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Eratosthenes' observation on 20 June 2013

by Charles-Henri Eyraud last modified Feb 06, 2014 07:32 PM
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On 20 june 2013 : Solstice celebration with Eratosthenes

Historical presentation

This event aims at a simple measurement which allows to determine the circumference of the Earth: Eratosthenes's measurement.
Eratosthenes is a Greek scientist who lived in the third century BC in Egypt. He observed that on the summer solstice at solar noon :

  • the sun was at its zenith in the city of Syene (Southern Egypt, now Aswan), so there was no shadow.
  • but in the North, in the city of Alexandria, the same day at the same moment, there was the shadow of an obelisk, which means the sun was inclined

Eratosthenes then measured in Alexandria the angle between the sun and the vertical. Then, knowing the distance between the two cities and assuming that, on Earth, the sun rays are parallel, he could set the earth's circumference.

The measurement

2200 years later, we are going to reproduce the observations of Eratosthenes with our partners. The event in Lyon is mainly dedicated to pupils of primary and secondary schools. The students have to come with their teacher and the device they prepared for the measurement.
In the ENS de Lyon, the measurement will take place at 13h43 HLF


The 20 of June's videoconference

  • Classes who want to participate in this videoconference have to
    1) Register on the Lamap website "Following in the foosteps of Eratosthene"
    2) Send an email to charles-henri.eyraud(at), replacing (at) by @, with some details:
    School : Name, Direction, Country, Latitude, Longitude. Age and number of students involved

Other measurements will have been done the previous days (17,18, and 19 June), and could be used for the videoconference


Other observations in the garden of the ENS de Lyon

  • Observation of sunspots
  • Observation of sun's corona

Participants will also be given the opportunity to make drawings of the sun and its spots as did Galileo. A drawing competition will be organised for scientific and artistic appreciation.

Help for calculations


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Arts and Stars
  • Responsable du site : Charles-Henri Eyraud
  • E-mail : charles-henri.eyraud [arobase]
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