At its core, the modern calendar is an attempt to track and predict the relationship between the sun and various regions of the earth. Historically, agricultural cycles, local climates, latitudes, tidal ebbs and flows and imperatives such as the need to anticipate seasonal change have shaped calendars. The Egyptian calendar, for example, was established in part to predict the annual rising of the Nile River, which was critical to Egyptian agriculture. This motivation is also why lunar calendars similar to the ones still used by Muslims fell out of favor somewhat -- with 12 lunar cycles adding up to roughly 354 days, such systems quickly drift out of alignment with the seasons.
The Gregorian calendar, introduced by Pope Gregory XIII in 1582, was itself an attempt to address the problems of its predecessor, the Julian calendar, which had been introduced by Julius Caesar to abolish the use of the lunar year and eliminate a three-month gap that opened up between the civil and astronomical equinoxes. It subsequently spread throughout the Roman Empire (and beyond as Christianity spread) and influenced the design of calendars elsewhere. Though it deviates from the time it takes the earth to revolve around the sun by just 11 minutes (a remarkable astronomical feat for the time), the Julian system overly adjusted for the fractional difference in year length, slowly leading to a misalignment in the astronomical and calendar years.
Read more: The Geopolitics of the Gregorian Calendar | Stratfor
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