Nicholas Nicastro 2015. Circumference. Eratosthenes and the ancient quest to measure the globe. New York: St Martin’s Press, 223 pp.

Apparently not much is known of Eratosthenes. His original works are lost (due to the destruction of the Library of Alexandria, where he worked and became chief librarian). What is know of him is through third parties who copied or summarised parts of his writings. That has not stopped Nicolas Nicastro putting together this short book, more than somewhat frustrating in that only towards the end of the book does the author begin to address Eratosthenes and his celebrated and surprisingly accurate estimate of the circumference of the Earth. Abandoning any attempt at suspense, the principal behind Eratosthenes’ method is explained on pages 27-28 but the methods of measurement, errors entailed, and other practical aspects are not discussed until almost the final pages. In between there is plenty of setting the scene, providing some context of Greek and Egyption history but although this is occasionally of interest it always feels like filler material which is there to turn an essay into a book. Having waded through most of it, I have to conclude that the wikipedia page on Eratosthenes is a more concise, informative and far less frustrating treatment of the same material.

It seems that Eratosthenes was big on ratios and proportions as a way of investigating and explaining phenomena. Thus his method: When the sun was directly overhead at Syrene, Eratosthenes determined that it was about 7° away from vertical a known distance away at Alexandria. The ratio of 7°/360° must then equal the ratio “distance to Alexandria”/”circumference of the Earth”. Depending on what assumptions are made about the modern equivalent of Eratosthenes’s unit of measurement (the stadion), the answer he got might have been somewhere between 1% and 10% of the accepted value (~40,000 km). Since he was a Platonist, he probably also rounded his resulting numbers. Anyway, the accuracy of measurement he was able to achieve is beside the point, the insight of his method Eratosthenes is what is important.

I could find no evidence in this book that the subtitle “ … the ancient quest to measure the globe” is anything but a grandiose misnomer. Surely there was no such quest. More likely the commonly-held quests were to get a decent feed and not get butchered by a visiting army.

Eratosthenes was also a significant mathematician and developed among other things an algorithm (“the Sieve of Eratosthenes”) for finding prime numbers, although this is not mentioned in the book (consult the above wikipedia page instead). Eratosthenes was a contemporary and friend of Archimedes and certainly one of the great minds of his day. If his works had not been destroyed no doubt he would be recognised as such, and for much more than just the circumference of the Earth achievement. Eratosthenes deserves a better biography than this, one with less of a focus on geodesy.

The bibliography is reasonably extensive.