Its shape is circular, for which reason it was once attributed to Vesta, the fire protector. The Sybil of Tibur is said to have pulled Augustus down a peg or two by announcing the imminent birth of Jesus. She even predicted that on that day, an oil well would gush forth in Rome. This apparently took place at the Trastevere, where Holy Mary's Basilica stands: it has been proved and you will see the exact place! Augustus doesn't seem to have been much affected by this prediction.