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Eratosthenes now continued from his knowledge about the Earth. Using his discoveries and knowledge of its size and shape, he began to sketch it. In the Library of Alexandria he had access to various travel books, which contained various items of information and representations of the world that needed to be pieced together in some organized ...
1,700 years after Eratosthenes's death, Christopher Columbus studied what Eratosthenes had written about the size of the Earth. Nevertheless, based on a map by Toscanelli , he chose to believe that the Earth's circumference was 25% smaller.
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 first scientific estimation of the radius of the Earth was given by Eratosthenes about 240 BC, with estimates of the accuracy of Eratosthenes's measurement ranging from −1% to 15%. The Earth is only approximately spherical, so no single value serves as its natural radius.
A parallel later ancient measurement of the size of the Earth was made by another Greek scholar, Posidonius (c. 135 – 51 BC), using a similar method as Eratosthenes. Instead of observing the Sun, he noted that the star Canopus was hidden from view in most parts of Greece but that it just grazed the horizon at Rhodes.
In this work, he calculated the sizes of the Sun and Moon, as well as their distances from the Earth in Earth radii. Shortly afterwards, Eratosthenes calculated the size of the Earth, providing a value for the Earth radii which 252,000 stades, which may be equivalent to 39,690 kilometers, rather close to the true figure of 40,120 kilometers. [16]
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