Moiety

latin: moietas / pars dimidia · arabic: نُور الكَوكَب (nūr al-kawkab, "the planet's light") / نِصْف قَوس (niṣf qaws, "half-arc")

Definition

In Arabic-Latin and traditional Western practice, a planet's moiety is half its allotted "orb of light" — the radius of the sphere of influence the planet carries with it. Sahl's standard tabulation: the Sun's body extends 15° in front and 15° behind (full orb 30°); the Moon 12° each side; Saturn and Jupiter 9° each side; Mars 8° each side; Venus and Mercury 7° each side. Two planets are read as connected by aspect when their orbs of light cross — i.e., when the distance between them is less than the sum of their two moieties.

In Tradition

In the Arabic-Latin tradition, the moiety is the per-planet orb component of a per-planet aspect doctrine: each planet carries its own orb-of-light independent of which aspect angle is being formed. This contrasts with modern Western practice, in which each aspect (sextile, square, trine, etc.) carries its own orb-tolerance independent of which planets form it. Robert Hand explicitly emphasizes the contrast as material to understanding Bonatti's per-planet aspect logic.

In Practice

Astrologers using moiety check whether two planets are aspecting by computing the average of their two moieties and treating that figure as the combined orb. For instance, a Sun-Saturn aspect carries (15° + 9°) / 2 = 12° as its combined-moiety orb, larger than the 8° or 10° most modern astrologers grant the same square or opposition. Bonatti and Lilly apply moiety to determine when corporal conjunctions and aspect-perfections take effect. Dykes notes (footnote on Sahl *Introduction* §5.3) that "these orbs are twice as big as those reported by later authorities such as Lilly," establishing Sahl as the ancestral authority and documenting the subsequent contraction of orb-values in the Western tradition. Hand attributes the shift from "orb of planet" to "orb of aspect" to early-20th-century American astrology accommodating the minor aspects.

Historical Origin

The per-planet orb-of-light doctrine is Arabic in origin — Hand notes that the Greek tradition did not carry a systematic orb framework before the Arabs developed it. The moiety scheme is attested in Sahl ibn Bishr (9th c.), Abu Ma'shar, and the Dorothean Arabic tradition; preserved through Bonatti's *Liber Astronomiae* (13th c.); and into Lilly's *Christian Astrology* (1647) with contracted values.

Etymology

Origin: Old French / Latin. Meaning: Half-portion.

Definition

In Tradition

In Practice

Historical Origin

Etymology

Further Reading