Pantheon

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The Pantheon is a domed Roman building on the Campus Martius in Rome, raised as a temple “to all the gods” and standing today as a Christian church. Its name is Greek — pan, all; theos, god — and it has remained roofed, walled, and in use for nearly nineteen centuries, which makes it the best-preserved major monument of the ancient city.

The name itself is older and looser than the building beneath it. Cassius Dio, writing in Greek a century after the rebuilding, offered two guesses at where it came from: either the rotunda once held the images of many gods — he names Mars and Venus among them — or its vaulted roof simply resembled the heavens. A historian who has to guess at the meaning of a name was working with a word already worn into a nickname; “Pantheon” may never have been the building’s formal dedication so much as the thing Romans called it. Both of Dio’s readings have governed how the place has been understood ever since — as a house of the assembled gods, and as an image of the sky.

The structure now standing is not the original. Marcus Agrippa, son-in-law and right hand of the first emperor, built a Pantheon on the site under Augustus, late in the first century BCE; he was consul for the third time in 27 BCE, the year the inscription across the portico still records in bronze letters: M·AGRIPPA·L·F·COS·TERTIVM·FECIT — Marcus Agrippa, son of Lucius, made this in his third consulship. The building behind that inscription is not his. Agrippa’s Pantheon burned in the great fire of 80 CE, was patched under Domitian, and burned again under Trajan around 110. After that the temple was rebuilt entire, and the visible Pantheon belongs to the early second century — about 113 to 125 CE on the usual reckoning, in the years of Trajan’s late reign and Hadrian’s early one.

What scholarship establishes, and what corrected a centuries-old error, comes from the bricks. In the 1890s the French architect Georges Chédanne read the brickstamps of the rotunda and found them too late for Agrippa by a century and a half; the building that bore his name was Hadrianic. A modern study by Lise Hetland of the stamped clay shows them clustering in the years 110 to 117, which pushes the planning back toward Trajan and suggests the Roman Empire’s most famous dome was conceived before Hadrian came to the purple and finished under him. Hadrian, who as a rule restored old buildings without writing his own name across them, kept Agrippa’s inscription on the front — a gesture that has misled observers about the building’s true age for the better part of two thousand years. The bronze of those letters is itself a later restoration; the original glittering lettering is long gone, its setting-holes still legible in the stone.

The dome is the reason the building matters to the history of architecture. Cast in unreinforced concrete — opus caementicium, lime mortar packed with aggregate and laid without steel — its coffered interior describes a perfect hemisphere whose diameter equals its height from the floor. That figure is 43.3 meters either way, so that a sphere of exactly that size would rest inside the rotunda touching the floor at a single point: the building is a half-sphere of air closed over by a half-sphere of stone. The Roman builders graded the concrete to its task, packing heavy travertine into the foundations, lighter tufa and broken brick up the walls, and at the crown a concrete of volcanic pumice so light it seems unwilling to fall. Five rings of sunken coffers — twenty-eight to a ring — cut still more weight from the shell. At the very top a single round opening, the oculus, nearly nine meters across, stands open to the sky with no glass and no cover. It remained the largest dome on earth for more than a thousand years, until Brunelleschi roofed the cathedral of Florence in 1436, and it is still the largest dome of unreinforced concrete ever built. Light enters only through that hole and through the bronze doors, so the disc of sun tracks across the coffers and down the wall through the day like a slow hand, and rain that falls through the oculus drains away through holes worked into the slightly convex floor.

What gods the temple served, and how, is less certain than the engineering. The dedication “to all the gods” is suggestive rather than documentary, and the ancient sources say almost nothing about the cult conducted inside. The reading of the dome as a deliberate image of the cosmos — the vault as the dome of heaven, the oculus as the sun, the seven coffer-rings sometimes tied to the seven planets of astrology — rests on Dio’s single line about the roof resembling the sky and on the inner logic of the form rather than on any surviving account of how the place was used. It is inference, attractive and old, but not record. The same caution holds for the sweep of deities it was said to gather. Where the building names a god at all, it names the Olympian order Rome had received from Greece — Jupiter, the Roman Zeus; Apollo; Mars and Venus, the divine ancestors the Julian house claimed — within the frame of ancient Roman religion as it stood at the height of the empire. Whether the eastern gods then spreading through Rome, Isis and Serapis among them, were ever counted into “all the gods” of this particular house is not something the stones or the texts settle. The idea of a single temple that contains the whole divine order — a building that is itself a kind of pantheism in masonry — belongs more to how later ages have read the Pantheon than to anything the Romans wrote down about it.

The building’s later life is firmer ground. In 609 the Byzantine emperor Phocas gave it to the papacy, and on the thirteenth of May that year it was consecrated as the church of Santa Maria ad Martyres — Saint Mary and the Martyrs — its pagan statues cleared out and, by the old account, the bones of martyrs brought in from the catacombs to refound it as Christian ground. The conversion almost certainly spared it the quarrying and decay that erased most pagan temples of the Roman Republic and the empire; a working church is not a quarry. What the centuries did take, the Church took. In 1625 Pope Urban VIII, of the Barberini family, ordered the bronze stripped from the portico’s roof and melted down for cannon at Castel Sant’Angelo, and Rome answered with a line that has stuck to his name ever since — that what the barbarians had not done, the Barberini did. The fabric itself held. The Pantheon has functioned as a church without interruption since the seventh century, the longest continuous use of any building in the city, and it holds inside it the tomb of the painter Raphael — Raffaello Sanzio of Urbino, buried there in 1520 at his own wish — and the tombs of the first two kings of a united Italy, Victor Emmanuel II and Umberto I, whose honor guard still keeps watch over them.

Reading the building

The Pantheon’s modern history is, in large part, the history of being studied. The brickstamp evidence that overturned the Agrippan attribution is laid out in Lise M. Hetland’s Dating the Pantheon, in the Journal of Roman Archaeology 20 (2007), which argues from the stamped clay that construction began under Trajan and not, as the long consensus held, wholly under Hadrian. The standard book remains William L. MacDonald’s The Pantheon: Design, Meaning, and Progeny (Harvard University Press, 1976), which traces the building’s afterlife as a model — the line of domed rotundas with temple porches that runs from the second century to the rotundas of the modern state. The fullest recent treatment is the volume edited by Tod A. Marder and Mark Wilson Jones, The Pantheon: From Antiquity to the Present (Cambridge University Press, 2015), which gathers the engineering, the dating, and the church history into one account; Wilson Jones’s earlier Principles of Roman Architecture (Yale, 2000) sets out the geometry of the sphere-in-the- rotunda in detail. Behind all of them stands the one ancient witness who tried to explain the place at all — Cassius Dio, Roman History 53.27, whose two guesses at the name, the gods and the sky, still frame every reading of the dome. The Pantheon is a monument of one overwhelming idea, cast at a stroke and never improved upon. Its hold on the imagination of Neoplatonism and of every later architecture that wanted to put the heavens indoors comes from that single gesture.

The Pantheon thus carries two religions in one fabric: a pagan temple whose purpose is half-lost, kept whole by the faith that replaced it. The dome built, perhaps, to gather all the gods has been kept in continuous use as a church; the oculus has never been closed.

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