Just learned how the crescent moon and star came to be widespread symbols of Islam. The crescent moon was the symbol of the pagan goddess Diana, goddess of the hunt. The city of Byzantium adopted Diana as its patron goddess, and with it her symbol, the crescent moon, and put the moon on their flags. When Byzantium was renamed Constantinople and became the Capitol of the Roman Empire in the 4th century, they added the star, symbol of Mary. For 1000 years, the star and crescent were the symbol of Constantinople -- a symbolic fusion of ancient paganism with Christianity.
In 1453, when the Ottomon's conquered Constantinople, they adopted the crescent + star symbol for the empire. Since the empire was over so much of the Muslim world, it became associated with Islam itself.
And now, it's widely used as the symbol of Islam itself -- on dozens of flags, and the Red Crescent (Islamic equivalent of the Red Cross), etc.
Evidently, there are a number of Muslims, aware of the pagan and Christian origins of the symbol, who disapprove of it. But obviously they're in the minority.
Fascinating how much history, religion, and metaphor is wrapped up in a symbol.