Dante says that Ptolemy revised Aristotle’s cosmos by adding another sphere beyond that of the fixed stars. This was demanded by the principles of philosophy that call for a perfectly simple Primum Mobile. [Definition—the outermost sphere of the universe whose motion moves the spheres within it.]

The order of the heavens is this: that of the Moon, then Mercury, Venus, the Sun, Mars, Jupiter, Saturn, and the Fixed Stars.

The ninth heaven is not perceptible to sense except by the motion of the others and wholly transparent. It is called Crystalline.

Beyond all these is the Empyrean Heaven, the Heaven of Flame, or Luminous Heaven which is immovable, because it has within itself, in every part, that is necessary. “And this is the reason that the Primum Mobile moves with immense velocity; because the fervent longing of all its parts to be united to those of this [tenth and] most divine and quiet heaven, makes it revolve with so much desire that its velocity is almost incomprehensible. And this quiet and peaceful heaven is the abode of that Supreme Deity who alone doth perfectly behold Himself. This is the abode of the beatified spirits, according to the holy Church, who cannot lie.”

The motion of the spheres is the work Intelligences, whom the common people call Angels. They cause the revolutions by their thought alone.

Aristotle proved that this world, that is, our earth, stands still and fixed for all eternity. Of the reasons he gives to affirm this truth, it is not my intention to speak; because it is quite enough to know on his great authority that this earth is immovable, and does not revolve, and that, with the sea, it is the centre of the heavens.