Bartolomeu Velho, Figure of the heavenly bodies. 1568. An illuminated illustration of the Ptolemaic geocentric conception of the universe.
The ninth sphere — the Primum Mobile — was contained only by the mind of God, and constituted all of physical creation and space. Unchanging, it is the structure in which all change is framed. It is the outermost limit of the universe, and the beginning of creation. The spiritual motion of intention or love creates physical motion.
Light and love enclose it in one circle
As it does all the rest, and this enclosing
He alone who circles it can comprehend.
Its motion is not measured by another’s,
But this sphere sets the others into motion,
As ten is factored into five and two.
— Dante Alighieri, Paradiso, Canto XXVII, l. 112-117.