The attempts by the Greeks to measure the circumference of the earth have generated much discussion. Two protagonists stand out among a cast of lesser actors: Eratosthenes, who produced a figure of 252,000 stades, and Posidonius, who is said to have produced one of 180,000 stades. Although both figures were variously accepted by later writers, it was the lower one that was adopted by Ptolemy. Backed by his immense authority, it came to rule the roost; and because it underestimated the size of the earth it was ultimately responsible for seducing Columbus into thinking that the Indies were readily accessible across the Atlantic. Eratosthenes' method is well known. What has not been clear, however, is how Posidonius, if it really was he, obtained his result. The answer can now in part be given.

The broad outline of the story, shorn of many of the much-debated details, must first be spelled out. The earliest figure for the circumference is given by Aristotle in the 340s BC in his De Caelo, quoting unnamed mathematicians, as 400,000 stades. A generation later this was refined, perhaps by his pupil Dicaearchus of Messana in Sicily, to 300,000 stades.