Ptolemy’s longitudes and Eratosthenes’ measurement of the earth’s circumference

International audienceA statistical analysis of the longitudes reported in Ptolemy's Geographia shows that many of them were obtained by distorting in a linear way data which were known with good accuracy. As a consequence, a new estimate of the value of the stadion used by Eratosthenes is obtained, supporting the thesis that his measurement of the Earth's circumference was remarkably accurate. Some conjectures about possible simplifications introduced by Cleomedes in his account of Eratosthenes' method are also proposed