Uncovering a masterpiece of Roman engineering: The project of Via Appia between Colle Pardo and Terracina

Via Appia, built around 312 BC, is an engineering masterpiece, its most striking feature being the segment between Colle Pardo and Terracina, which goes “straight” for about 61 km. We investigate this segment by GPS techniques: results lead to uncover that the original project of the road was based on a complex interplay between geometry and astronomy. The project was indeed carried out with the help of an orthogonal centuriation grid, with all probabilities starting from a node located at the south easternmost point of the grid itself. The road however does not run along the grid's diagonal: it was orientated astronomically to the setting of the star Castor at the time of construction. Since the Gemini twins were patrons of the Roman army, the project turns out to be a work entertained for both practical and symbolic reasons, during a key moment of the Roman history.