Istanbul, historically known as Byzantium and Constantinople is the largest city in Turkey and 5th largest city proper in the world with a population of 13 million, also making it the largest metropolitan city proper in Europe and the second largest metropolitan area in Europe by population. Istanbul is also a megacity, as well as the cultural, economic, and financial centre of Turkey. The city covers 39 districts of the Istanbul province. It is located on the Bosphorus Strait and encompasses the natural harbour known as the Golden Horn, in the northwest of the country. It extends both on the European Thrace and on the Asian Anatolia sides of the Bosphorus, and is thereby the only metropolis in the world that is situated on two continents. Istanbul is a designated alpha world city.