astronomical text is the Babylonian Venus tablet of Ammisaduqa, a 7th-century BC copy of a list of observations of the motions of the planet Venus that probably dates as early as the second millennium BC.