19-Year Metonic Cycle

The 19-year Metonic cycle is a period after which the lunar phases repeat on the same calendar dates almost exactly. This happens because the Moon's orbit around Earth (the lunar month) and Earth's orbit around the Sun (a year) align closely over 19 years. In this cycle, the phases of the Moon (new, first quarter, full, last quarter) recur on similar dates, helping lunar and solar calendars stay synchronized. Historically, this cycle has been used to set the timing of lunar months and festivals, ensuring that lunar-based events remain consistent with the solar year over long periods.