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| Lunar calendar | |
|---|---|
| Name | Lunar calendar |
| Type | Calendar based primarily on lunar cycles |
| Epoch | Various |
| First | Antiquity |
| Structure | Months based on synodic months |
| Usage | Religious, cultural, civil |
Lunar calendar
A lunar calendar is a calendar system that bases its months chiefly on the phases of the Moon. It contrasts with solar calendar systems and with lunisolar calendar systems that intercalate to stay aligned with the Sun. Societies from Mesopotamia to East Asia and the Islamic world have used lunar-based reckoning for civil, ritual, and agricultural purposes.
Lunar calendars count time by successive lunar phases such as new moons and full moons observed across cycles defined by the synodic month, the interval between successive new moons measured in relation to Earth and the Sun. Variants arose independently in regions including Ancient Egypt, Babylon, China, India, Maya, Arabia, and Greece, each integrating the lunar cycle with local institutions like royal courts, temple rites, and agricultural festivals. Prominent historical actors tied to lunar observance include the Sumerians, Babylonians, Zhou dynasty, Gupta Empire, Byzantine Empire, and the medieval scholars of al-Andalus.
Strict lunar calendars, lunisolar calendars, and ritual or ecclesiastical lunar systems form principal categories. A strict lunar calendar such as that used in the Islamic calendar omits intercalation, causing months to migrate through seasons, marking observances like Ramadan and the Hajj relative to lunar months. Lunisolar systems like the traditional calendars of China, Hebrew, and pre-modern India insert intercalary months following rules developed by bodies such as the Sanctuary authorities and later by astronomers including Al-Battani, Al-Biruni, and Aryabhata to reconcile lunar months with solar years. Ecclesiastical lunar rules used by Christianity for determining Easter rely on computus methods developed in the First Council of Nicaea and refined by figures such as Dionysius Exiguus and Cambridge University scholars in medieval Europe.
Lunar calendars structure months around the synodic month (~29.53 days), producing alternating 29- and 30-day months in many traditions to approximate the true lunar cycle. Calculation methods involve observational sighting of the crescent moon used historically by communities under the influence of authorities like the Ottoman Empire and modern national bodies such as the Saudi Arabian Supreme Court for declaring months. Computational alternatives employ astronomical mean lunations, epacts, and the Metonic cycle attributed to Meton of Athens to align lunar months with solar years in lunisolar calendars; the Metonic cycle underpins the Hebrew calendar and was known to Hipparchus and later to Maimonides. Precision improvements emerged from the adoption of positional astronomy by scholars like Ptolemy, Ulugh Beg, and European observatories including Greenwich Observatory.
Lunar calendars underpin major rites and festivals across many faiths and cultures. In Islam, the observation or calculation of the new moon governs fasting and pilgrimage timings, while in Judaism months such as Nisan and holidays like Passover depend on a lunisolar reckoning maintained by rabbinic institutions including the Sanhedrin historically and modern rabbinic authorities. East Asian festivals such as the Chinese New Year and Mid-Autumn Festival derive from the traditional Chinese lunisolar calendar and are embedded in dynastic histories like the Han dynasty. In South Asia, Hindu festivals such as Diwali and monthly observances are scheduled by Panchangam traditions linked to astronomical treatises by Varahamihira. Indigenous calendars such as those of the Maya also integrate lunar counts into ritual cycles recorded on stelae and codices.
Lunar reckoning appears in early written records from Sumer, Akkad, and Babylon, where priestly astronomers developed intercalation schemes to keep religious months aligned with seasons for agriculture and temple rites. Greek and Hellenistic astronomers transferred lunar theory into Mediterranean practice via figures like Aristarchus of Samos and Hipparchus, influencing Roman reforms and later Byzantine computation. Islamic Golden Age scholars preserved and extended lunar and planetary tables in works by Al-Khwarizmi and Al-Battani, shaping medieval European calendrical reform through translations in Toledo and institutions such as University of Paris. The early modern adoption of the Gregorian calendar by Catholic states affected the interface between lunar-based liturgical observances and civil timekeeping across Europe and its colonies.
Lunisolar calendars explicitly mediate between lunar months and the solar year by adding intercalary months or days; institutions like the Hebrew calendar and the traditional Chinese calendar use cycles and agricultural markers such as solstices and equinoxes observed at sites like Stonehenge historically. Strict lunar systems like the Islamic calendar lack solar correction, producing a 33-year cycle through the seasons and prompting administrative challenges in states such as the Ottoman Empire and modern nations when aligning fiscal or agricultural planning with lunar months. Hybrid approaches appear in civil reforms—examples include attempts by Kemal Atatürk in Turkey and by revolutionary governments in Iran to modify or standardize calendar systems for administrative coherence.
Contemporary lunar or lunisolar calendars remain central to religious life and cultural identity worldwide: the Islamic calendar for Muslim communities, the Hebrew calendar for Jewish communities, the traditional Chinese calendar for festivals in China, Taiwan, Vietnam, and Korea, and regional systems such as the Burmese calendar and the Bengali calendar for cultural and agricultural events. National governments and religious authorities—examples include the Saudi government, State of Israel, and municipal councils in Hong Kong—issue official declarations or rely on fixed computational rules maintained by observatories like the US Naval Observatory and national academies of sciences. Scholarly work in astronomy and computational calendrics continues at institutions such as International Astronomical Union and university departments worldwide to improve precision and reconcile tradition with modern civil timekeeping.