Primum Mobile


In classical, medieval, and Renaissance astronomy, the Primum Mobile was the outermost moving sphere in the geocentric model of the universe.
The concept was introduced by Ptolemy to account for the apparent daily motion of the heavens around the Earth, producing the east-to-west rising and setting of the sun and stars, and reached Western Europe via Avicenna.

Appearance and rotation

The Ptolemaic system presented a view of the universe in which apparent motion was taken for real – a viewpoint still maintained in common speech through such everyday terms as moonrise and sunset. Rotation of the Earth on its polar axis – as seen in a heliocentric solar system, which was not to be
widely accepted until well after Copernicus – leads to what earlier astronomers saw as the real movement of all the heavenly bodies around the Earth every 24 hours.
Astronomers believed that the seven naked-eye planets were carried around the spherical Earth on invisible orbs, while an eighth sphere contained the fixed stars. Motion was provided to the whole system by the Primum Mobile, itself set within the Empyrean, and the fastest moving of all the spheres.

Spherical variations

The total number of celestial spheres was not fixed. In this 16th-century illustration, the firmament is eighth, a "crystalline" sphere is ninth, and the Primum Mobile is tenth. Outside all is the Empyrean, the "habitation of God and all the elect".

Copernicus and after

Copernicus accepted existence of the sphere of the fixed stars, and that of the Primum Mobile, as too did Galileo – though he would later challenge its necessity in a heliocentric system.
Francis Bacon was as sceptical of the Primum Mobile as he was of the rotation of the earth. Once Kepler had made the sun, not the Primum Mobile, the cause of planetary motion, however, the Primum Mobile gradually declined into the realm of metaphor or literary allusion.

Literary references