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International Year of Astronomy: 800 students measure the Earth’s radius

On 26 March, 800 students throughout Spain took part in the same experiment: they repeated the experiment that Eratosthenes performed in 240 BC to measure the Earth's radius.

The ancient Greeks had long known that the Earth was round - but not how big it was. Eratosthenes, who was director of the Library of Alexandria, an astronomer, philosopher, poet and mathematician, worked out a way of calculating its radius.

He started by observing that at midday during the summer solstice a stick cast a shadow of differing length in different places in Egypt. In some places the sun rays were perpendicular to the ground, and therefore the stick cast no shadow.

He then came up came up with a mathematical formula which calculates the radius of the Earth by making some simple measurements.

• Calculate the ratio of two measurements taken in different places at the same time.
• The angle of the Sun towards the ground
• The distance between the two observations

This experiment took place in the framework of the International Year of Astronomy 2009 

To learn more about this experiment you can visit the project’s portal:
http://astronomia2009.es

Further information of the International Year of Astronomy 2009:
http://www.astronomy2009.org/