Ellipses and Ovals in the Physical Space of St Peter’s Square in Rome

We discuss the spatial and perceptive implications of the geometry at urban scale of St. Peter’s square in Rome. The case study of Bernini's project for the colonnade makes explicit the relation between the geometry of the plan and the spatial development. We will show that the geometry of the circle affects intentionally the perception of the walking visitor. There is an ambiguity of use between the terms ellipse and oval, especially in touristic literature and some school textbooks. The specific mathematical properties, of one curve respect to the other, were well known at the time and were obviously intentionally exploited in the design of the square along with the colonnade. We also report of a “geometric flash mob”, we organized in occasion of the 2015 European Researcher’s Night.