| abstract
| - The ancient civil Egyptian calendar, known as the Annus Vagus or "Wandering Year", 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 into 3 "weeks" of ten days each. This calendar was in use by at least 2700 BCE, but probably before that. A text from the reign of First Dynasty King Djer indicates that the Egyptians had already established a link between the heliacal rising and the beginning of the year. The Egyptians seem to have used a lunar calendar at an earlier date, but when they discovered the discrepancy between the lunar calendar and the actual passage of time, they switched to a calendar based on the Nile inundation. 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 (Egyptian Sopdet, Greek Sothis). The Egyptian year was divided into the three seasons of akh.t (Inundation), pr.t (Growth - Winter) and shomu (Harvest - Summer). The heliacal rising of Sothis returned to the same point in the calendar every 1460 years (a period called the Sothic cycle). The difference between a seasonal year and a civil year was therefore 365 days in 1460 years, or 1 day in 4 years. Similarly, the Egyptians were aware that 309 lunations nearly equalled 9125 days, or 25 Egyptian years, which was likely used in the construction of the secondary lunar calendar. For much of Egyptian history, the months were not given individual names but rather were numbered within the three seasons. As early as the Middle Kingdom, however, each month was given its own name. These finally evolved into the New Kingdom months, which in turn gave rise to the Hellenized names that were used among others for chronology by Ptolemy in his Almagest. Astronomers in the Middle Ages used it as well because of its mathematical regularity—Copernicus for example constructed his tables for the motion of the planets based on the Egyptian year. The convention amongst modern Egyptologists is to number the months consecutively using Roman numerals. According to Roman writer Censorinus, the Egyptian New Year's Day fell on July 20 in the Julian Calendar in 139 CE, which was a heliacal rising of Sirius in Egypt. From this it is possible to calculate that the previous occasion on which this occurred was 1322 BCE, and the one before that was 2782 BCE. This latter date has been postulated as the time when the calendar was invented, but Djer's reign preceded that date. Other historians push it back another whole cycle, to 4242 BCE. In 238 BCE, the Ptolemaic rulers decreed that every fourth year should be 366 days long rather than 365. The Egyptians, most of whom were farmers, did not accept the reform as it was the agricultural seasons that made up their year. The reform eventually went into effect with the introduction of the "Alexandrian calendar" by Augustus in 26/25 BCE, which included a sixth epagomenal day for the first time in 22 BCE. The reformed Egyptian calendar continues to be used in Egypt as the Coptic calendar of the Egyptian Church and by the Egyptian populace at large, particularly the fellahin to calculate the agricultural seasons. Contemporary Egyptian farmers, like their ancient predecessors, divide the year into three seasons, namely winter, summer and inundation. It is also associated with local festivals such as the annual Flooding of the Nile and the ancient Spring festival sham en nisim. The Ethiopian calendar is based on this calendar but uses Amharic names for its months and uses a different era. The French Republican Calendar was similar, but began its year at the autumnal equinox. British orrery maker John Gleave represented the Egyptian calendar in a reconstruction of the Antikythera mechanism.
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