On the Design of St. Peter’s Square in Rome

<title>Abstract</title> <p>St. Peter’s Square in Rome, designed between 1656 and 1667 by Gian Lorenzo Bernini, is bordered on either side by a semicircular colonnade. The geometry of the square has been the subject of numerous studies aimed at determining whether its plan is elliptical or oval. It has been shown that Bernini employed the famous ovato tondo (“rounded oval”), proposed approximately half a century earlier by the Italian architect Sebastiano Serlio. This construction approximates an ellipse by means of an oval composed of four circular arcs. An aerial view of the square reveals three ovals traced on the pavement. The first consists of eight angular sectors, the second defines the inner boundary of the colonnade, and the third its outer boundary. While the centers and radii of the four small and four large circles forming the second oval are well established, the corresponding parameters of the other two ovals remain uncertain. In this paper, we investigate the hypothesis that Bernini used only two small circles, which we call “generating circles”, and simply modified their radii to construct the other two ovals. The excellent agreement between the geometry resulting from this hypothesis and the actual dimensions of St. Peter’s Square strongly suggests that this was indeed the method employed by Bernini in its design.</p>