This article is about the strait in Turkey. For the strait in Russia, see Eastern Bosphorus. For the surrounding neighbourhoods of Istanbul, see Boğaziçi. For the university in Turkey, see Boğaziçi University. For the ancient Hellenic state, see Bosporan Kingdom.

Satellite image of the Bosphorus strait, taken from the ISS in April 2004.

Aerial view of the Bosphorus strait from north (bottom) to south (top), with the city center of Istanbul at the southern end.

The Bosphorus or Bosporus ( Ancient Greek: Βόσπορος, Bósporos; Turkish: Boğaziçi) 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 Bosporus 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 limits of the Bosphorus 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.

The depth of the Bosphorus varies from 13 to 110 m (43 to 361 ft) in midstream with an average of 65 m (213 ft). The deepest location is between Kandilli and Bebek with 110 m (360 ft). The most shallow locations are off Kadıköy İnciburnu on the northward route with 18 m (59 ft) and off Aşiyan Point on the southward route with 13 m (43 ft). The Golden Horn is an estuary off the main straits that acted as a moat to protect Old Istanbul from attack, as well as providing a sheltered anchorage for the imperial navy until the 19th century.

Most of the shores of the strait are heavily populated, straddled as it is by the city of Istanbul (with a metropolitan area population in excess of 12 million inhabitants) which extends inland from both coasts.

It has been known since before the 20th century that the Black Sea and the Sea of Marmara flow into each other in an example of a density flow and in August 2010 a continuous ‘underwater channel’ of suspensioncomposition was discovered to flow along the floor of the Bosphorus which would be the sixth largest river on Earth if it were to be on land. The study of the water and wind erosion of the straits relates to that of its formation. Sections of the shore have been reinforced with concrete or rubble and sections of the strait prone to deposition are periodically dredged.


The Bosphorus is also known as “Strait of Constantinople”, or as “Istanbul Strait” (Turkish: İstanbul Boğazı). To distinguish it from the Cimmerian Bosporus, it was anciently known as the Thracian Bosphorus (Herodotus 4.83; Bosporus Thracius, Bosporus Thraciae , Βόσπορος Θρᾴκιος, also Chalcedonian Bosporus, Bosporus Chalcedoniae, Bosporos tes Khalkedonies, Herodotus 4.87, or Mysian Bosporus, Bosporus Mysius). The term could also be used as common noun βόσπορος, meaning “a strait”, and was also applied to the Hellespont in Classical Greek (Aeschylus, Sophocles).

The Greek name Βόσπορος (Bosporos) was folk-etymologized as from βοὸς πόρος, i.e. “cattle strait” (or “Ox-ford”), from the genitive of bous βοῦς “ox, cattle” + poros πόρος “passage”, thus “cattle-passage”, or “cow passage” in reference to Io from Greek mythology who was transformed into a cow and condemned to wander the earth until she crossed the Bosphorus where she met Prometheus. This folk etymology was canonized by Aeschylus in Prometheus Bound (v. 734f.), where Prometheus prophesies to Io that the strait would be named after her. The site where Io supposedly went ashore was near Chrysopolis, was named he Bous “the Cow”. The same site was also known as Damalis, as it was where the Athenian general Chares had erected a monument to his wife Damalis. This monument included a colossal statue of a cow (the name Damalis translating to “calf”). The actual etymology of the name is more likely from the verb βύζω or βύω, “to fill up, clog up, plug, stop”, referring to a “plugged” or stopped-up passage, perhaps also cognate with the name of Byzantium (Hesychius has βύζαντες: πλήθοντες, i.e. buzantes meaning “filled up”).

The spelling with -ph-, as Bosphorus, has no justification in the ancient Greek name, but it occurs as a variant in medieval Latin (as Bosphorus, and occasionally Bosforus, Bosferus), and in medieval Greek sometimes as Βόσφορος, giving rise to the French formBosphore, Spanish Bósforo, and Russian Босфор. The 12th-century Greek scholar John Tzetzes calls it Damaliten Bosporon (after Damalis), but he also reports that in popular usage the strait was known as Prosphorion during his day, the name of the most ancient northern harbour of Constantinople.


Main article: Black Sea deluge theory

A map depicting the location of the Bosphorus (red) relative to theDardanelles (yellow) and the Sea of Marmara, which together form theTurkish Straits.

The exact cause for the formation of the Bosphorus remains the subject of debate among geologists. Thousands of years ago, the Black Sea became disconnected from the Aegean Sea. The Black Sea deluge theory(reinforced in credibility by a study of the same name of 1997 by two scientists from Columbia University) contends that the Bosphorus was formed in circa 5600 BC when the rising waters of the Mediterranean Sea and the Sea of Marmara breached through to the Black Sea, which at the time (according to the theory) was a low-lying body of fresh water.

It is also said in myth that floating rocks known as the Symplegades or Clashing Rocks once crushed any ship that attempted passage of the Bosphorus until the hero Jason obtained passage, whereupon the rocks became fixed, and Greek access to the Black Sea was opened.

Ancient Greece, Persia, Rome, the Byzantines and the Ottoman Empire

The Bosphorus with the Castles of Europe and Asia. 19th-century engraving by Thomas Allom. The castles are Rumelihisarı andAnadoluhisarı, respectively.

As part of the only passage between the Black Sea and the Mediterranean, the Bosphorus has always been of great commercial and strategic importance. The Greek city-state of Athens in the 5th century BC, which was dependent on grain imports from Scythia, maintained critical alliances with cities which controlled the straits, such as the Megarian colony Byzantium.

Persian King Darius I the Great, in an attempt to subdue the Scythian horsemen who roamed across the north of the Black Sea, crossed through the Bosphorus, then marched towards the Danube River. His army crossed the Bosphorus over an enormous bridge made by connecting Achaemenid boats. This bridge essentially connected the farthest geographic tip of Asia to Europe, encompassing at least some 1000 meters of open water if not more. Years later, a similar boat bridge would be constructed by Xerxes I on the Dardanelles (Hellespont) strait, during his invasion of Greece. Byzantines called the Bosphorus “Stenon” and most important toponyms of it Bosporios Akra, Argyropolis, St. Mamas, St. Phokas, Hestiai or Michaelion, Phoneus, Anaplous or Sosthenion in European side and Hieron tower, Eirenaion, Anthemiou, Sophianai, Bithynian Chryspolis in Asian side in this era

The strategic significance of the strait was one of the factors in the decision of the Roman Emperor Constantine the Great to found there in AD 330 his new capital, Constantinople, which came to be known as the capital of theEastern Roman Empire. On 29 May 1453 it was conquered by the emerging Ottoman Empire. In fact, as the Ottoman Turks closed in on Constantinople, they constructed a fortification on each side of the strait, Anadoluhisarı(1393) and Rumelihisarı (1451).

By | 2017-02-20T12:30:20+00:00 May 17th, 2015|Categories: Istanbul Main Attractions|

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