Primum Mobile

In classical, medieval, and Renaissance astronomy, the Primum Mobile (Latin: "first movable") was the outermost moving sphere in the geocentric model of the universe.

[1] 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.

[2] 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.

[4] Astronomers believed that the seven naked-eye planets (including the Moon and the Sun) were carried around the spherical Earth on invisible orbs, while an eighth sphere contained the fixed stars.

Copernicus accepted existence of the sphere of the fixed stars, and (more ambiguously) that of the Primum Mobile,[6] as too (initially) did Galileo[7] – though he would later challenge its necessity in a heliocentric system.