The Pantheon, Temple of All the Gods and Sky of Stone

Few buildings in the world have passed through almost nineteen centuries while remaining in use as the Pantheon has. On the façade one still reads the name of Marcus Agrippa, son-in-law of Augustus, but the temple we admire was rebuilt under the emperor Hadrian in the early decades of the second century. Its dome, opened to the sky by a great circular eye, was read for centuries as an image of the cosmos, and it still arouses the wonder that struck Renaissance travellers.

From Agrippa's dedication to Hadrian's work

The first Pantheon was built by Agrippa in the twenties of the first century BC. Damaged by fires, it was entirely rebuilt between about 118 and 125, during the reign of Hadrian, who chose to keep the original inscription on the front. The bronze letters record that Agrippa built it during his third consulship. This gesture of apparent modesty long confused the dating, which was clarified only through the study of brick stamps.

The historian Cassius Dio, in the third century, wrote that he did not know the origin of the name for certain. He proposed two explanations: the statues of many deities placed inside, or the shape of the vault, resembling the heavens. Even for the ancients, then, the building had a symbolic dimension tied to the totality of the gods and to the image of the celestial vault.

The dome and the open eye

The dome has a diameter of about forty-three metres, equal to the interior height of the building: an ideal sphere of these dimensions could fit inside it perfectly. It is the largest unreinforced concrete dome ever built. The Romans lightened the structure as it rose, using ever lighter materials such as pumice, and carving into the inner surface five rows of coffers that reduce the weight.

At the centre opens the oculus, about nine metres wide, the only source of light. The disc of sunlight moves during the day across the walls and coffers like the hand of a huge clock. Some scholars have suggested that the building was oriented to create light effects on significant dates of the Roman calendar, but these hypotheses are debated. What is certain is that rain falls in freely, drained by small holes in the slightly convex floor.

From pagan temple to church of the martyrs

In 609 the Byzantine emperor Phocas gave the building to Pope Boniface IV, who consecrated it as Santa Maria ad Martyres. Tradition says that many cartloads of martyrs' bones were brought there from the catacombs. This transformation saved the Pantheon from the ruin that befell so many temples. In memory of the descent of the Holy Spirit, at Pentecost red rose petals are dropped through the oculus, a tradition still alive today.

A popular legend says that the oculus was opened by the devil as he fled at the moment of the Christian consecration. In 1625 Pope Urban VIII Barberini had the bronze removed from the portico beams, and a famous pasquinade remarked: «Quod non fecerunt barbari, fecerunt Barberini». Raphael, who died in 1520, and the first kings of Italy lie buried in the Pantheon.

Entering the Pantheon means passing from a noisy square into a suspended space, where light falls from above and marks the passing of time. Pagans and Christians alike read in it, in different languages, the same intuition: that a building can be a scale model of the universe, and that geometry can become a form of contemplation.