Metonic cycle (nineteen-year lunisolar period)

Key characteristics

• Duration: 19 solar years, during which the Moon completes about 235 synodic months.
• Intercalation pattern: most lunisolar systems insert seven extra (leap) months within each 19‑year span to keep months in step with seasons.
• Practical accuracy: close but not exact—the small residual difference accumulates over centuries and requires additional corrections or longer cycles.

Named for the fifth‑century BCE Athenian astronomer Meton, the cycle was known to earlier cultures such as the Babylonians and came into practical use in a variety of calendars. Classical Greek sources credit Meton with promulgating a 19‑year scheme for civic calendars around the 5th century BCE.

Uses and examples

The Metonic idea underlies several historical and modern systems. The traditional Hebrew calendar uses a 19‑year cycle with seven leap months to keep the lunar months aligned with Jewish festivals. Early Christian computus for determining the date of Easter also relied on Metonic relationships. Many lunisolar calendars and ecclesiastical tables adopt Metonic rules or refinements of them in civil and religious timekeeping (lunisolar calendars).

Practical implementations typically follow a preset sequence of leap years within each 19‑year block. For example, one widespread pattern places intercalary months in years 3, 6, 8, 11, 14, 17 and 19 of the cycle. Such patterns preserve the alignment of months and seasons for many decades.

Limitations and refinements

Although useful, the Metonic cycle is an approximation. The small mismatch between 19 tropical years and 235 synodic months accumulates, so long‑term accuracy benefits from corrections. Ancient astronomers proposed refinements—most famously the Callippic cycle, which adjusts a four‑fold Metonic span—and modern calendars apply further rules to maintain seasonal and astronomical alignment. For technical studies of lunar months and calendar design see resources on lunar months.