Called either Ay Yιldιz (meaning "moon star") or Alsancak ("red banner"), the flag of the Republic of Turkey depicts a white crescent moon and star on a red background. Although the crescent moon is a common symbol for Islam, it predates Muhammad by thousands of years.
The star and crescent have been found on artifacts from ancient Central Asia and Siberia and are believed to have been used in worship of the gods of the sun, moon, and stars. The symbol also has a place in ancient Greek symbology. The city of Byzantium, at the mouth of the Black Sea, was dedicated to the Greek goddess Artemis, whose symbol was the crescent moon. The flag of Byzantium thus became a white crescent moon against a field of red.
In 330 AD, Roman Emperor Constantine I made Byzantium the capital of what historians today call the Byzantine Empire. An eight-pointed star was added to the city's flag during Constantine's rule, possibly as a symbol of the Virgin Mary. After Constantine's death, Byzantium was renamed Constantinople. In 1453, Constantinople was conquered by Ottoman Turks, and Sultan Mehmed II adopted its flag for his own. In 1844, the eight-pointed star was replaced with a five-pointed star.
Constantinople became the center of the Ottoman Empire, which ruled over the Muslim world for centuries. Its flag thus became synonymous with Islam in most of the world, though many Muslims refuse to recognize the symbol because of its pagan origins.
With 1923's Treaty of Lausanne, the Republic of Turkey was officially recognized throughout the world, and the flag of the Ottoman Empire became the flag of the Republic of Turkey. The capital of Turkey was moved to Ankara, though Constantinople (which officially was renamed Istanbul in 1930) was and still is the largest city in Turkey. The proportions of the flag's crescent and star were standardized by law in 1936.