In art and symbolism, a crescent is generally the shape produced when a circular disk has a segment of another circle removed from its edge, so that what remains is a shape enclosed by two circular arcs of different diameters which intersect at two points (usually in such a manner that the enclosed shape does not include the center of the original circle).
In astronomy, a crescent is the shape of the lit side of a spherical body (most notably the moon) that appears to be less than half illuminated by the Sun as seen by the viewer. Mathematically, assuming the terminator lies on a great circle, such a crescent will actually be the figure bounded by a half-ellipse and a half-circle, with the major axis of the ellipse coinciding with a diameter of the semicircle.
The direction in which the "horns" (the points at the intersection of the two arcs) face indicates whether a crescent is a "young" increasing moon or an "old" decreasing moon. So horns pointing left ')' denote a waxing crescent and pointing right 'C' a waning crescent (at least in the northern hemisphere).
The crescent used to be a symbol of the Byzantine Empire. After the Fall of Constantinople in 1453 it was taken as a symbol by the Ottoman Empire. The crescent is now often used to symbolize the Islamic faith. However, it should be noted that the crescent was not a symbol used for Islam by Muhammad or any other early Muslim rulers, as the Islamic religion is, in fact, against appointing "Holy Symbols" (so that during the early centuries of Islam, Muslim authorities simply didn't want any geometric symbols to be used to symbolize Islam, in the way that the cross symbolizes Christianity, the menorah was a commonly-occurring symbol of Judaism, etc.). This is why early Islamic coins were covered with Arabic writing, but contained no visual symbols.
Despite this mixed history, many Islamic nations and charities use the crescent symbol on their flags or logos (e.g. Pakistan, The Red Crescent, etc. — though currently none of the Arab states in Arabia or the Mashreq have crescents on their flags). In this manner it could be argued that its modern usage is meant to signify identity and, for devout Muslims, brotherhood.
Many Christian fundamentalists such as Jack Chick use this symbolism to make a claim that Allah was in fact Hubal who was, in turn, a moon-god worshipped by the pre-Islamic Arabs. This argument is sometimes used to argue that the god of Islam is different from that of the Jews or Christians.
Leliwa. Polish noble coat of arms with crescent as a heraldic symbol.
The crescent as a neo-pagan symbol of the Triple Goddess.Note that in the case of an astronomical crescent, such as the moon observed in the sky, the outer arc will be 180° (a half-circle as previously mentioned), while the Islamic crescent symbol (Arabic ÙÙا٠hilÄl) is generally shown with an outer arc significantly greater than 180° (as seen in the illustrations above).
The crescent is also used as a heraldic symbol in some Slavic countries. However, this usage is not affiliated in any way with Islam. The roots of the Slavic crescent can be traced to the old Slavic pagan beliefs. In English and Canadian heraldry a crescent is the cadence mark of a second son.
The crescent symbol is also used to represent the moon in astronomy and astrology, and to represent silver (the metal associated with the moon) in alchemy, where, by inference, it can also be used to represent qualities that silver possesses.