Primum Mobile

Definition

The Primum Mobile (Latin for "the first moved") is the outermost shell of sky in the Earth-centered model that older astrology assumes — usually the ninth sphere, just beyond the eighth that holds the fixed stars. It is the source of the primary motion: the daily turning of the whole heavens from east to west, once in about twenty-four hours. As it turns it carries the inner spheres — fixed stars, Sun, Moon, planets — around the Earth, setting when each body rises, culminates, sets. Its Greek ancestor is Aristotle's "first mover" (proton kinoun).

In Tradition

In this older cosmic picture the Primum Mobile gives astrology its day-frame, the backdrop against which the zodiac-frame is read. Its steady daily turning is the primary motion — separate from a planet's slower drift through the zodiac, the secondary motion. Because it turns at constant speed and, in the medieval account, has no variation of light, astrologers took its action as even and unchanging: the uniform engine that makes rising, culminating, setting — and the houses — mean anything.

In Practice

The Primum Mobile is the conceptual frame for everything in astrology that depends on the daily turning of the sky rather than on a planet's position in the zodiac. The split between primary motion (the whole sky wheeling east to west, driven by the Primum Mobile) and secondary motion (a planet creeping forward through its sign) is what separates the house framework from the sign framework: houses come from the daily rotation, signs from the zodiac band. Primary directions — a predictive method — take both their name and their mechanism from this primary motion: the technique advances chart points along the daily arc as if continuing the Primum Mobile's turning, and it was the main longevity method of classical and medieval astrology. Knowing the Primum Mobile also explains why rising times and quadrant house division change with latitude. Today the nine-sphere model is not taken as literal astronomy, but the primary-versus-secondary-motion distinction it carries still does real work whenever an astrologer separates house-based testimony from sign-based testimony.

Historical Origin

The Primum Mobile is a medieval-Latin coinage built on Aristotelian-Ptolemaic cosmology, its underlying "first mover" being Aristotelian. It is standard in the traditional cosmological frame carried through the Arabic Almagest tradition and the medieval Latin reception — Guido Bonatti's Liber Astronomiae (13th century) treats the ninth sphere as the uniform, undifferentiated outermost heaven. Modern traditional reconstructions by Helena Avelar and Luis Ribeiro and by Charles Obert keep it as the source of primary motion.

Etymology

Origin: Latin. Meaning: The first moved.

Further Reading

• Guido Bonatti, Liber Astronomiae
• Helena Avelar and Luis Ribeiro, On the Heavenly Spheres
• Charles Obert, The Classical Seven Planets