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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).
With a stade of 185 m (607 ft), 804,000,000 stadia is 149,000,000 km (93,000,000 mi), approximately the distance from the Earth to the Sun. Eratosthenes also calculated the Sun's diameter. According to Macrobius , Eratosthenes made the diameter of the Sun to be about 27 times that of the Earth. [ 17 ]
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.
Units of surface measurement Unit Greek name Equal to Modern equivalent Description pous πούς: 0.095 m 2 (1.02 sq ft) square foot hexapodēs ἑξαπόδης: 36 podes 3.42 m 2 (36.8 sq ft) square six-foot akaina ἄκαινα: 100 podes 9.50 m 2 (102.3 sq ft) rod hēmiektos ἡμίεκτος: 833 + 1 ⁄ 3 podes 79.2 m 2 (853 sq ft) half ...
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
Posidonius's method for calculating the circumference of the Earth, relied on the altitude of the star Canopus Posidonius was informed in his approach to finding the Earth's circumference by Eratosthenes , who a century earlier arrived at a figure of 252,000 stadia; both men's figures for the Earth's circumference were uncannily accurate.
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).