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The sublunary sphere is a concept derived from Greek astronomy. It is the region of the cosmos from the Earth to the Moon, consisting of the four classical elements: earth, water, air, and fire. Beginning with the Moon, up to the limits of the universe, everything is made of aether. The planets and stars are located in the region of aether, where everything is permanent, regular, and unchanging. Plato and Aristotle helped to formulate the theory of a sublunary sphere in antiquity. Thomas Aquinas is among those who picked up on the ideas of an existing sublunary sphere, and charted them in his work Summa Theologica. This idea usually goes hand in hand with geocentrism and the concept of a spherical Earth.
