The Moon is the second luminary, ruling Cancer as its sole domicile, exalted in Taurus, in fall in Scorpio and in detriment in Capricorn. Ptolemy gives it a moist nature, and it is the chief of the nocturnal sect. Because it moves fastest of all the bodies, roughly thirteen degrees a day, it is the principal timekeeper of the classical system: it carries the applying and separating aspects on which horary judgment turns, and its position relative to the Sun defines the phases, which the tradition reads as a cycle of increase and decrease. Its natural significations are the mother in a night chart, the body, the common people, the household, liquids, and everything that changes, receives and reflects. In medical astrology it governs the humours and the flow of fluids, and in electional practice the phase of the Moon is one of the first things considered.
Ptolemy's Tetrabiblos explains the Moon's assigned quality on physical rather than merely traditional grounds, stating that she is chiefly humidifying because she stands nearest the earth and to the moist exhalations that rise from it, the same four-quality framework of hot, cold, wet and dry that classical medicine used to describe bodies and temperaments generally. That reasoning belongs to the Hellenistic synthesis of roughly the first century BCE to the second century CE in Greco-Roman Egypt, but the Moon's astrological importance is far older, rooted in Babylonian celestial omen-reading and horoscopy reaching back to perhaps 1000 BCE, where her monthly appearance, disappearance and eclipses were tracked for omens concerning the state long before Hellenistic astrology built the personal, chart-based system of phases and aspects now used to read an individual nativity.