According toStrabo, Bithynia was bounded on the east by the river Sangarius (modernSakarya river), but the more commonly received division extended it to theParthenius, which separated it fromPaphlagonia, thus comprising the district inhabited by theMariandyni. On the west and southwest it was separated fromMysia by theriver Rhyndacus and on the south it adjoinedPhrygia andGalatia.[1]
It is occupied by mountains and forests, but has valleys and coastal districts of great fertility. The most important mountain range is the (so-called)"Mysian" Olympus (8,000 ft, 2,400 m), which towers aboveBursa and is clearly visible as far away asIstanbul (70 miles, 110 km). Its summits are covered with snow for a great part of the year.[1]
East of this the range extends for more than 100 miles (160 km), from theSakarya toPaphlagonia. Both of these ranges are part of the border of mountains which bound the great tableland ofAnatolia,Turkey. The broad tract which projects towards the west as far as the shores of the Bosporus, though hilly and covered with forests—theTurkishAğaç Denizi, or "The sea of Trees"—is not traversed by any mountain chain. The west coast is indented by two deep inlets, the northernmost, theGulf of İzmit (ancient Gulf of Astacus), penetrating between 40 and 50 miles (64 and 80 km) into the interior as far asİzmit (ancientNicomedia), separated by anisthmus of only about 25 miles (40 km) from theBlack Sea; and theGulf of Mudanya orGemlik (Gulf of Cius), about 25 miles (40 km) long. At its extremity is situated the small town of Gemlik (ancientCius) at the mouth of a valley, communicating with the lake of Iznik, on which was situatedNicaea.[1]
The principal rivers are theSangarios which traverses the province from south to north; theRhyndacus, which separated it from Mysia; and theBillaeus (Filyos), which rises in the Aladağ, about 50 miles (80 km) from the sea, and after flowing by modernBolu (ancient Bithynion-Claudiopolis) falls into the Euxine, close to the ruins of the ancientTium, about 40 miles (64 km) northeast ofHeraclea Pontica (the modernKaradeniz Ereğli), having a course of more than 100 miles (160 km). The Parthenius (modernBartın), the eastern boundary of the province, is a much less considerable stream.[1]
The valleys towards the Black Sea abound in fruit trees of all kinds, such as oranges, while the valley of the Sangarius and the plains near Bursa and Iznik (Nicaea) are fertile and well cultivated. Extensive plantations ofmulberry trees supply the silk for which Bursa has long been celebrated, and which is manufactured there on a large scale.[1]
Bithynia is named for theThracian tribe of theBithyni, mentioned byHerodotus (VII.75) alongside theThyni. The "Thraco-Phrygian" migration from the Balkans to Asia Minor would have taken place at some point following theBronze Age collapse or during the early Iron Age. The Thyni and Bithyni appear to have settled simultaneously in the adjoining parts of Asia, where they expelled or subdued theMysians,Caucones and other minor tribes, theMariandyni maintaining themselves in the northeast. Herodotus mentions the Thyni and Bithyni as settling side by side.[1] No trace of their original language has been preserved, but Herodotus describes them as related to the tribes of Thracian extraction.
Later theGreeks established on the coast the colonies of Cius (modern Gemlik);Chalcedon (modernKadıköy), at the entrance of the Bosporus, nearly oppositeByzantium (modernIstanbul) andHeraclea Pontica (modern Karadeniz Ereğli), on the Euxine, about 120 miles (190 km) east of the Bosporus.[2]
The Bithynians were incorporated by kingCroesus within theLydian monarchy, with which they fell under the dominion ofPersia (546 BC), and were included in thesatrapy ofPhrygia, which comprised all the countries up to the Hellespont and Bosporus.[1]
Even before the conquest byAlexander the Bithynians appear to have asserted their independence, and successfully maintained it under two native princes,Bas andZipoites, the latter of whom assumed the title of king (basileus) in 297 BC.
As aRoman province, the boundaries of Bithynia changed frequently. During this period, Bithynia was commonly united for administrative purposes with the province ofPontus within theRoman Empire. This was the situation at the time ofEmperorTrajan, whenPliny the Younger was appointed governor of the combined provinces (109/110 – 111/112), a circumstance which has provided historians with valuable information concerning the Roman provincial administration at that time.
Under theByzantine Empire, Bithynia was again divided into two provinces, separated by theSangarius. Only the area to the west of the river retained the name of Bithynia.[2]
Bithynia attracted much attention because of its roads and its strategic position between the frontiers of theDanube in the north and theEuphrates in the south-east. To secure communications with theeastern provinces, the monumentalbridge across the river Sangarius was constructed around 562. Troops frequently wintered at Nicomedia.
During this time, the most important cities in Bithynia wereNicomedia, founded by Nicomedes, andNicaea. The two had a long rivalry with each other over which city held the rank of capital.
As found in theNotitia Dignitatum. Provincial administration reformed anddioceses established byDiocletian,c. 293. Permanentpraetorian prefectures established after the death ofConstantine I. Empire permanently partitioned after 395. Exarchates ofRavenna andAfrica established after 584. After massive territorial losses in the 7th century, the remaining provinces were superseded by thetheme system in c. 640–660, although inAsia Minor and parts of Greece they survived under the themes until the early 9th century.