Posidonius of Apameia (c. 135–51 BC), was a Greek Stoic philosopher who traveled throughout the Roman world and beyond and was a celebrated polymath throughout the Greco-Roman world, like Aristotle and Eratosthenes.
His work “about the ocean and the adjacent areas” was a general geographical discussion, showing how all the forces had an effect on each other and applied also to human life. He measured the Earth’s circumference by reference to the position of the star Canopus. His measure of 240,000 stadia translates to 24,000 miles, close to the actual circumference of 24,901 miles. He was informed in his approach by Eratosthenes, who a century earlier used the elevation of the Sun at different latitudes. Both men’s figures for the Earth’s circumference were uncannily accurate, aided in each case by mutually compensating errors in measurement. However, the version of Posidonius’ calculation popularised by Strabo was revised by correcting the distance between Rhodes and Alexandria to 3,750 stadia, resulting in a circumference of 180,000 stadia, or 18,000 miles. Ptolemy discussed and favored this revised figure of Posidonius over Eratosthenes in his Geographia, and during the Middle Ages scholars divided into two camps regarding the circumference of the Earth, one side identifying with Eratosthenes’ calculation and the other with Posidonius’ 180,000 stadion measure.
Digital Maps of the Ancient World 