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| Lunisolar calendar | |
|---|---|
| Name | Lunisolar calendar |
| Type | Calendar system |
| Epoch | Various epochs (e.g., Anno Domini, Islamic calendar epoch, Hebrew calendar epoch) |
| Region | Worldwide (notably China, India, Israel, Japan, Korea) |
| Start | Varies by tradition (e.g., Chinese calendar New Year, Rosh Hashanah) |
| Units | Months and years based on lunar phases and solar cycles |
Lunisolar calendar A lunisolar calendar is a calendar system that synchronizes the cycle of the Moon with the Earth's orbit around the Sun, combining lunar months with a solar year to align months with seasons. It has been adopted and adapted by diverse societies including China, India, Israel, Greece, Rome, Babylon, Mesopotamia, and Japan, producing civil, religious, and agricultural norms. Complex rules of intercalation and epoch selection have produced many regional variants and historical reforms, influencing festivals, law, and astronomy across eras such as the Hellenistic period, Medieval Europe, and the Early Modern period.
A lunisolar calendar defines a year by reconciling the synodic month, observed in cycles like those tracked by Metonic cycle observers and Hipparchus, with the tropical year used by Hipparchus and Ptolemy in Alexandria. It uses lunar months pegged to phases noted by Babylonian astronomy, Indian astronomical tradition, or Chinese astronomy, while inserting intercalary months or days following algorithms inspired by authorities such as Meton of Athens and later mathematicians in Byzantium and Islamic Golden Age centers like Baghdad and Córdoba. Epochs and month names often derive from cultural sources such as Hebrew calendar, Vedic literature, Roman reforms, and the Chinese sexagenary cycle.
Lunisolar systems emerged in antiquity among literate civilizations concerned with agriculture and ritual, including Sumer, Babylonia, and Ancient Egypt's neighbors. The Babylonian calendar influenced the Hebrew calendar and Hellenistic mechanics employed by Meton of Athens and Eudoxus of Cnidus. Roman reforms under Numa Pompilius and Julius Caesar interacted with lunisolar practices before the Julian calendar codified a solar year. In South Asia, Vedic and later classical texts such as the Surya Siddhanta and works by Aryabhata informed Hindu calendar systems. East Asian development centered on the Chinese calendar and astronomical bureaus in dynasties like the Han dynasty and Tang dynasty, later adapted by Japan and Korea. Medieval scholasticism and astronomers like Al-Biruni and Nasir al-Din al-Tusi transmitted intercalation theory across Persia and Europe.
Major families include the Hebrew calendar of Judaism, the Chinese calendar used for Chinese New Year and festivals, the Hindu calendar with multiple regional almanacs like the Panchang, and the traditional lunisolar calendars of Japan before the Meiji reforms. Other variants include the Burmese calendar in Myanmar, the Vietnamese calendar, the Korean calendar, and medieval European ecclesiastical computations culminating in the Computus for determining Easter. Differences arise in month naming (e.g., Nisan, Adar, Chaitra, Phalguna, Adar II), epoch choice (e.g., Anno Mundi, regnal eras like Taika era), and week or festival anchoring (e.g., Passover, Lent, Mid-Autumn Festival).
Intercalation reconciles lunar months with solar years via methods such as the 19-year Metonic cycle, 8-year cycles, and observational insertion used by ancient Babylonian and some Islamic-era scholars. Algorithms include arithmetic rules in the Hebrew calendar (adding an intercalary month 7 times in 19 years), astronomical rules in the Chinese calendar using solar terms (jieqi) defined by Shoujing Guo-era reforms, and complex rules in the Hindu calendar based on ayanamsa adjustments and tithis tracked by observatories like Jantar Mantar. Intercalation has been contested in councils and reforms such as the Council of Nicaea discussions influencing Computus and later state reforms in Tokugawa Japan and Meiji Restoration adjustments.
Lunisolar calendars structure major religious observances: the Hebrew calendar fixes Rosh Hashanah, Yom Kippur, and Sukkot; the Hindu calendar organizes Diwali, Holi, and regional rites; the Chinese calendar schedules Lunar New Year and the Mid-Autumn Festival; ecclesiastical lunisolar calculations determine Easter for Christianity. These calendars underpin liturgical cycles in institutions such as Temple Mount rites historically, monastic timetables in Benedictine communities, and civic agriculture timing in empires like the Ottoman Empire and Qing dynasty. Disputes about intercalation have produced schisms and reform movements, involving figures like Rabbi Hillel II and reformers in Meiji-era Japan.
Many modern states adopted the Gregorian calendar while retaining lunisolar systems for cultural and religious observance, as seen in Israel (civil vs. religious calendars), India (national holidays vs. regional almanacs), and China (official Gregorian use with traditional festivals). Reforms occurred in the 19th century and 20th century—notably the Meiji Restoration adoption of the Gregorian system, the Soviet calendar experiments, and standardizations in Ottoman and Republic of China transitions. Contemporary astronomy and software implementations draw on datasets from institutions like International Astronomical Union and national observatories (e.g., Royal Greenwich Observatory, National Astronomical Observatory of Japan), and interfaith calendrical committees continue to negotiate dates for shared observances.
Compared with purely lunar calendars like the Islamic calendar, lunisolar systems maintain seasonal alignment by adding months, preventing drift seen in lunar-only systems that cause festivals to cycle through seasons. Compared with solar calendars like the Gregorian calendar and Julian calendar, lunisolar calendars preserve the connection of months to lunar phases and cultural months (e.g., Tishri, Chaitra), at the cost of greater complexity in intercalation and variability in month lengths. Hybrid systems reflect compromises between agricultural needs in societies such as Mesopotamia, liturgical requirements in Christendom, and astronomical precision pursued from Ancient Greece through the Renaissance and into modern computational calendrics.