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Concept

Metonic cycle

A compass-like diagram illustrates the phases of the moon, with a central full moon surrounded by its waxing and waning phases, symbolizing the Metonic cycle.

The astronomical observation, named for the Athenian astronomer Meton (5th century BCE), that 19 solar years equal almost exactly 235 lunar months. The relation lets a single calendar reconcile the Sun and Moon. The Antikythera Mechanism's back face is dominated by a five-turn spiral dial of 235 lunar months — one full Metonic cycle — letting the user read off the date of any new or full moon for nineteen years at a stretch.

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