On the Heavens (Greek: Περὶ οὐρανοῦ, Latin: De Caelo or De Caelo et Mundo) is Aristotle's chief cosmological treatise: written in 350 BC it contains his astronomical theory and his ideas on the concrete workings of the terrestrial world. It should not be confused with the spurious work On the Universe (De mundo, also known as On the Cosmos).