Domes are remarkable spaces whether large or small. There is no experience quite like that of walking through a vast domed space, a shape that conjures up hosts of associations in a way that no other roof form does. The flat beamed ceiling or the arched vaults shelter us from the weather, but the dome makes us imagine a vast extension of ourselves as cosmic creatures under the sky, as beings created from the primordial elements looking up at the sun, moon, and stars. Domed spaces put us in mind of our place in the world both physically and spiritually.
Rome did not invent the dome. The history of domed structures reaches far back into prehistory to some of the earliest constructed human dwellings. But no city developed and exploited dome architecture more extensively than Rome throughout its history from its origins to the present day. Many of the forms and engineering of dome construction first appeared in the Eternal City and spread quickly throughout the world.
For many centuries the Pantheon in Rome boasted the largest dome in the world. It remained the largest dome from its completion in 125 CE until the construction of the dome of Florence Cathedral in the mid 15th century. It is as high as it is broad, about 142 feet by 142 feet.
The Emperor Hadrian built the present Pantheon -- the temple to all the gods -- to replace a smaller earlier Pantheon built by Marcus Agrippa, the son-in-law of the Emperor Augustus after the Battle of Actium in 31 BCE. Agrippa likely built a more conventional four square Roman temple with a windowless cella and a portico on a podium. The great fire of 80 CE destroyed that building leaving nothing but the facade. The Emperor Domitian rebuilt that temple only to see it destroyed by fire again in 110 CE.
Hadrian built the current vastly expanded and altered temple. Exactly who was the architect remains controversial. The current consensus says that Apollodorus of Damascus -- the architect to Hadrian's predecessor, the Emperor Trajan -- probably designed the building. Apollodorus was a Nabatean Arab from what is now Jordan. The Pantheon is consistent with a lot of his work for Trajan. Other scholars argue that the Emperor Hadrian himself designed the building. Hadrian was indeed a very capable architect designing and building much of his extensive villa at Tivoli. He also designed the largest temple in the city of Rome, the Temple of Venus and Roma in the Forum next to the Colosseum.
For reasons that remain unknown, the Emperor Hadrian kept the original dedication inscription of the Pantheon. It says in abbreviated Latin in restored bronze letters "Marcus Agrippa three times Consul made this." However, Hadrian stamped all the bricks used in the building with his insignia lest there be any doubt. That inscription stands on top of a row of columns, all single undivided shafts made from Egyptian blue granite from the quarries east of Aswan in the far south of Egypt. Transporting all these columns up the Nile, across the Mediterranean, and through the streets of Rome and set up on the site was itself a marvel of engineering. The bases and capitals of these columns are marble from the quarries of Mt. Pentelikos near Athens, the same quarries that supplied the marble for the Parthenon built almost 6 centuries earlier.
The bronze doors on the Pantheon today are the original doors. They are much restored and modified, but they are the doors Hadrian had installed on the building when it was completed in 125. Walking though the columned portico and through the doors into that immense round room is still one of the most spectacular experiences of any building in the world.
The dome of the Pantheon is an immense hemisphere of structural concrete resting not on a solid wall but upon a concealed series of brick arches and stone piers created to stabilize the dome as each pour of concrete dried and contracted. The Romans invented concrete, a mixture of gravel, pulverized stone, and mortar that could be poured into molds and that would solidify into hard durable shapes. They invented concrete to serve what was at the time was a radical re-conception of building design. Most large ancient buildings were roofs resting on forests of posts or columns, buildings such as Egyptian or Greek temples, or the wooden buildings of posts and brackets holding up heavy ceramic roofs used continuously in China. The Romans imagined buildings as giant shells enclosing great uninterrupted spaces, buildings like the Pantheon.
The oculus in the top of the dome measures about 27 feet in diameter and was always open to the sky. Sunlight, rain and snow pour in from the top today as in ancient times. Small channels in the floor keep the flooding to a minimum. The Pantheon is today a church and has been since the 7th century. Today statues of Christian saints now stand where images of the Roman gods once stood. Now as then, the vast concrete dome overwhelms all of the images in the building. The dome in ancient times had a veneer of white marble with blue ceramic tile mosaic filling the inside of the coffers. Inside each coffer a gilded bronze star or rosette sparkled in the reflected sunlight from the marble floor. A sunburst of gilded bronze surrounded the oculus. The outside of the dome was originally covered in gilded copper and visible throughout the city. Today the dome is no longer quite so prominent. The ground level around the Pantheon is 12 feet higher than in ancient times thanks to centuries of river floods after the ancient drainage system collapsed.
The Pantheon was the temple to all the gods, not only the gods of Rome but of all the gods throughout the Empire. The gods of Egypt, North Africa, Syria, the Greek gods, the Celtic gods, and the Jewish God all had their place in this temple.
But especially the Pantheon was the temple of the seven Roman planetary gods, Apollo the sun, Diana the moon, Mercury, Venus, Mars, Jupiter, and Saturn. The Earth would not be considered a planet until Copernicus and Galileo in the 16th century. We still use those names for the planets and continue to name newly discovered planets (such as Uranus and Neptune) after Classical deities.
The Pantheon formed out of a transition in religion at the time. Educated people of the day such as the Emperor Hadrian no longer believed in the old sacrificial religions, the idea that the gods were invisible powerful beings who controlled all those things we can't control such as the weather and luck, that religion was a straightforward business of keeping those beings happy, trying to divine their will, and seeking to influence them.
Hadrian and others like him saw the gods as aspects of a larger cosmic and moral order that they believed pervaded everything and that they called Providence. The dome that over-awed the images of the gods in the Pantheon as it dominates the saints now visualizes this universal cosmic order.
A section and floor plan of the Pantheon.
The circle dominates the design of the Pantheon throughout, a shape that ancient peoples including the Romans identified with the sky. But the two other basic geometric shapes, the square and the triangle are also present in the floor plan. Link all the flat-backed niches and you get a square. Link all the round backed niches and you get a triangle. The circle represented the heavens in the mind of the Romans, but the square represented earth and the triangle fire, the element that passed between earth and heaven in the Roman cosmos.
The Pantheon carried a lot of political meaning too that was more apparent in the building's original state. The Pantheon was an image of the stated mission of the Roman Empire, to bring peace to the world. The Pantheon shows us the perfect harmonious order of the stars and planets that the Empire seeks to realize upon earth.
How big is the Pantheon really? This is a photo taken in 1925 during a restoration survey of the dome.
The Pantheon is now the oldest surviving dome in Rome, but it was far from the first, and while it was the biggest, it was not the only dome in the ancient city.
Domes remained exceptional features on otherwise very conservative and tradition bound Roman religious architecture. But domes commonly appeared in those vast pleasure palaces that Roman emperors built to appease public opinion in the city that we call baths.
A reconstruction of the Baths of Caracalla.
People did indeed go to these structures to bathe, and bathing facilities formed the center of these complexes. Three chambers formed the center of every Roman bath, the calidarium or hot bath, the tepdidarium or luke-warm bath, and the frigidarium or the cold bath. In most public baths in the Empire these were usually relatively intimate sized rooms not much different from a modern sauna. The ones in the city of Rome were as vast as the main hall of Grand Central Station in New York.
The calidarium of the Baths of Caracalla was a domed steam bath the size of a state capitol rotunda. A network of ceramic pipes in the floor and walls carried hot steam throughout the vast room. Slaves worked long hours stoking the furnaces that kept the boilers running, very hard and brutal labor.
Today only the foundations and two support piers remain of the calidarium of Emperor Caracalla's baths.
The two remaining support piers of the domed calidarium that once dominated the large complex of the Baths of Caracalla.
These ruins are still strikingly large and grand. The three bathing rooms form the center and most prominent part of this structure, but people came here for more than to just get clean. This complex had gyms and athletic fields. It had a library (probably not so much for scholarly literature, but for popular novels, usually romance or pornography just as popular then as now). And it had public gardens and rooms for meeting friends and soliciting sex.
The Villa Gordiani on the outskirts of Rome is a collection of ruins from the 3rd century that may be the remains of the estate of the Gordiani, a short lived imperial dynasty during the time of the Soldier Emperors made up of three emperors in succession all of whom were named Gordian. All of them lasted very briefly and died violently.
In the midst of the ruins is a small domed temple known by locals as The Mausoleum, though it was likely not a burial monument.
The Mausoleum's brick and concrete domed interior.
These first domes of Rome inspired generations of later architects and fired the ambitions of later rulers. The Eastern Roman or Byzantine Empire inherited the dome, placed it on top of four large arches and turned it into a central feature of Eastern Christian churches. Islam would inherit the Byzantine dome and modify it further carrying the legacy of Roman architecture east through Central Asia, to the Indian subcontinent, and eventually to the frontiers of China.
In the city of Rome itself, these ancient domes were only the beginning of a local fascination with this architectural feature that would last down to the 20th century.