The Turkish national flag is mostly red, with a white star and a crescent in the center. Ottoman Sultan Selim III formalized the look in 1793, but the flag is actually much older.
The crescent-and-star combination has been used in Turkey since Hellenistic times (400s to 100 BCE). It likely came from ancient Mesopotamian iconocraphy. Ancient depictions of the symbol always show the crescent with horns pointing upward and with the star placed inside the crescent, for reasons that have been lost to time. When it came to Turkey, they gave it their own meanings. For Byzantium the moon symbolized Diana, also known as Artemis, the patron goddess of the city.
In 1453, when the city was conquered by the Ottoman Empire, the flag remained unchanged. With time, it became not just Istanbul’s flag but the Ottoman flag, with its design formalized in 1793 and its status as national flag formalized in 1844. Turks affectionately call the flag “ay yildiz” – the “moon star” flag.
Many nations that were once part of Ottoman Empire adopted the star-and-crescent when they gained independence, including Libya, Tunisia, and Algeria. In the 1900s the symbol became associated with not just the Ottomans, but with Islam in general, and many states that were never part of the Ottoman Empire adopted it too, including Pakistan, Malaysia, and the Maldives. Pretty amazing that an ancient Mesopotamian symbol is flown around the world today.