The smooth functioning of an ordered society depends on the possession of a means of regularizing its activities over time. That means is a calendar, and its regularity is a function of how well it models the more or less regular movements of the celestial bodies -of the moon, the sun or the stars.

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http://www.rarefile.net/7utk9ol4e2k6/Greek.RomanCalendars.ConstructionsOfTime.rar
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