Circumference: Eratosthenes and the Ancient Quest to Measure the Globe

Circumference, by Nicholas Nicastro, tells a story about Eratosthenes and the times in which he lived.  While it purports to be about his remarkable measurement of the circumference of the Earth, it is mostly about other things and at times it  feels like the author has gotten seriously sidetracked.  Surprisingly, the author shies away from math, which is ridiculous, given that Eratosthenes’s method for estimating the Earth’s circumference only depends on elementary geometry.  The method is summed up, incompletely, in one paragraph and a diagram in a sidebar.  The assumption that the Sun’s rays arrive at the Earth as parallel lines is not mentioned.

There are serious problems with evaluating Eratosthenes’s measurement.  His unit of measurement was the stade, whose length we do not know.  We also do not know how accurate was the measurement of the distance between Alexandria and Syene, the basis of his calculation.

None of Eratosthenes original writings survive and what other writers have said about him is sparse.  This does not make him a very promising topic for a book, which perhaps explains the author’s frequent digressions.