The Moon is the second light of astrology and the ruler of Cancer. Its swift motion through the signs made it a primary marker of time and change in both Mesopotamian and Hellenistic practice. The crescent that marks it is correspondingly old: classical deities associated with the Moon, such as Artemis and Selene, are shown wearing a crescent across the brow or shoulders with its points turned upward, the form later heraldry would call a crescent increscent, and the sign is attested in this basic shape from late antiquity onward with little variation.
Unlike several of the other classical planets, whose glyphs were built from initials or emblematic weapons and instruments, the Moon's sign is a direct picture of its own changing appearance in the sky, which may be why it needed no abbreviation or reinterpretation to remain legible across two millennia of astronomical and astrological use. The steady watching of the Moon's motion for calendrical and omen purposes in ancient Mesopotamia is among the best-documented strands of that tradition, recorded on cuneiform tablets centuries before the Hellenistic Greek world adopted the planet, with its sign already essentially fixed, into the horoscopic astrology that shaped medieval and modern practice.