Inside the Hagia Sophia, one cannot help but gaze upwards in awe at the immense dome—a brick hemisphere, 31 metres in diameter, that rises approximately 55 metres above the floor. This architectural marvel rests on four massive arches, each supported by four pendentives. These pendentives, triangular sections of a spherical surface, transition the weight from the circular base of the dome down to the square plan of the four enormous piers at the building's centre. Encircling the base of the dome is a ring of forty windows, piercing the structure just below the curve. This ingenious design creates the illusion of the dome floating above a band of light. Procopius, writing around 555 CE, described the dome as appearing 'suspended by a golden chain from heaven'. Such is the brilliance of its visual effect that the dome appears to hover, unattached to the building it crowns. For 1,489 years, the Hagia Sophia has held this ethereal form, a testament to both faith and physics.

Justinian's commission

The Hagia Sophia we know today is the third church to stand on its site. The original church, commissioned by Constantius II around 360 CE, fell victim to flames during a riot in 404. A second church, completed by Theodosius II in 415, met a similar fate during the Nika Riots of 532—a devastating rebellion against Emperor Justinian, which he narrowly survived. Determined to replace the destroyed church, Justinian envisioned a structure of unprecedented scale and grandeur in Christian architecture. He appointed two remarkable architects: Anthemius of Tralles, renowned for his work in mathematics and geometry, and Isidore of Miletus, a professor of stereometry and physics at the University of Constantinople. Remarkably, neither architect had extensive experience in construction. Yet Justinian's choice was deliberate; he sought a building so structurally ambitious that it required the sophisticated mathematical expertise they offered. Construction began promptly in 532, and on 27 December 537, the church was consecrated. Procopius records Justinian entering the completed church, exclaiming, 'Solomon, I have surpassed you.'

What the dome rests on

The structural challenge of supporting a hemispherical dome over a square base was not entirely new; the Romans had previously tackled similar problems. The Pantheon in Rome, completed under Hadrian in 126 CE, features a dome resting on a circular drum, a comparatively simpler task. The Hagia Sophia, however, attempts a far more complex arrangement: a hemisphere over a square. The Roman solution often involved the use of 'squinches'—small arches bridging the corners of a square, creating an octagonal base for the dome. This method, however, limited the potential diameter of the dome. Anthemius and Isidore introduced a more innovative solution that would become a hallmark of Byzantine architecture: the pendentive. A pendentive is a triangular segment of a sphere, and four of these meeting at the corners of a square plan form a circular base for the dome. Pendentives effectively transfer the dome's outward thrust onto the robust piers at each corner. In the Hagia Sophia, these pendentives span over 20 metres on each side, forming a prominent feature of the interior's grand design. Decorated with gold-mosaic angels in the 9th century, one of these original mosaics was uncovered in 2009 after being concealed by plaster.

The first collapse

Originally, the dome of the Hagia Sophia was lower and shallower than the one we see today. Just two decades after its consecration, the dome partially collapsed following an earthquake in 558 CE, likely the result of the significant seismic activity that occurred during the Constantinople earthquake of December 557, estimated at a magnitude of 6.5-7.0. The collapse affected the eastern arch, leading to the failure of the eastern half of the dome. To address this catastrophe, Isidore the Younger, the nephew of the original architect Isidore of Miletus, was brought in to oversee repairs. His solution was to rebuild the dome with a higher and steeper curvature, raising it from about 15 metres to approximately 20 metres. This change redistributed the structural loads more effectively, directing more force onto the vertical piers and thereby reducing lateral thrust on the arches. The reconstruction, completed by 562, has endured through centuries, remaining largely intact with subsequent repairs.

Subsequent repairs and standing earthquakes

Situated in a seismically active region, the Hagia Sophia has withstood numerous significant earthquakes over the centuries, notably in 869, 989, 1346, 1509, 1894, and 1999. These events predominantly damaged the dome rim, pendentives, and the western arch. After the 989 earthquake, the Armenian architect Trdat, known for his work on the cathedral at Ani, was engaged to rebuild parts of the western dome. The 1346 collapse of the eastern half-dome was repaired through financial contributions from the Orthodox world. During the 'Little Judgement Day' earthquake of 1509, estimated at a magnitude of 7.2, the building sustained extensive damage but the dome remarkably held. In the aftermath of the Ottoman conquest in 1453, the authorities initiated a significant restoration under the guidance of the architect Sinan in the 1570s. Sinan added the four substantial exterior buttresses and the minarets we see today. Through meticulous maintenance, the Hagia Sophia has retained much of its original 6th-century core. The piers and lower walls remain intact from Justinian's era, while the dome features elements from both Isidore the Younger's reconstruction in 562 and Trdat's in 989. The building's decorative schemes have shifted to reflect its changing roles—from Byzantine Christian to Ottoman Islamic, to museum, and back to mosque, most recently in 2020.

Why the construction is genuinely unusual

Three features distinguish the Hagia Sophia's construction as genuinely unusual. Firstly, the use of pendentives at such a monumental scale was unprecedented according to surviving records. Though smaller examples might have existed in earlier Byzantine constructions, none survive to attest to the technique's prior use. Hagia Sophia remains the earliest extant example of pendentive architecture on such a grand scale. Secondly, the mortar employed by Anthemius and Isidore—a lime-based mix with volcanic-pumice aggregate—possesses unique characteristics. This mortar sets slowly and maintains a slight elasticity for decades, while its compressive strength continues to increase over centuries, a property confirmed by modern materials testing (Mainstone, 1988). This allowed the builders to construct without concern for thermal contraction during the cooling process and provided the flexibility needed to absorb seismic shocks. Lastly, the windows encircling the dome's base are not only aesthetic features but also critical to reducing the structure's weight at a crucial juncture, enhancing both the visual and structural integrity of the dome.

What it became

For 916 years, the Hagia Sophia served as the principal church of the Byzantine Empire, until Sultan Mehmed II rode in on horseback and ordered its conversion into a mosque following the Ottoman conquest of Constantinople on 29 May 1453. The mosaics, though plastered over, were not destroyed, preserving Byzantine artistry beneath. The Ottomans added four minarets over the subsequent century, reinforcing its new religious function. The building remained a mosque until 1934, when Mustafa Kemal Atatürk's secular Turkish Republic transformed it into a museum, initiating restorations that eventually uncovered its stunning mosaics. However, in July 2020, the Turkish government reconverted it into a mosque, with the mosaics now covered during prayers. Throughout its long history, the Hagia Sophia has functioned as a church, mosque, museum, and mosque once more. Despite these transformations, the structure's core—its dome, pendentives, and piers—remains a constant, enduring through time. Its faith and its physics are interwoven, each upholding the other.

Anthemius of Tralles and Isidore of Miletus conceived a building nearly 1,500 years ago that has outlasted the Byzantine Empire, the religious practices it was built for, the language of its inscriptions, and the construction techniques that shaped it. Much of the original 6th-century brickwork remains intact. The piers that support the arches, which in turn support the pendentives and the dome, consist of stones meticulously cut by anonymous craftsmen. Inhabited continuously since 537—by worshippers, tourists, scholars, and archaeologists—the Hagia Sophia remains a living monument. The mathematical principles that Anthemius devised to achieve the pendentive geometry endure unchanged. The volcanic-pumice mortar continues to gain strength. And when sunlight filters through the dome’s windows, casting a glow upon the golden mosaics, the dome seems to defy gravity, perpetually floating above. Here, faith and physics are inextricably linked, and it is this very intertwining that the building was designed to illustrate.