Lunisolar

Lunisolar refers to calendar systems or timekeeping methods that use both the Moon’s cycles and the Sun’s position to divide time. In a lunisolar calendar, months are generally based on lunar phases, while the calendar year is kept roughly in step with the solar year by inserting intercalary months or adjusting month lengths. This aims to keep the calendar aligned with the seasons over time.

Lunisolar calendars differ from purely lunar calendars, which ignore the solar year and drift relative to the seasons, and from purely solar calendars, which do not rely on the lunar cycle. By combining both celestial cycles, lunisolar systems seek to preserve regular monthly intervals while ensuring that seasonal seasonal markers remain relatively stable.

Well-known lunisolar calendars include the Chinese calendar, the Hebrew calendar, and several traditional Hindu calendars. These systems are used for civil timekeeping as well as the dating of religious festivals and agricultural events. For example, the Chinese New Year is tied to the lunar new moon but is also kept near the start of spring through calendar adjustments; the Hebrew calendar uses leap months to ensure that Passover occurs in spring.

Astronomically, a synodic month is about 29.53 days and a solar year about 365.24 days. Because these lengths are not commensurate, intercalary months are added periodically to realign the calendar with the solar year. The Metonic cycle, a 19-year period containing 235 lunar months, provides a historical framework for this alignment in several lunisolar systems.