Lunar calendar

A calendar whose months follow the Moon’s phases, usually alternating between 29 and 30 days. A purely lunar year is shorter than the solar year.

Synodic month: The average time between successive identical phases of the Moon, about 29.53 days. Its length sets the duration of a month in a phase-based calendar.
Islamic calendar: A religious calendar of twelve lunar months, with no regular leap month to keep it aligned with the seasons. It is a prominent example of a calendar that remains purely lunar.
Babylonian astronomy: The systematic observation and mathematical study of celestial phenomena in ancient Babylonia. Babylonian records helped establish methods for predicting lunar cycles and managing months.
Ramadan: The ninth month of the Islamic calendar, observed by Muslims as a month of fasting. Its dates shift across the solar year because the Islamic calendar is purely lunar.
Solar calendar: A calendar whose year is designed to track the seasonal cycle of the Sun. It keeps dates near the same seasons, unlike a purely lunar calendar.
Lunar phase: The changing appearance of the Moon’s illuminated portion as seen from Earth. Visible phases provide the cycle that lunar months track.
Hebrew calendar: A lunisolar calendar that keeps months tied to lunar phases and years aligned with the seasons. Its leap months show how lunar months can coexist with seasonal dating.
Metonic cycle: A period of 19 solar years that is nearly equal to 235 synodic months. This relationship offered a practical cycle for coordinating lunar months with seasons.
Eid al-Fitr: The Muslim festival marking the end of Ramadan and the beginning of Shawwal. Its date depends on the lunar month boundary that follows Ramadan.
Lunisolar calendar: A calendar that follows lunar months and uses intercalation to keep its year aligned with the solar seasons. It shares lunar months but avoids the seasonal drift of a purely lunar system.

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