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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).
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
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.
On Sizes and Distances (of the Sun and Moon) (Greek: Περὶ μεγεθῶν καὶ ἀποστημάτων [ἡλίου καὶ σελήνης], romanized: Peri megethon kai apostematon) is a text by the ancient Greek astronomer Hipparchus (c. 190 – c. 120 BC) in which approximations are made for the radii of the Sun and the Moon as well as their distances from the Earth.
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.
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.
The Earth's radius is the distance from Earth's center to its surface, about 6,371 km (3,959 mi). While "radius" normally is a characteristic of perfect spheres, the Earth deviates from spherical by only a third of a percent, sufficiently close to treat it as a sphere in many contexts and justifying the term "the radius of the Earth".
Later arc measurements aimed at determining the flattening of the Earth ellipsoid by measuring at different geographic latitudes. The first of these was the French Geodesic Mission , commissioned by the French Academy of Sciences in 1735–1738, involving measurement expeditions to Lapland ( Maupertuis et al.) and Peru ( Pierre Bouguer et al.).