The moon goddess Hecate was credited with protecting Byzantium in 341BCE when a crescent moon suddenly broke through the clouds above the city as hostile Macedonian forces were approaching under the cover of darkness. Defenders manned the walls and the attack was repelled. To commemorate Hecate, a crescent and star image was stamped on the city’s coinage. The decorative device continued to be popular and was adopted centuries later by the Turks and many subsequent Islamic nations, with protective and other symbolic meanings.
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 waxing (also young, or increasing) or waning (also old, or decreasing). Eastward pointing horns (pointing to the left, as seen from the Northern hemisphere) indicate a waxing crescent, whereas westward pointing horns (pointing to the right, as seen from the Northern hemisphere) indicate a waning crescent. Note that the directions the horns point relative to the observer are reversed in the Southern hemisphere.
The word crescent is derived etymologically from the present participle of the Latin verb crescere “to grow”, thus meaning “waxing” or “increasing”, and so was originally applied to the form of the waxing moon (luna crescens). The English word is now commonly used to refer to either the waxing or waning shape. In the technical language of blazoning used in heraldry, the word “increscent” refers to a crescent shape with its horns to the left, and “decrescent” refers to one with its horns to the right.
The crescent is one of the oldest symbols known to humanity. Together with the sun, it appeared on Akkadian seals as early as 2300 BC and from at least the second millennium BC it was the symbol of the Mesopotamian Moon gods Nanna in Sumer and Sin in Babylonia, Sin being the “Lamp of Heaven and Earth”. The crescent was well known in the Middle East and was transplanted by the Phoenicians in the 8th century BC as far as Carthage (now in Tunisia). The crescent and star also appears on pre-Islamic coins of South Arabia.