On the Heavens

In On the Heavens (De Caelo) Aristotle sets out his picture of the universe: a finite, eternal, spherical cosmos, with the unchanging heavens composed of a fifth element, the aether, moving in perfect circles.

Below the moon lie the four elements — earth, water, air, fire — each seeking its natural place. This ordered, geocentric world, argued also in the Physics, governed astronomy until the seventeenth century.