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| Calendar | |
|---|---|
| Name | Calendar |
| Introduced | Antiquity |
| Type | System for organizing days |
Calendar
A calendar is a system for organizing days for social, commercial, and religious purposes; it coordinates timekeeping across societies such as Ancient Egypt, Babylon, Rome, China, and India and underpins institutions like the United Nations, International Organization for Standardization, European Union, Islamic Republic of Iran and People's Republic of China. Calendars enable scheduling for events like the Olympic Games, Easter, Ramadan, Diwali, and national observances such as Independence Day (United States), Bastille Day, and Chinese New Year, and they are implemented in technologies produced by companies like Microsoft, Google, Apple Inc., and standards bodies including ISO 8601.
A calendar translates astronomical cycles observed by cultures including the Maya civilization, Babylonian Empire, Roman Republic, Han dynasty, and Vedic period into recurring civil intervals used by authorities such as the Roman Catholic Church, Ottoman Empire, British Empire, Soviet Union, and Japanese government. It reconciles motions of bodies like the Sun, Moon, and planets observed by astronomers such as Hipparchus, Ptolemy, Al-Battani, and Tycho Brahe to create systems used in legal codes like the Gregorian reform and administrative structures in states including France (French Republic), United Kingdom, Ottoman Empire, and Mongol Empire.
Development traces to ancient calendars such as the Egyptian calendar, Sumerian calendar, Babylonian calendar, and the Maya calendar, refined by reforms under figures like Julius Caesar, Pope Gregory XIII, Akbar the Great, and Ulugh Beg. The Julian calendar introduced by Julius Caesar replaced earlier Roman systems administered by officials such as the Pontifex Maximus; the later Gregorian calendar promulgated by Pope Gregory XIII corrected drift against equinoxes noted by astronomers like Christopher Clavius. Other historical systems include the Hebrew calendar used by Kingdom of Judah, the Islamic calendar established in the early Rashidun Caliphate, the Chinese calendar shaped across dynasties such as the Han dynasty and Tang dynasty, and the Hindu calendar traditions codified in texts linked to the Maurya Empire and Gupta Empire.
Calendrical schemes fall into solar, lunar, lunisolar, and other categories exemplified by the Gregorian calendar (solar), Islamic calendar (lunar), Hebrew calendar (lunisolar), and the Maya Long Count (Mesoamerican cyclical). Systems include the Julian calendar, Revised Julian calendar, Persian calendar (Solar Hijri), Baha'i calendar, Ethiopian calendar, Coptic calendar, and regional systems such as the Japanese calendar (era name system) and the Korean calendar. Specialized constructs appear in proposals like those from Isaac Newton-era scholars, the Committee for the Scientific Examination of the Gregorian Reform-style groups, and modern standardizers like International Organization for Standardization producing ISO 8601.
Core elements include intercalation methods used by authorities like Meton of Athens and Sosigenes of Alexandria, epoch choices as in the Anno Domini system devised by Dionysius Exiguus, and naming conventions such as Roman month names preserved in systems used by Venice, Byzantine Empire, and contemporary states. Units comprise days counted by observatories like Greenwich Observatory and institutions such as the Royal Greenwich Observatory, weeks aligned with religious cycles in Judaism, Christianity, and Islam, months tied to lunar phases studied by Al-Biruni and Nasir al-Din al-Tusi, and years synchronized with the tropical year determined by astronomers including Simon Newcomb and G. V. Schiaparelli.
Religious calendars govern liturgical and festival cycles in Roman Catholic Church, Eastern Orthodox Church, Sunni Islam, Shia Islam, Judaism, Hinduism, Buddhism, and Sikhism, scheduling observances like Easter, Passover, Yom Kippur, Ramadan, Eid al-Fitr, Vesak, Navaratri, and Guru Nanak Gurpurab. Cultural calendars include the Chinese calendar used in festivals across Mainland China, Taiwan, Singapore, and Malaysia, the Ethiopian calendar in Ethiopia, the Mayan calendar in Mesoamerican heritage sites such as Tikal and Chichen Itza, and regional agricultural timetables found in societies like the Inca Empire and the Aztec Empire.
Reform efforts span interventions like the Julian calendar reform under Julius Caesar, the Gregorian calendar reform under Pope Gregory XIII implemented across polities including Spain, Portugal, Poland, and later Great Britain; proposals include the World Calendar, International Fixed Calendar advocated by Augustus B. Blakelock-era movements, and 20th-century debates involving bodies such as the League of Nations and United Nations. Standardization relies on institutions like ISO with ISO 8601 for international data interchange, and on observatories such as the International Earth Rotation and Reference Systems Service coordinating leap seconds affecting time systems like Coordinated Universal Time.
Modern civil calendars underpin global commerce, finance, and computing in contexts like New York Stock Exchange, London Stock Exchange, Tokyo Stock Exchange, and institutions such as World Bank, International Monetary Fund, European Central Bank, and Federal Reserve System. Calendars are embedded in software from Microsoft Outlook to Google Calendar and mobile platforms by Apple Inc., and they guide scheduling for international events like the United Nations General Assembly, FIFA World Cup, Summer Olympic Games, and COP climate conferences. Scientific applications include ephemerides produced by Jet Propulsion Laboratory, timekeeping standards at NIST, astronomical almanacs from Royal Observatory, Greenwich, and calendrical research by scholars at universities such as University of Cambridge, Harvard University, Peking University, and University of Tokyo.