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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 made several important contributions to mathematics and science, and was a friend of Archimedes. Around 255 BC, he invented the armillary sphere. In On the Circular Motions of the Celestial Bodies, [11] Cleomedes credited him with having calculated the Earth's circumference around 240 BC, with high accuracy. [2]
Arc measurement of Eratosthenes. Arc measurement, [1] sometimes called degree measurement [2] (German: Gradmessung), [3] is the astrogeodetic technique of determining the radius of Earth and, by extension, its circumference.
Earth radius (denoted as R 🜨 or R E) is the distance from the center of Earth to a point on or near its surface. Approximating the figure of Earth by an Earth spheroid (an oblate ellipsoid), the radius ranges from a maximum (equatorial radius, denoted a) of nearly 6,378 km (3,963 mi) to a minimum (polar radius, denoted b) of nearly 6,357 km (3,950 mi).
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
The angle between the sunbeam and a gnomon (vertical pole) at Alexandria allowed Eratosthenes to estimate Earth's circumference. Under the assumption that the Sun is very far away, the ancient Greek geographer Eratosthenes performed an experiment using the differences in the observed angle of the Sun from two different locations to calculate ...
Eratosthenes could only measure the circumference of Earth by assuming that the distance to the Sun is so great that the rays of sunlight are practically parallel. [24] Measure of Earth's circumference according to Cleomedes' simplified version, based on the wrong assumption that Syene is on the Tropic of Cancer and on the same meridian as ...
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".