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The Sothic cycle is a period of 1461 ancient Egyptian years, the time that it takes for their 365-day year to lose enough time that the start of the year would once again coincide with the heliacal rising of the star Sirius, called Sothis in Greek. A single year between heliacal risings of Sothis is a Sothic year. This rising occurred within a month or so of the beginning of the Nile flood, a matter of primary importance to this agricultural society. It is believed that Ancient Egyptians followed both a 365-day solar civil calendar and a lunar religious calendar.
This cycle is based on the assumption that the ancient Egyptian year was not only 365 days long, but did not have any intercalary days added to keep it in alignment with the Sothic year. The Sothic year is a kind of sidereal year. Normally, a sidereal year is considered to be 365.25636 days long, but that only applies to stars on the ecliptic, or the apparent path of the Sun. Because Sirius is not on the ecliptic, the wobbling of the celestial equator and hence of the horizon at the latitude of Egypt causes the Sothic year to be slightly smaller. Indeed, it is almost exactly 365.25 days long, the average number of days in a Julian year. Thus 1461 Egyptian years equal 1460 Julian years.
This cycle was first noticed by Eduard Meyer in 1904, who then carefully combed the known Egyptian inscriptions and written materials to find any mention of the calendar date when Sirius rose. He found six of them, on which the dates of much of the conventional chronology are based, one of which is believed to date to the 7th year of Senusret III . Meyer concluded that the Egyptian civil calendar was created in 4121 BC, a date that appears in a number of old books. But research and discoveries have since shown that the first dynasty of Egypt did not begin before c.3100 BC, and the claim that 4121 BC is the "earliest fixed date" has been discredited.
A number of criticisms have been levelled against the reliablity of dating by the Sothic cycle. Some are serious enough to warrant consideration (e.g., was the civil year unchanged through the thousands of years of Egyptian history?), while others are not (e.g., there is no certain mention of the Sothic Cycle in contemporary Egyptian writing).
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