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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).
Hipparchus (c. 190 – c. 120 BC), a Greek mathematician who measured the radii of the Sun and the Moon as well as their distances from the Earth. Posidonius (c. 135 – c. 51 BC), a Greek astronomer and mathematician who calculated the circumference of the Earth.
Eratosthenes (c. 276 – c. 194/195 BC), a Greek mathematician who calculated the circumference of the Earth and also the distance from the Earth to the Sun. Hipparchus (c. 190 – c. 120 BC), a Greek mathematician who measured the radii of the Sun and the Moon as well as their distances from the Earth. On the Sizes and Distances
192.6 mm (7.58 in) distance from thumb-tip to tip of outstretched index finger [2] orthodōron ὀρθόδωρον: 11 daktyloi 211.9 mm (8.34 in) straight hand's width spithamē σπιθαμή: 12 daktyloi 231.2 mm (9.10 in) span of all fingers pous πούς: 16 daktyloi 308.2 mm (12.13 in) foot: pygmē πυγμή: 18 daktyloi 346.8 mm (13.65 in)
He measured the Earth's circumference by reference to the position of the star Canopus. His measure of 240,000 stadia translates to 24,000 miles (39,000 km), close to the actual circumference of 24,901 miles (40,074 km). [11] He was informed in his approach by Eratosthenes, who a century earlier used the elevation of the Sun at different latitudes.
Traditional Greek units of measurement were standardized and used in modern Greece before and alongside the adoption of the metric system in 1836. Metric units were finally made legally compulsory in 1922.
Eratosthenes measure of Earth circumference: Image title: Illustration of the method Eratosthenes used to calculate the circumference of the Earth by CMG Lee.
This book is the original source for the well-known story of how Eratosthenes measured the Earth's circumference. Many modern mathematicians and astronomers believe the description to be reasonable (and believe Eratosthenes' achievement to be one of the more impressive accomplishments of ancient astronomy).