History of calendars

The history of calendars traces how societies organized days, months, and years using astronomical cycles, political authority, and cultural traditions.

Lunar calendar: A calendar whose months follow the phases of the Moon. Lunar cycles provided one of the earliest widely observed measures of time.
Lunisolar calendar: A calendar that follows lunar months while using adjustments to keep its year aligned with the seasons. Many historical calendars reconciled Moon-based months with the solar year this way.
Julius Caesar: Roman general and statesman who instituted a solar calendar reform in 46 BCE. His reform established the Julian calendar across the Roman world.
Calendar era: A system for numbering years from a chosen event or reference point. Different year-counting origins preserve distinct religious and political histories.
Egyptian calendar: An ancient civil calendar of twelve 30-day months and five additional days. Its fixed 365-day year linked timekeeping to the Nile’s seasonal cycle.
Intercalation: The insertion of an extra day or month to keep a calendar aligned with an astronomical cycle. Extra months or days corrected drift in calendars built from uneven natural cycles.
Sosigenes of Alexandria: An ancient astronomer traditionally credited with advising Julius Caesar on calendar reform. His astronomical advice is associated with the Julian calendar’s leap-year system.
Anno Domini: A year-numbering convention that counts from the traditionally calculated birth of Jesus. Its spread made one Christian chronology influential far beyond its original setting.
Maya calendar: A system of interlocking Maya cycles for tracking ritual and solar time. Maya timekeeping developed independently into a notably complex calendar system.
Metonic cycle: A period of 19 solar years that closely matches 235 lunar months. This cycle offered a regular rule for inserting months into lunisolar calendars.

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