On the HeavensAristotle
About On the Heavens
Aristotle's (De Caelo) presents his cosmology: the structure of the physical universe, the nature of celestial motion, and the properties of the elements. It is the work in which Aristotle most fully develops the distinction between the sublunary world of generation and corruption and the superlunary world of eternal, circular motion.
The heavens are composed of a fifth element, aether, whose natural motion is circular and which neither comes to be nor passes away. The celestial spheres carry the stars and planets in perfect, unceasing rotation. Below the moon, four elements (earth, water, air, fire) move naturally in straight lines toward their proper places: heavy bodies toward the center, light bodies away from it. This framework explains why the earth is spherical and stationary at the center of the cosmos.
Aristotle also argues that the universe is finite in extent, that there is no void beyond it, and that it is unique (there cannot be more than one cosmos). He defends the eternity of the world against those who claim it was created, arguing that circular motion has no beginning and no end.
The geocentric, finite, hierarchical cosmos of dominated Western and Islamic astronomy until Copernicus, and Galileo's is in large part a sustained argument against it.