Archive - Eratosthenes, the Solstice and the Size of the Earth

June 19, 2016


Each Sunday we present a notable item from our archives. This EPOD was originally published June 21, 2001.

Provided by: Martin Ruzek, USRA; MODIS Science Team
Summary authors & editors: Martin Ruzek

It was near the summer solstice of 240 BC that Eratosthenes, curator of the famed Library of Alexandria and renowned mathematician and geographer, performed his famous experiment in Egypt to calculate the diameter of the Earth. The bottom of a deep well in the city of Syene, Egypt (near the present day Aswan Dam and very near the Tropic of Cancer) was known to be illuminated by the sun directly at mid-day on the longest day of the year (the solstice). But on the same day, a vertical pole in Alexandria, some 800 km to the north, cast a distinct shadow. By measuring the shadow and applying the geometry of a sphere, Eratosthenes calculated the Earth's diameter with remarkable accuracy. Sadly, the concept of a spherical Earth was lost from common thought for over a thousand years until Christopher Columbus and others proved the fact by sailing west to go east. The background reference image of Egypt and the Nile River is provided by the NASA MODIS instrument.

[Sep 5, 2006 - Donald Etz notes: "From reading Jeffrey Burton Russell's book, Inventing the Flat Earth (1991), I was persuaded that most educated Europeans of Columbus' time believed the earth is round. The main debate seems to have been over its dimensions. Columbus ventured on his voyage because he believed the earth was much smaller than it is." -ed]

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