The Bosphorus or Bosporus, also known as the Istanbul Strait, is a strait that forms part of the boundary between Europe and Asia. The Bosphorus, the Sea of Marmara, and the Dardanelles strait to the southwest together form the Turkish Straits. The world’s narrowest strait used for international navigation, the Bosphorus connects the Black Sea with the Sea of Marmara, which is connected by the Dardanelles to the Aegean Sea, and thereby to the Mediterranean Sea.
The Bosphorus’ limits are defined as the connecting line between the lighthouses Rumeli Feneri and Anadolu Feneri in the north and between the Ahırkapı Feneri and the Kadıköy İnciburnu Feneri in the south. Between the limits, the strait is 31 km (17 nmi) long, with a width of 3,329 m (1.798 nmi) at the northern entrance and 2,826 m (1.526 nmi) at the southern entrance. Its maximum width is 3,420 m (1.85 nmi) between Umuryeri and Büyükdere Limanı, and minimum width 700 m (0.38 nmi) between Kandilli Point and Aşiyan. A 45-degree course alteration is required for the ships at this point. The current can reach 7–8 knots at this point. It is a highly dangerous point for ship navigation. At Yenikoy, the necessary course alteration is 80 degrees. All the dangers and obstacles characteristic of narrow waterways are present and acute in this critical sea lane. At the above mentioned turns (Kandilli and Yenikoy) where significant course alterations have to be made, the rear and forward sights are totally blocked prior to and during the course alteration. The ships approaching from the opposite direction cannot be seen round these bends. There is also very heavy ferry traffic in the Strait of Istanbul, which crosses between European and Asiatic sides of the city.
The shores of the strait are heavily populated as the city of Istanbul (with a metropolitan area in excess of 12 million inhabitants) straddles it.
The name comes from Greek Bosporos (Βόσπορος), which the ancient Greeks analysed as bous βοῦς ‘ox’ + poros πόρος ‘means of passing a river, ford, ferry’, thus meaning ‘ox-ford’, which is a reference to Io (mythology) from Greek mythology who was transformed into a cow and condemned to wander the earth until she crossed the Bosphorus where she met Prometheus. Although it has been known for a while that the Black Sea and the Sea of Marmara flow into each other in an example of a density flow, findings of a study by the University of Leeds in August 2010 reveal that there is in fact an underwater channel of high density water flowing across the floor of the Bosphorus (caused by the difference in density of the two seas), which would be the sixth largest river on Earth if it were to be on land.
