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Inclassical, medieval, and Renaissance astronomy, thePrimum Mobile (Latin: "first movable") was the outermost movingsphere in thegeocentric model of theuniverse.[1]
The concept was introduced byPtolemy to account for the apparentdaily motion of the heavens around the Earth, producing the east-to-west rising and setting of the sun and stars, and reached Western Europe viaAvicenna.[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 asmoonrise andsunset.[3] Rotation of the Earth on its polar axis – as seen in aheliocentric solar system, which (while anticipated byAristarchus) was not to be widely accepted until well afterCopernicus[3] – leads to what earlier astronomers saw as the real movement of all the heavenly bodies around the Earth every 24 hours.[4]
Astronomers believed that the sevennaked-eye planets (including the Moon and the Sun) were carried around thespherical Earth on invisible orbs, while an eighth sphere contained the fixedstars. Motion was provided to the whole system by the Primum Mobile, itself set within theEmpyrean, and the fastest moving of all the spheres.[5]
The total number ofcelestial spheres was not fixed. In this 16th-century illustration, thefirmament (sphere of fixed stars) is eighth, a "crystalline" sphere (posited to account for the reference to "waters ... above the firmament" inGenesis 1:7) is ninth, and the Primum Mobile is tenth. Outside all is theEmpyrean, the "habitation of God and all theelect".
Copernicus accepted existence of the sphere of the fixed stars, and (more ambiguously) that of the Primum Mobile,[6] as too (initially) didGalileo[7] – though he would later challenge its necessity in a heliocentric system.[8]
Francis Bacon was as sceptical of the Primum Mobile as he was of the rotation of the earth.[9] OnceKepler had made the sun, not the Primum Mobile, the cause of planetary motion, however,[10] the Primum Mobile gradually declined into the realm of metaphor or literary allusion.