Calendar

A system for organizing days into months and years according to astronomical cycles or conventional rules. Calendars structure time for civil, religious, and agricultural purposes.

Solar day: The interval between successive passages of the Sun across a local meridian, averaging about 24 hours. The day is the basic unit calendars count and group.
Gregorian calendar: The internationally dominant solar calendar, introduced in 1582 with a revised leap-year rule. Its century-year exceptions limit the drift inherited from the Julian calendar.
Sumerian calendar: An early Mesopotamian lunisolar calendar used by Sumerian city-states. It illustrates some of the earliest recorded attempts to coordinate months and seasons.
Calendar era: A system for numbering years from a chosen reference event or starting point. A calendar's year count depends on which event its society treats as year one.
Lunar month: The interval between successive occurrences of the same lunar phase, averaging about 29.53 days. Its length motivates the alternating month patterns of lunisolar calendars.
Julian calendar: A solar calendar introduced by Julius Caesar in 46 BCE, with a leap day every fourth year. Its simple leap-year cycle gradually displaced dates from the seasons.
Egyptian calendar: An ancient civil calendar of 365 days, organized into twelve months and five additional days. Its fixed year shows an early solution that did not track leap years.
Anno Domini: A year-numbering convention that counts from a traditional estimate of the birth of Jesus. Its adoption helped make one era common in international historical dating.
Tropical year: The time Earth takes to complete a seasonal cycle, about 365.24 days. Calendars tied to seasons must approximate this non-integer number of days.
Islamic calendar: A lunar calendar of twelve months whose year is about eleven days shorter than the solar year. Its months follow lunar phases without intercalation to track the seasons.

Linked from 24 pages