metónico

Meton of Athens was a 5th-century BCE Greek astronomer and mathematician who, together with his colleague Euctemon, described the Metonic cycle. This 19-year period is notable for the near coincidence between the solar year and the lunar month, such that the phases of the Moon recur on or near the same days of the year after 19 years.

The Metonic cycle posits that in 19 solar years there are about 235 synodic lunar months. Because of this close alignment, calendars can stay in sync with the seasons by adding intercalary months in certain years within the cycle. In practice, seven of the nineteen years include an extra month, a system used by various lunisolar calendars.

Historically, the cycle influenced the design of calendars and astronomical computations. It was adopted in ancient and medieval times to keep lunar months aligned with the solar year, and it particularly shaped the structure of the Hebrew calendar, which employs a 19-year cycle with seven leap years to maintain seasonal consistency. The Metonic cycle also informed later astronomical methods for predicting new moons and determining dates tied to lunar phases.

The term metónico (or Meton) is used in reference to the astronomer and, by extension, to the cycle he described. The cycle remains a foundational concept in the history of calendar construction and the study of lunisolar synchronization.