Metonic cycle

The Metonic cycle is a period of about 19 years after which the lunar phases (new moon, full moon, etc.) repeat on the same calendar dates, nearly perfectly aligned with the solar year. This happens because 19 solar years closely match 235 lunar months. Essentially, it allows calendars to synchronize lunar months with the solar year, which is why some ancient calendars, like the Greek and Jewish calendars, use this cycle to coordinate months and seasons. It’s a key concept for understanding how lunar and solar cycles interconnect over time.