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| Calendars | |
|---|---|
| Name | Calendars |
| Type | Timekeeping system |
| Introduced | Various |
Calendars are organized systems for naming and arranging days for social, religious, agricultural, and administrative purposes. They provide repeating cycles of days, weeks, months, and years used to coordinate events, record history, and align civil activity with astronomical phenomena. Major calendar systems have influenced law, commerce, navigation, and ceremonial life across regions such as Ancient Egypt, Babylon, Rome, China, and Maya civilization.
Calendars reconcile cycles apparent in the sky—such as those observed by Hipparchus, Ptolemy, and Nicolas Copernicus—with human social needs. Systems like the Julian calendar and Gregorian calendar reflect reforms by figures and institutions including Julius Caesar, Pope Gregory XIII, and the Roman Senate. Regional variants such as the Chinese calendar, Hebrew calendar, Islamic calendar, and Maya calendar demonstrate adaptations to agricultural cycles, lunar phases, and ritual cycles in places like Imperial China, Ancient Israel, Medina, and Mesoamerica.
Solar calendars, including the Egyptian calendar and the Gregorian calendar, base years on the Earth's orbit around the Sun as studied by astronomers like Johannes Kepler and Edmond Halley. Lunar calendars, such as the Islamic calendar used in Saudi Arabia and by communities in Indonesia and Pakistan, track months by the phases exploited by observers from Babylon to Mecca. Lunisolar calendars, exemplified by the Hebrew calendar and the Chinese calendar, insert intercalary months similar to the practice in Ancient Greece and reforms by Meton of Athens. Fixed and reform proposals include the French Republican Calendar instituted after the French Revolution and proposals by Isaac Newton and John Herschel for perennial systems.
Core components include the day, the week, the month, and the year as formalized in legal codes and scholarly works from Justinian I to William of Ockham. Week structures—such as the seven-day week linked to planetary nomenclature in Babylon and adopted in Roman Empire Christianity—affect scheduling in places like United Kingdom and Japan. Month lengths and leap rules—e.g., the centurial rules in the Gregorian calendar—derive from observations of the tropical year by astronomers at institutions like the Vatican Observatory and the Royal Greenwich Observatory. Epochs and era systems—such as Anno Domini introduced by Dionysius Exiguus, regnal years used in England and Japan, and the Mesoamerican Long Count—anchor historical dating used by historians of Medieval Europe and Pre-Columbian America.
Early timekeeping appears in monuments from Göbekli Tepe and artifacts from Neolithic Europe while calendrical inscriptions appear in Sumer and on Babylonian tablets attributed to scribes of Hammurabi's era. The Egyptian 365-day civil year influenced Ptolemaic Egypt and was central to agricultural planning on the Nile River. Roman reforms by Numa Pompilius and later by Julius Caesar produced the Julian system that governed Europe until the papal reform of Pope Gregory XIII corrected accumulated drift. Innovations in Mesoamerica by the Maya produced interlocking cycles like the Tzolk'in and Haab', while East Asian calendars evolved through astronomical treatises commissioned by dynasties such as the Han dynasty and the Tang dynasty.
Religious observances depend on calendars: Passover timing in Judaism uses the Hebrew lunisolar rules; Ramadan in Islam follows the purely lunar cycle observed in Mecca and communities worldwide; Easter calculation (computus) was systematized by church councils including the Council of Nicaea and influenced by scholars like Bede and Dionysius Exiguus. Other ritual years—such as the Hindu calendar variations across India and the Buddhist calendar in Thailand—guide festivals, monastic seasons, and pilgrimage itineraries to sites like Varanasi and Lumbini.
The widespread adoption of the Gregorian calendar was driven by nation-states like Spain, France, United Kingdom, and later Russia and Greece, affecting legal systems and international commerce mediated by bodies such as the International Telegraph Union and conferences leading to the ISO 8601 standard. Industrialization and global trade tied civil calendars to economic centers including London and New York City. Calendar reform movements—advocated by organizations such as the World Calendar Association and individuals like Elijah H. Wang—have proposed alternatives to better align fiscal, educational, and administrative cycles across entities like United Nations agencies and multinational corporations.
Astronomical bases for calendars employ observations and models developed by Tycho Brahe, Galileo Galilei, and Edmond Halley to measure solar year and lunar month lengths; modern ephemerides produced by agencies like NASA and the Jet Propulsion Laboratory refine these parameters. Computational algorithms—for example those taught in texts by Jean Meeus and implemented in software libraries used by European Space Agency missions—calculate leap years, Easter dates, and conversion between systems such as Julian Day Numbers used by astronomers at the Harvard-Smithsonian Center for Astrophysics and historical chronologists working with archives from Vatican Secret Archives and the British Library.