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Why did the ancient Egyptian 365-day civil calendar drift through the seasons?
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calendar
First complete printed title page for the Kalendarium (“Calendar”) by Regiomontanus, 1476.
The ancient Egyptian civil calendar consisted of 365 days, organized into 12 months of 30 days each plus five additional epagomenal days. This system was designed for administrative and accounting purposes rather than to track the solar year precisely.12
The calendar drifted through the seasons because the civil year was approximately one-fourth of a day shorter than the actual solar year. Because there was no leap-year mechanism to compensate for this discrepancy, the calendar fell behind the solar year by one day every four years. Consequently, the civil calendar slowly rotated through the seasons, completing a full cycle every 1,460 years, a period known as the Sothic cycle.12