Posidonio a Gadeira: l’osservazione diretta e la testimonianza dei marinai

Ancient geographers followed two methods to construct their maps: they could apply geometrical or astronomical knowledge and make use of the testimony of travellers and sailors. Posidonius of Apamea exploited both methods, while Strabo tended to mistrust travellers’ and sailors’ accounts when these conflicted with data derived from geometrical-astronomical speculation. In this contribution, I have turned my attention to some of Strabo’s accounts of Posidonius’ research in the city of Gadeira/Gades, a privileged place of observation for astronomers and geographers.