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Wellsprings
cosmology

The Sublunary and Superlunary Realms

τὰ ὑπὸ σελήνην / τὰ ὑπὲρ σελήνην

Below the Moon, things are born and decay; above it, the heavens turn forever in unchanging crystalline circles.

Aristotle split the cosmos in two at the orbit of the Moon. Everything below — earth, water, air, and fire — is made of the four elements that mix, perish, and move in straight lines, which is why our world is a place of birth, change, and death. Everything above is built from a fifth substance, aether, which neither grows nor decays and moves only in perfect circles, so the stars and planets wheel eternally and unchanged. This two-tier picture, later absorbed into Ptolemy's geocentric astronomy, governed how Europe imagined the universe until Galileo's telescope found change in the heavens.

Where this idea shows up

1 Greek source·3 Jewish-canon citations

Where to read it