Istanbul is a transcontinental city in Europe and Asia, located between the Black Sea and the Sea of Marmara in northwestern Turkey, 64.7% of whose residents live on the European and 35.3% on the Asian side. Within city limits, Istanbul has an area of between 1166 and 1830 km2, it is the administrative center of the Istanbul Metropolitan Municipality (coterminous with Istanbul Province), which covers an area of 5343 km2 , both areas hosting a population of around 14 million residents. Istanbul is one of the world's most populous cities and ranks as world's 5th-largest city proper, dropping the rank to the 24th place as an urban area and to the 18th place as a metro area due to a minor difference between the population within city limits and the agglomeration.
Hagia Sophia is a former Christian patriarchal basilica (church), later an imperial mosque, and now a museum (Ayasofya Müzesi) in Istanbul, Turkey. From the date of its construction in 537 until 1453, it served as an Eastern Orthodox cathedral and seat of the Patriarchate of Constantinople, except between 1204 and 1261, when it was converted to a Roman Catholic cathedral under the Latin Empire. The building was a mosque from 29 May 1453 until 1931. It was then secularized and opened as a museum on 1 February 1935.
Famous in particular for its massive dome, it is considered the epitome of Byzantine architecture and is said to have "changed the history of architecture". 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.
The church contained a large collection of holy 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 on the part of Pope Leo IX in 1054, an act which is commonly considered the start of the Great Schism.
In 1453, Constantinople was conquered by the Ottoman Turks under Sultan Mehmed II, who ordered this main church of the Orthodox Christianity converted into a mosque. By that point, the church had fallen into a state of disrepair. Nevertheless, the Christian cathedral made a strong impression on the new Ottoman rulers and they decided to convert it into a mosque. The bells, altar, iconostasis, and sacrificial vessels and other relics were removed and the mosaics depicting Jesus, his Mother Mary, Christian saints and angels were also removed or plastered over. Islamic features—such as the mihrab, minbar, 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 is currently (2014) the second-most visited museum in Turkey, attracting almost 3.3 million visitors annually.
From its initial conversion until the construction of the nearby larger Sultan Ahmed Mosque (Blue Mosque of Istanbul) in 1616, it was the principal mosque of Istanbul. The Hagia Sophia served as inspiration for many other Ottoman mosques, such as the Blue Mosque, the Şehzade Mosque, the Süleymaniye Mosque, the Rüstem Pasha Mosque and the Kılıç Ali Paşa Mosque.