Egyptian calendar

The ancient civil Egyptian calendar had a year that was 365 days long. The year consisted of 12 months of 30 days each, plus five extra days (epagomenae, from Greek ἐπαγόμεναι) at the end of the year. The months were divided into three weeks (decans) of ten days each. Because the ancient Egyptian year was almost a quarter of a day shorter than the solar year and stellar events therefore "wandered" through the calendar, it has been referred to as the annus vagus, or "wandering year".

A tablet from the reign of First Dynasty King Djer (c. 3000 BC) was conjectured by early Egyptologists to indicate that the Egyptians had already established a link between the heliacal rising of Sirius (Egyptian Sopdet, Greek Σείριος Seirios) and the beginning of the year. However, more recent analysis of the pictorial scene on this tablet has questioned whether it actually refers to Sothis at all. Current knowledge of this period remains a matter more of speculation than of established fact.

The Egyptians may have used a luni-solar calendar at an earlier date, with the intercalation of an extra month regulated either by the heliacal rising of Sothis or by the inundation of the fields by the Nile. The first inundation according to the calendar was observed in Egypt's first capital, Memphis, at the same time as the heliacal rising of Sirius. The Egyptian year was divided into the three seasons of Akhet (Inundation), Peret (Growth - Winter) and Shemu (Harvest - Summer).

The heliacal rising of Sothis returned to the same point in the calendar every 1,460 years (a period called the Sothic cycle). The difference between a seasonal year and a civil year was therefore 365 days in 1,460 years, or one day in four years. Similarly, the Egyptians were aware that 309 lunations nearly equaled 9,125 days, or 25 Egyptian years, which was later used in the construction of a secondary lunar calendar that did not depend on observations.

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