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Egyptian calendar

The ancient Egyptian civil calendar had a year that was 365 days long, consisting of 12 months of 30 days each, plus 5 extra days at the end of the year. The months were divided in to 3 "weeks" of ten days each.

This calendar was in use prior to 2400 B.C.E, and possibly before that. It was used throughout antiquity. It was used by astronomerss in the Middle Ages because of it's mathematical regularity.

The Egyptian calendar was simple, but it is neither a lunar nor a solar calendar. Months do not correspond to lunar months, and years do not correspond to solar years. The Egyptians were aware of this, and calculated their seasonal year by the stars, to be the time between successive heliacal risings of the star Sirius (which the Egyptians called Sothis). The heliacal rising of Sothis returned to the same point in the calendar every 1460 years (a period called the Sothis period). The difference between a seasonal year and a civil year was therefore 365 days in 1460 years, or 1 day in 4 years. In 238 B.C.E., it was decreed that every 4th year should be 366 days long rather than 365. That practice was not followed, however, until the introduction of the "Alexandrian calendar" in 25 B.C.E. Calendars in use today (the Coptic calendar and the Ethiopian calendar) are similar, as was the French Revolutionary calendar.