The crescent moon, sometimes with one or more stars, is widely used as a symbol of Islam by both Muslims and non-Muslims. However, its origin and symbolism are older and broader than just Islam, potentially going back to prehistoric times. In addition, since Islam prohibits images of any kind as they may lead to idolatry, some Muslims object to the use of this or any symbol for Islam.The Ottoman Empire, one of the three great Islamic empires, used the crescent on their flags, some say from a historic association between Turkey, the origin of the Ottoman Empire, and the crescent. The crescent and star became a symbol of the Ottoman Empire, one of the three great Islamic empires. The Persian Sassanian empire, which predated Islam, also used the crescent as its symbol. It is from these associations that the crescent has become known as a symbol of Islam. The crescent appears on the flag of many Islamic countries past and present, likely more from their association with the Ottoman empire than any other symbolism of the crescent. This is similar to the use of the Union Jack (British flag) or colors from the Union Jack in countries that are former colonies of the British Empire – for instance – the US and Canada use colors from the Union Jack and Australia and New Zealand use an image of the flag itself on their flags.
However, the use of the crescent in the Middle East predates the Ottoman Empire. The habitable area of the Middle East forms a crescent, the aptly named Fertile Crescent. The city of Byzantium, now Istanbul, was dedicated to the Roman goddess Diana; her symbol was the crescent. The various polytheistic religions in the Middle East, as in Europe, that were supplanted by monotheistic religions, including Islam, had moon goddesses who where often represented by a crescent moon.
The crescent moon is clearly important in Islam and Middle Eastern history, which of course overlap. The Islamic calendar is a lunar calendar and months begin when the first crescent moon is visible to the human eye. Visibility depends on a number of issues beyond astronomy including weather, so unlike the Jewish calendar which is also lunar, using astronomy to calculate the beginning and ending of the months is not permissible. The crescent's use as a symbol of Islam is more a matter of convenience than faith or fact. A visual marker to identify mosques and other Muslim buildings, services and information often useful. The crescent's long association with the Middle East means that it isn't used by other groups. For instance, the crescent symbol was used on many European shields and battle flags prior to the crusades, but removed from many of them to remove confusion as to which side they were on. Symbols more in line with Islam might be a green or black flag such as the Prophet Muhammad (Peace Be Upon Him) is reported to have carried or a related word in Arabic calligraphy; however, these are less likely to be recognized my non-Muslims and being more delicate, are not appropriate in as many situations. Understanding-Islam.com points out that there is an important distinction between Islamic symbols and symbols used by Muslims, the crescent definitely falls in the later category.
