Sofia is the capital and largest city of Bulgaria. Sofia is located at the foot of Mount Vitosha in the western part of the country. It occupies a strategic position at the centre of the Balkan Peninsula. Sofias history spans 2,400 years. Its ancient name Serdica derives from the local Celtic tribe of the Serdi who established the town in the 5th century BC. It remained a relatively small settlement until 1879, when it was declared the capital of Bulgaria. Sofia is the 15th largest city in the European Union with a population of around 1.4 million people. Sofia has been ranked by the Globalization and World Cities Research Network as a Beta city. Many of the majoruniversities, cultural institutions, and businesses of Bulgaria are concentrated in Sofia. The name Serdica or Sardica was popular in Latin, Ancient Greek and Byzantine Greeksources from Antiquity and the Middle Ages; it was related to the local Celtic tribe of the Serdi. The name was last used in the 19th century in a Bulgarian text, Service and hagiography of Saint George the New of Sofia. Another of Sofias names, Triaditsa, was mentioned in Greek medieval sources. The Bulgarian name Sredets, which is related to sreda (middle), first appeared in the 11th-centuryVision of Daniel and was widely used in the Middle Ages. The current name Sofia was first used in the 14th-century Vitosha Charter of Bulgarian tsar Ivan Shishman or in aRagusan merchants notes of 1376; it refers to the famous Holy Sophia Church, an ancient church in the city named after the Christian concept of the Holy Wisdom. Although Sredets remained in use until the late 18th century, Sofia gradually overcame the Slavic name in popularity. During the Ottoman rule it was called Sofya by the Turkish conquerors of Bulgaria. The citys name is pronounced by Bulgarians with a stress on the o, in contrast with the tendency of foreigners to place the stress on i. The female given name "Sofia" is pronounced by Bulgarians with a stress on the i.