Preface

Extract In the late third century bc, Eratosthenes, then chief librarian of the Great Library of Alexandria in Egypt, calculated the circumference of the Earth based upon the difference between the shadow cast by the pointer (gnomon) of a sundial in Alexandria at midday on the Summer Solstice, and compared the angle created by the shadow to the knowledge that at the city of Syene to the south no shadow was cast by a gnomon at the same time on the same day. Accounts of the attempt by Eratosthenes to accurately calculate the circumference of the Earth have taken on a near-legendary status—appearing in academic texts and journals, online videos, popular television programmes, and children’s books.1 However, the accounts of Eratosthenes’ work are, unfortunately, lacking several specific details which would allow for the accuracy of his results to be initially determined. This is particularly the case in trying to convert the results, which were given in an ancient unit of measure (the stade), into a modern equivalent.

This is because there were several different systems of measurement in use across the ancient Greek world that incorporated a unit called a stade, but these units were all of different lengths. This has led to considerable scholarly debate, almost since the time that the calculations were made, over the size of the unit that Eratosthenes was using, and subsequently the accuracy of his results.