III · THE WORK · ART HISTORY
Hagia Sophia
▶ Listen · narrated
A dome thirty metres across, made of brick, resting on nothing you can see from the floor. The windows that make it hover also made it fall.
At a glance
- Completion
- 27 December 537
- Material
- Brick
- Width
- 73 metres (239 ft 6 in)
- Current use
- Mosque
- Previous use
- Church (360–1453), museum (1935–2020)
Look closer
Forty windows at the dome's base
The dome rests on a ring pierced by windows set so closely that, from the floor, the solid masonry between them nearly disappears. The effect is that the dome's edge dissolves into light. This was deliberate: the windows were meant to make the structure appear weightless, as though the hemisphere were suspended from above rather than supported from below. The visual trick worked so well that it may have encouraged a structural miscalculation. The first dome, completed in 537, was shallower than the present one. It collapsed in 558.
Pendentives carrying a circle onto a square
A dome is round. The floor is square. Pendentives are the curved triangular surfaces that resolve this mismatch, allowing a circular base to rise from four arches. Hagia Sophia did not invent the pendentive, but it was among the first large buildings to use them to carry a full dome over a vast open interior. The geometry is visible from below: four concave triangles, each spanning the corner between two arches, curving upward to meet the circular base of the dome. They make the transition look inevitable.
Gold mosaic ground reflecting light downward
Much of the interior surface that is not marble is covered in mosaic set on a gold ground. Gold tesserae are made by sandwiching gold leaf between glass, then setting the pieces at slight angles to the wall. The result is a surface that does not simply reflect light but scatters it, so that the walls seem to emit a soft glow rather than bounce it back sharply. In a building designed around the manipulation of light, this mattered. The gold was not ornament. It was part of the lighting system.
The story
Hagia Sophia is the third church building erected on this site. The first was completed on 15 February 360. The second, completed on 10 October 415, burned in 532 during the Nika riots. The present structure was begun on 23 February 532 and completed on 27 December 537, built for the Byzantine emperor Justinian I and designed by Isidore of Miletus and Anthemius of Tralles, both described as geometers rather than architects in the usual sense.
The building they produced was a basilica plan — a long rectangular nave — but one dominated by a central dome. That dome, approximately thirty metres across, was the engineering problem. Domes had been built before, but not at this scale over an open interior, and not using the particular combination of pendentives and windowed drum that Hagia Sophia employed. The pendentives allowed the dome to rest on four arches spanning a square, rather than on a circular drum rising from a circular wall. This opened the floor.
The windows were both an aesthetic and a structural choice. Forty of them ring the base of the dome, and their effect from below is to make the dome's support disappear into light. Byzantine writers described the dome as hanging from heaven on a golden chain. That was the intention. But the windows also weakened the structure. The first dome, shallower than the present one, collapsed in 558 following an earthquake. It was rebuilt with a steeper profile, which has stood since, though repairs have been frequent and the structure has never been entirely stable.
The building served as the cathedral of Constantinople from 537 until 1453, except for the period from 1204 to 1261, when it was a Catholic cathedral under the Latin Empire established by the Fourth Crusade. After the Ottoman conquest of Constantinople in 1453, it was converted to a mosque. Minarets were added. Christian mosaics were plastered over, though not destroyed. It remained a mosque until 1931, when it was closed, then reopened in 1935 as a museum under the secular Turkish Republic. In 2020, it was reclassified as a mosque, a decision that remains disputed.
The building is seventy-three metres wide. It is made of brick. These are the facts that survive documentation. What is harder to verify is the extent to which the original designers understood the forces they were manipulating. The collapse of the first dome suggests they did not, entirely. The survival of the second suggests they learned.
Why it mattered then
Hagia Sophia was built to assert that the Christian Roman Empire, centred now in Constantinople rather than Rome, remained the inheritor of both imperial and divine authority. Justinian I, who commissioned it, had ambitions to reconquer the western provinces lost to Germanic kingdoms. The building was a statement in brick and gold that Constantinople was the centre of the world, and that its emperor ruled by divine sanction. It was also an architectural experiment. The combination of a basilica plan with a central dome, carried on pendentives and lit by a ring of windows, had not been attempted at this scale. If the building succeeded — and despite the collapse it did succeed — it would demonstrate that Byzantine engineering could surpass anything built in Rome. The scale mattered. For more than five hundred years after its completion, Hagia Sophia remained the largest enclosed space in Christendom. That was not incidental to its purpose. It was the purpose.
Why it matters now
Hagia Sophia has been a church, a mosque, a museum, and a mosque again. Each reclassification has been contested, and the 2020 decision to reconvert it from a museum to a mosque drew international criticism, including from UNESCO and the World Council of Churches. The building is now a site where questions about secular versus religious control of cultural heritage, about the ownership of historical monuments, and about the relationship between national identity and imperial history are all active rather than settled. Architecturally, it remains the model for a particular solution to the problem of covering a large interior space with a dome. Ottoman mosques, built a thousand years after Hagia Sophia, returned to its combination of pendentives, windowed drum, and central dome. The building demonstrated that a dome could float, or appear to, and that appearance has been worth replicating ever since.
The surprising detail
The building has been lent out precisely once. In 1935, when it reopened as a museum, the Republic of Turkey's position was that Hagia Sophia would remain in Istanbul. It has not travelled since, and given its current status as a functioning mosque, it will not. The structure that was designed to be the centre of the Byzantine world has become, by law and by physical immobility, permanently fixed to the site where it was built. The building that was meant to represent universal Christian authority is now the subject of a legal and diplomatic argument about who has the right to determine what it represents.
What is disputed
The precise reasons for the collapse of the first dome in 558 are not documented in detail. Earthquake damage is mentioned in sources, but whether the design was inherently unstable or whether the earthquake alone caused the failure is not certain. The attribution of the design to Isidore of Miletus and Anthemius of Tralles is well attested, but the extent of their individual contributions and whether they worked with a larger team of builders whose names were not recorded is unknown.
Remember this
A dome can be made to hover by placing windows where its weight should be visible. The trick is optical, but the consequences are structural.
Test yourself
The first dome collapsed in 558 and was rebuilt steeper. Why would increasing the steepness of a dome make it more stable, given that it also makes it heavier?
A steeper dome directs more of its weight downward through the pendentives and into the supporting piers, rather than outward against the walls. The outward thrust — the horizontal force trying to push the walls apart — is what causes domes to fail. A shallow dome has a greater horizontal component to its thrust. A steeper dome converts more of that force into vertical compression, which masonry handles well. The trade-off is that a steeper dome is heavier and requires stronger piers, but the original piers at Hagia Sophia were evidently adequate for the increased vertical load. They were not adequate for the horizontal thrust of the shallower dome. The engineers who rebuilt it in 558 understood this, or learned it from the failure.
Go deeper
Image: Arild Vågen, CC BY-SA 3.0. Licence: CC BY-SA 3.0. Source.