Glyph and Origin

Its glyph is a crescent. The Moon is one of the seven classical planets, watched and deified in ancient Mesopotamia and absorbed into Hellenistic astrology as a foundational body. As with the Sun's pairing to gold, alchemy assigned the Moon its own metal, silver, and the association is old enough to have left a mark on ordinary language: the Latin name for the Moon's day, dies Lunae, gives English Monday and the corresponding day in every Romance language, a naming pattern inherited from the same seven-day planetary week that fixed the classical glyphs in the first place.

The crescent sign travelled into modern computing with the rest of the classical set when Unicode encoded it in the Miscellaneous Symbols block in 1991, and it remains one of the few astrological glyphs whose modern printed form is essentially unchanged from its late antique original, unlike the Sun's circumpunct or Mercury's staff, both of which show real evolution across the Byzantine and medieval periods before settling into their present shapes. That stability may also owe something to necessity: because the Moon moves so quickly through the zodiac compared with the other classical bodies, its sign appears far more often in any working chart or ephemeris than most of its neighbours, and a mark that is drawn constantly has less room to drift than one consulted only rarely.