The Metonic Cycle
A period of about nineteen years after which the phases of the Moon fall on very nearly the same days of the solar year. 235 lunar months are almost exactly nineteen solar years — a near-perfect reconciliation of the Moon’s months with the Sun’s year.
The problem it solves
A lunar month lasts about 29.5 days, which does not divide evenly into a solar year of about 365.25 days. Any calendar that wants to keep months tied to the Moon and years tied to the Sun has to reconcile the two, and the mismatch drifts quickly.
The near-coincidence
Nineteen solar years come to almost exactly 235 synodic (phase-to-phase) months — the two totals differ by only about two hours. Meton of Athens is credited with fixing this in the fifth century BCE, and the cycle carries his name.
Where it is used
The Metonic cycle underlies lunisolar calendars from the Hebrew calendar to the reckoning of the date of Easter, and it is one of the cycles built into the dials of the Antikythera mechanism.
Connections
- The Antikythera Mechanism — An ancient Greek geared device, recovered in 1901 from a shipwreck off the island of Antikythera, that mechanically modelled the movements of the Sun and Moon and predicted eclipses. It is the oldest known analogue astronomical computer.
- The Synodic Month — The average time from one New Moon to the next — about 29.53 days. It is the cycle of the Moon’s phases as seen from Earth, and the ‘month’ at the root of lunar and lunisolar calendars.