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Image: Posidonius earth circumference

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Description: Following works by en:Eratosthenes, en:Posidonius measured the Earth's circumference by reference to the position of the star en:Canopus C{\displaystyle C}, which touches horizon at Rhodes R{\displaystyle R}, but at Alexandria A{\displaystyle A} ascends as far as φ=712∘{\displaystyle \varphi =7{\tfrac {1}{2}}^{\circ }} above the horizon. Using his assessment of lateral arc length AR{\displaystyle AR} he could calculate the Earth's circumference.
Title: Posidonius earth circumference
Credit: Own work, incorporating parts of File:Canopus.jpg (p.d.), File:Eratosthenes.svg (own work).
Author: Erzbischof
Usage Terms: Creative Commons Attribution-Share Alike 3.0
License: CC BY-SA 3.0
License Link: https://creativecommons.org/licenses/by-sa/3.0
Attribution Required?: Yes

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