Built in AD 360 at the beginning of the Middle Ages, it was famous in particular for its massive dome. It was the world's largest building and an engineering marvel of its time.
It is considered the epitome of Byzantine architecture and is said to have changed the history of architecture.
The Hagia Sophia construction consists of mostly masonry. The structure is composed of brick and mortar joints that are 1.5 times the width of the bricks.
The mortar joints are composed of a combination of sand and minute ceramic pieces displaced very evenly throughout the mortar joints. This combination of sand and ceramic pieces could be considered to be the equivalent of modern concrete at the time.
From the date of its construction's completion in 537 until 1453, it served as an Eastern Orthodox cathedral and the seat of the ecumenical patriarch of Constantinople, except between 1204 and 1261, when it was converted by the Fourth Crusaders to a Roman Catholic cathedral under the Latin Empire.
The building was later converted into an Ottoman mosque from 29 May 1453 until 1931. It was then secularized and opened as a museum on 1 February 1935. It remained the world's largest cathedral for nearly a thousand years, until Seville Cathedral was completed in 1520.
The current building was originally constructed as a church between 532 and 537 on the orders of the Byzantine emperor Justinian I and was the third Church of the Holy Wisdom to occupy the site, the prior one having been destroyed by rioters in the Nika riots.
It was designed by the Greek geometers Isidore of Miletus and Anthemius of Tralles. The church was dedicated to the Wisdom of God, the Logos, the second person of the Trinity, its patronal feast taking place on 25 December, the commemoration of the birth of the incarnation of the Logos in Christ.
Although sometimes referred to as Sancta Sophia as though it were named after Sophia the Martyr, sophia being the phonetic spelling in Latin of the Greek word for wisdom, or Shrine of the Holy Wisdom of God.
The church contained a large collection of relics and featured, among other things, a 15-metre (49 ft) silver iconostasis.
The focal point of the Eastern Orthodox Church for nearly one thousand years, the building witnessed the excommunication of Patriarch Michael I Cerularius officially communicated by Humbert of Silva Candida, the papal envoy of Pope Leo IX in 1054, an act that is commonly considered the start of the East–West Schism.
In 1453, Constantinople was conquered by the Ottoman Empire under Mehmed the Conqueror, who ordered this main church of Eastern Orthodox Christianity converted into a mosque.
Although some parts of the city of Constantinople had fallen into disrepair, the cathedral had been maintained with funds set aside for this purpose, and the Christian cathedral made a strong impression on the new Ottoman rulers who conceived its conversion.
The bells, altar, iconostasis, and other relics were destroyed and the mosaics depicting Jesus, his Mother Mary, Christian saints, and angels were also destroyed or plastered over.
Islamic features such as the mihrab a niche in the wall indicating the direction toward Mecca, for prayer, minbar or pulpit, and four minarets were added. It remained a mosque until 1931 when it was closed to the public for four years.
It was re-opened in 1935 as a museum by the Republic of Turkey. Hagia Sophia was, as of 2014, the second-most visited museum in Turkey, attracting almost 3.3 million visitors annually.
According to data released by the Turkish Culture and Tourism Ministry, Hagia Sophia was Turkey's most visited tourist attraction in 2015.
From its initial conversion until the construction of the nearby Sultan Ahmed Mosque, aka the Blue Mosque of Istanbul, in 1616, it was the principal mosque of Istanbul.
The Byzantine architecture of the Hagia Sophia served as inspiration for many other Ottoman mosques, including the Blue Mosque, the Şehzade Mosque, the Süleymaniye Mosque, the Rustem Pasha Mosque and the Kılıç Ali Pasha Complex.
In 1935, the first Turkish President and founder of the Republic of Turkey, Mustafa Kemal Ataturk, transformed the building into a museum. The carpets were removed and marble floor decorations such as the Omphalion appeared for the first time in centuries, while the white plaster covering many of the mosaics was removed.
Nevertheless, the condition of the structure deteriorated, and the World Monuments Fund placed Hagia Sophia on 1996 World Monuments Watch, and again in 1998. The building's copper roof had cracked, causing water to leak down over the fragile frescoes and mosaics.
Moisture entered from below as well. Rising ground water had raised the level of humidity within the monument, creating an unstable environment for stone and paint. The WMF secured a series of grants from 1997 to 2002 for the restoration of the dome.
The first stage of work involved the structural stabilization and repair of the cracked roof, which was undertaken with the participation of the Turkish Ministry of Culture.
The second phase, the preservation of the dome's interior, afforded the opportunity to employ and train young Turkish conservators in the care of mosaics.
By 2006, the WMF project was complete, though many other areas of Hagia Sophia continue to require significant stability improvement, restoration and conservation.Hagia Sophia is currently the second most visited museum in Turkey, attracting almost 3.3 million visitors annually.
Although use of the complex as a place of worship - mosque or church was strictly prohibited, in 2006 the Turkish government allowed the allocation of a small room in the museum complex to be used as a prayer room for Christian and Muslim museum staff.
Since 2013 from the minarets of the museum the muezzin sings the call to prayer twice per day, in the afternoon.
In 2007, Greek American politician Chris Spirou launched an international organization - Free Agia Sophia Council championing the cause of restoring the building to its original function as a Christian church.
Since the early 2010s, several campaigns and government high officials, notably Turkey's deputy prime minister Bulent Arınç in November 2013, have been demanding that Hagia Sophia be converted into a mosque again.
In 2015, in retaliation for the acknowledgment by Pope Francis of the Armenian Genocide, the Mufti of Ankara, Mefail Hızlı, stated that he believes the conversion of Hagia Sophia into a mosque will be accelerated.
On 13 May 2017 a large group of people, organized by the Anatolia Youth Association (AGD), gathered in front of Hagia Sophia and prayed the morning prayer with a call for the reconversion of the museum into a mosque.
On 21 June 2017 Presidency of Religious Affairs or Diyanet organized a special program, which included the recitation of the Quran and prayers in Hagia Sofia, to mark the Laylat al-Qadr, the program was broadcast live by state-run television TRT.
On 31 March 2018 Turkish president Recep Tayyip Erdoğan recited the first verse of the Quran in the Hagia Sophia, dedicating the prayer to the souls of all who left us this work as inheritance, especially Istanbul's conqueror.
Strengthening the political movement to make the Hagia Sophia a mosque once again, which would reverse Atatürk's measure of turning the Hagia Sophia into a secular museum.
In March 2019 Erdogan said that he will change the status of Hagia Sophia from a museum to a mosque, adding that it was a very big mistake to turn it into a museum.
As a UNESCO World Heritage site, this change would require approval from UNESCO's World Heritage Committee.
On 1 July 2016 Muslim prayers were held again in the Hagia Sophia for the first time in 85 years.
Many religious structures have been modeled on the Hagia Sophia's core structure of a large central dome resting on pendentives and buttressed by two semi-domes. A notable Venetian example is the High Altar of Santa Maria della Salute.
Several mosques commissioned by the Ottoman dynasty closely mimic the geometry of the Hagia Sophia, including the Suleymaniye Mosque and the Bayezid II Mosque. In many cases, Ottoman architects preferred to surround the central dome with four semi-domes rather than two.
This is true in the Sultan Ahmed Mosque, the New Mosque - Istanbul, and the Fatih Mosque. Like the original plan of the Hagia Sophia, many of these mosques are also entered through a colonnaded courtyard. However, the courtyard of the Hagia Sophia no longer exists.
Many Neo-Byzantine churches are modeled on the Hagia Sophia. The Kronstadt Naval Cathedral and Poti Cathedral closely replicate the internal geometry of the Hagia Sophia. The interior of the Kronstadt Naval Cathedral is a nearly 1-to-1 copy of the Hagia Sophia.
The marble revetment also closely mimics the source work. Like Ottoman mosques, many churches based on the Hagia Sophia include four semi-domes rather than two, such as the Church of Saint Sava in Belgrade.
Several churches combine the layout of the Hagia Sophia with a Latin cross plan. For instance, the Cathedral Basilica of Saint Louis (St. Louis), where the transept is formed by two semi-domes surrounding the main dome.
This church also closely emulates the column capitals and mosaic styles of the Hagia Sophia. Other similar examples include the Alexander Nevsky Cathedral, Sofia, Saint Clement Catholic Church, Chicago, and Basilica of the National Shrine of the Immaculate Conception.
The Catedral Metropolitana Ortodoxa in Sao Paulo and the Eglise du Saint-Esprit (Paris) closely follow the interior layout of the Hagia Sophia.
Both include four semi-domes, but the two lateral semi-domes are very shallow. In terms of size, the Eglise du Saint-Esprit is about two thirds the scale of the Hagia Sophia.
Synagogues based on the Hagia Sophia include the Congregation Emanu-El (San Francisco), Great Synagogue of Florence, and Hurva Synagogue.
