Hagia Sophia

Hagia Sophia, the Church of the Holy Wisdom, was completed in 537 CE under the Byzantine emperor Justinian I as the cathedral of Constantinople, replacing two earlier churches on the same site that had each been destroyed by riot and fire. Designed by the mathematician Anthemius of Tralles and the architect Isidore of Miletus, its enormous dome, carried on four massive pendentives over a nearly square nave, created the largest enclosed interior space in the world at the time, a rank the building held for close to a thousand years and an engineering achievement still studied today.

Sultan Mehmed II converted the building into a mosque within days of the Ottoman conquest of Constantinople in 1453, adding minarets, a mihrab oriented toward Mecca, and eventually the great calligraphic roundels bearing the names of Allah, Muhammad and the early caliphs that still hang inside. The Turkish Republic's founding government secularized the building in 1934 and opened it as a museum the following year, a status that held for eighty five years and made Hagia Sophia one of the most visited cultural sites in the world, its Byzantine mosaics and Ottoman Islamic elements displayed together as a single layered monument to both traditions.

In July 2020, Turkey's Council of State annulled the 1934 secularization and President Erdogan reclassified the building as an active mosque, a decision that took effect within weeks and returned regular Muslim prayer to the space for the first time since 1935, covering rather than removing the building's most prominent Christian mosaics during prayer times.

Rowland Mainstone's architectural study of Hagia Sophia traces how its builders solved the problem of covering a vast square space with a round dome using pendentives, curved triangular sections that translate the dome's circular base onto the four supporting arches beneath it, a solution so influential it became the standard method for large domed construction across the later Byzantine and Ottoman worlds. Ottoman architects, including the great Mimar Sinan, studied the building directly and built Istanbul's imperial mosques in deliberate conversation with it, so much of the city's skyline exists in visual dialogue with Justinian's original.

The interior still carries both traditions' devotional art in the same space: ninth and tenth century mosaics depicting Christ Pantocrator, the Virgin and Child, and Byzantine emperors were plastered over after the 1453 conversion in keeping with Islamic prohibitions on figural imagery in a mosque, then partially uncovered during the museum era, and remain visible today alongside the Ottoman calligraphic medallions and mihrab, covered during prayer rather than removed since the 2020 reconversion.

The 2020 decision to restore active mosque status drew a sharply divided response: welcomed within Turkey as a restoration of Ottoman and Muslim heritage, and criticized by UNESCO, the government of Greece and some Christian communities on grounds ranging from World Heritage protections to interfaith relations. This atlas records the building's full layered history, Byzantine cathedral, Ottoman mosque, secular museum, and mosque again, and its current active status, without adjudicating the 2020 decision's merits.