Lunisolar calendar

A calendar that uses lunar months and periodically adds an intercalary month to keep its year aligned with the solar year.

Synodic month: The average time between successive new moons, about 29.53 days. Its length sets the basis for lunisolar calendar months.
Hebrew calendar: A traditional Jewish calendar with lunar months and leap months in a 19-year cycle. It uses the Metonic cycle to align festivals with their seasonal dates.
Babylonian astronomy: The systematic observation and recording of celestial phenomena in ancient Mesopotamia. Long observational records supported decisions about when to insert extra months.
Lunar calendar: A calendar organized around lunar months without systematically keeping its year aligned with the solar seasons. Unlike a lunisolar calendar, it allows calendar dates to drift through the seasons.
Tropical year: The time Earth takes to complete a seasonal cycle, averaging about 365.24 days. The calendar must track this cycle to keep months in their seasons.
Chinese calendar: A traditional Chinese calendar based on lunar months, with leap months keeping months seasonally aligned. Its leap-month rules provide a distinct East Asian implementation.
Enūma Anu Enlil: A Mesopotamian compendium of celestial omens and astronomical observations. Its traditions reflect the importance of lunar and planetary observation in calendar keeping.
Solar calendar: A calendar organized to track the seasonal year rather than the sequence of lunar months. It preserves seasonal dates without using lunar months as its basic units.
Intercalation: The insertion of an extra unit of time into a calendar to keep it aligned with an astronomical cycle. Adding a month corrects the growing gap between twelve lunar months and a solar year.
Babylonian calendar: An ancient Mesopotamian calendar of lunar months, with intercalation used to maintain seasonal alignment. Babylonian practice helped establish the recorded tradition of lunisolar intercalation.

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