In November 324CE the Roman Emperor Constantine settled on Byzantium, the location of a Greek colony on the banks of the Bosporus, as the capital of the Eastern Roman Empire. Power was shifting away from Rome, and Constantine was seeking to bolster his defences against the Persians and from barbarian incursions along the Danube by establishing a stronghold in a strategic Eastern location.1 In May 330, both pagan and Christian rituals were performed to dedicate the city and it was renamed to Constantinople (modern-day Istanbul). On a hill in the city (modern-day Sarayburnu Hill) he found a Greek temple dedicated to the goddess Athena. Constantine, whose mother (Helena) was a Christian Greek woman, is best known as the first Christian Roman emperor, and during his reign had introduced several measures in support of Christianity — to stop the persecution of Christians and to develop a theological understanding of Christianity. He had also embarked on building several significant churches, including Old Saint Peter’s Basilica in Rome and The Church of the Holy Sepulchre in Jerusalem. In Byzantium Constantine built a church with a wooden roof on the site of the Greek temple, removing 427 mostly pagan statues from the site. These origins, however, are uncertain, as there is no archaeological evidence, and this account only comes to us through the 6th century chronicler Hesychius of Miletus.Constantine died in 337 and his son Emperor Constantius II later built a larger church on the site calling it Magna Ecclesia or the Great Church. The church was consecrated on 15 February 360. Two centuries later, in 532, during Emperor Justinian I’s reign, one of the empire’s most violent uprisings, the Nika revolt, which began as a factional rivalry between supporters of chariot racing teams before spilling over to a rebellion against the palace, broke out. Justinian brutally suppressed the revolt but not before many of the city’s buildings, including the Great Church, had been razed to the ground. Justinian would have seen the prompt reconstruction of the church as a factor in maintaining order and unity.2 The growing influence of Christianity and the desire to reinforce the idea that imperial power derived directly from God provided the impetus for a grand construction project. Justinian commissioned the architects Isidoros of Miletus and Anthemius of Tralles the same year to transform the wooden-roofed structure into a monumental church. For the construction, columns were quarried and shipped from sites along the Mediterranean; and marble, stone, and glass were salvaged and dispatched from ruins and monuments cross the empire, including Anatolia, Syria, and Egypt.3 To respond to the Emperor’s demand for a superior and unprecedented structure, the architects decided upon a magnificent dome, 100 feet in diameter. Procopius, Justinian’s historian, says of the dome that it ‘does not appear to rest upon a solid foundation, but to cover the place beneath as though it were suspended from heaven by the fabled golden chain’.4 To surmount the architectural challenge posed by a dome of this size, Isidoros and Anthemius developed one of the earliest known examples of a pendentive. A pendentive is a supplementary triangular segment that curves upward into the central dome to transfer lateral pressure into the piers below.5 This allows the dome to seamlessly connect to the building’s base, creating visual harmony and engendering an open and airy atmosphere in the interior space.6 The mosaics on its surface reflect light from the surrounding windows adding to the aesthetic appeal. Acoustically, the concave shape of the pendentives serves as a natural reflector, directing sound towards the centre of the structure, improving resonance. Conversely, the use of marble elsewhere induces a delay in sound, causing a natural reverberation effect, enhancing song and prayer.The church was completed in 537 and became known as Hagia Sophia (pronounced ay-ah soh-fee-ah), Greek for Holy Wisdom (the wisdom as embodied in Christ), and would remain the world’s largest cathedral for the next 1000 years.In 1453, following the capture of Constantinople by the Ottoman Empire under Sultan Mehmed II, the church was transformed into a mosque. Hagia Sophia was highly valued by the Ottoman Turks as a symbol of their conquest, Constantinople being a significant centre of power for one of the most formidable empires in history. The conversion of Hagia Sophia into a mosque represented the Ottoman devotion to Islam, Islamic scholarship and culture.7 The Ottomans removed Christian paintings and iconography and incorporated Islamic architectural features, including four minarets, a mihrab (a Mecca-facing niche in the wall), and a minbar (pulpit) to accommodate the ceremonies of prayer. However, the dome remained the most singular feature of the structure.It is remarkable that an edifice that had represented the Christian faith for nearly a millennium could transition to serve an entirely different faith with such little alteration. In this respect, the dome of Hagia Sophia played a crucial role in the transition. Circularity is a universal constant across sacred architecture, and can be seen throughout history in such structures as The Great Stupa of Sanchi (3rd century BCE Buddhist monument in India), The Pantheon (125CE, Rome, Italy), and The Dome of the Rock (691CE, Jerusalem).8 The universality of the circle as a symbol of completeness and unity makes it particularly fitting for environments that foster a connection with the divine.9 The essence of circularity is captured by a dome, which with its graceful curvature is reminiscent of the vastness of the heavens enclosing the earth below. A central dome can also establish a point from which worshippers are equidistant, promoting equality and unity.10 The success of Hagia Sophia’s pendentive dome also saw it emulated in other grand buildings, including the nearby Blue Mosque (1616).The Ottoman Empire was eventually replaced by the Turkish Republic in 1922, with the remainder splitting into various independent states in North Africa and West Asia. On 24 November 1934, the Council of Ministers of the Republic of Turkey issued a decree to convert Hagia Sophia into a museum; it would hold relics from the Byzantine and Ottoman eras.11 This was part of a wider movement towards secularisation by president Mustafa Kemal Atatürk, the founder of modern Turkey. Eighty-six years later, on 10 July 2020, an hour after the Turkish courts had annulled the 1934 decree, Turkey’s current president Recep Tayyip Erdogan declared Hagia Sophia open to Islamic worship.12 The first Friday prayers were held on 24 July 2020, during which the Christian iconography was veiled with white cloth. At other times the iconography would be unveiled and the building would remain open to tourists — and its feline residents. One such resident was Gli, a cat that lived on the grounds of Hagia Sophia for 16 years and became an integral part of the Hagia Sophia’s reputation.13 Tourists who visited the museum grew fond of Gli so that she eventually garnered an online presence through social media, becoming known as the Hagia Sophia Cat.