Chalcedon in the context of Anabasis Alexandri

The Council of Chalcedon (/kælˈsiːdən, ˈkælsɪdɒn/; Latin: Concilium Chalcedonense) was the fourth ecumenical council of the Christian Church. It was convoked by the Roman emperor Marcian. The council convened in the city of Chalcedon, Bithynia (modern-day Kadıköy, Istanbul, Turkey) from 8 October to 1 November 451. The council was attended by over 520 bishops or their representatives, making it the largest and best-documented of the first seven ecumenical councils. The principal purpose of the council was to re-assert the teachings of the ecumenical Council of Ephesus against the teachings of Eutyches and Nestorius. Such doctrines viewed Christ's divine and human natures as separate and distinct (Nestorianism), or viewed Christ as solely divine (monophysitism). The Council of Chalcedon issued the Chalcedonian Definition, stating that Jesus is "perfect both in deity and in humanness; this selfsame one is also actually God and actually man." The Council's judgments and definitions regarding the divine marked a significant turning point in the Christological debates.

Herophilos (/hɪˈrɒfɪləs/; Ancient Greek: Ἡρόφιλος; 335–280 BC), sometimes Latinised Herophilus, was a Greek physician regarded as one of the earliest anatomists. Born in Chalcedon, he spent the majority of his life in Alexandria. He was the first scientist to systematically perform scientific dissections of human cadavers. He recorded his findings in over nine works, which are now all lost. The early Christian author Tertullian states that Herophilos vivisected at least 600 prisoners; however, this account has been disputed by many historians. He is often seen as the father of anatomy.

The Battle of Chrysopolis was fought on 18 September 324 between the Roman emperors Constantine I and Licinius near Chrysopolis (modern Üsküdar), opposite Byzantium (modern Istanbul) on the Asiatic shore of the Bosphorus. The battle followed Constantine's victory at the Battle of the Hellespont, where his son Crispus destroyed the Licinian fleet. Licinius withdrew his forces from Byzantium to Chalcedon and gathered the remnants of his army, reinforced by Visigothic auxiliaries and the troops of the co-emperor Martinian.

Constantine crossed the Bosphorus into Asia Minor and marched on Chrysopolis. Licinius drew up his army with the traditional pagan standards, while Constantine advanced under the labarum. Constantine launched a direct attack that routed Licinius' troops. Ancient sources report heavy losses for Licinius, and he retreated with the survivors to Nicomedia.

The Chalcedonian Definition (also called the Chalcedonian Creed or the Definition of Chalcedon) is the declaration of the dyophysitism of Christ's nature, adopted at the Council of Chalcedon in AD 451. Chalcedon was an early centre of Christianity located in Asia Minor. The council was the fourth of the ecumenical councils that are accepted by Chalcedonian churches which include the Catholic and Orthodox churches.

It was the first council not to be recognised by any Oriental Orthodox church; for this reason these churches may be classified as Non-Chalcedonian.

Kadıköy (Turkish pronunciation: [kaˈdɯkøj] ) is a municipality and district in Istanbul Province, Turkey. Its area is 25 km, and its population is 467,919 (2023). It is a large and populous area in the Asian side of Istanbul, on the northern shore of the Sea of Marmara. It partially faces the historic city centre of Fatih on the European side of the Bosporus. It is bordered by the districts of Üsküdar, to the northwest, Ataşehir, to the northeast, and Maltepe, to the southeast.

Kadıköy was known in classical antiquity and during the Roman and Byzantine eras as Chalcedon (Greek: Χαλκηδών). Chalcedon was known as the 'city of the blind'. The settlement has been under control of many empires, finally being taken by the Ottomans before the fall of Constantinople. At first, Chalcedon was rural, but with time it urbanized. Kadıköy separated from the Üsküdar district in 1928.

In the 9th century, during the Arab–Byzantine wars, the Byzantine Empire used a semaphore system of beacons to transmit messages from the border with the Abbasid Caliphate across Asia Minor to the Byzantine capital, Constantinople.

According to Byzantine sources (Constantine Porphyrogenitus, Theophanes Continuatus and Symeon Magister), the line of beacons began with the fortress of Loulon, on the northern exit of the Cilician Gates, and continued with Mt. Argaios (identified mostly with Keçikalesı on Hasan Dağı, but also with Erciyes Dağı near Caesarea), Mt. Samos or Isamos (unidentified, probably north of Lake Tatta), the fortress of Aigilon (unidentified, probably south of Dorylaion), Mt. Mamas (unidentified, Constantine Porphyrogenitus has Mysian Olympus instead), Mt. Kyrizos (somewhere between Lake Ascania and the Gulf of Kios, possibly Katerlı Dağı according to W. M. Ramsay), Mt. Mokilos above Pylae on the southern shore of the Gulf of Nicomedia (identified by Ramsay with Samanlı Dağı), Mt. Saint Auxentius (modern Kayış Dağı) south-east of Chalcedon (modern Kadıköy) and the lighthouse (Pharos) of the Great Palace in Constantinople. This main line was complemented by secondary branches that transmitted the messages to other locations, as well as along the frontier itself.