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In his well-known measurement of the Earth, and according to Cleomedes, Eratosthenes estimated in 5000 stades the distance between Aswan and Alexandria. Modern accounts state that he calculated the distance from the time taken by caravans (improbable, as the Nile was navigable all the way, downstream from Aswan, so caravans weren't needed) or that he hired walking surveyors for that task. For example, in https://www.aps.org/publications/apsnews/200606/history.cfm

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I've been unable to confirm this in old, Greek or Latin accounts.

Any comments?

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    $\begingroup$ The main sources are Cleomedes and Strabo, and neither of them is reliable, so basically we do not know what Eratosthenes actually did. And not just for the distance, see How did Eratosthenes determine that Alexandria and Syene were on the same meridian? $\endgroup$ – Conifold Mar 28 '20 at 10:41
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    $\begingroup$ There is no evidence that he specially hired someone to measure this distance. He probably relied on travelers accounts. $\endgroup$ – Alexandre Eremenko Mar 28 '20 at 12:04
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    $\begingroup$ Martianus Capella, VI, 598 hs-augsburg.de/~harsch/Chronologia/Lspost05/Martianus/… $\endgroup$ – sand1 Mar 28 '20 at 20:23
  • $\begingroup$ @sand1 It's an interesting text, but I can find no mention to the distance between Alexandria and Aswan/Syene... However, the fact that there were royal surveyors that had worked out the distance between Meroe and Syene is a valuable indirect clue... $\endgroup$ – xxavier Mar 29 '20 at 16:46
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Based on the discussion of Eratosthenes' method in Daniel Špelda's Astronomie v Antice (Astronomy in Antiquity, ISBN 80-7225-210-0), the distance of 5000 stadia was estimated by Eratosthenes based on the time traders took to travel between the cities. A round value was used, similar to other values Eratosthenes used in his computation, suggesting that even he himself was aware that the numbers were just estimations.

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