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The Sothic cycle is a period of 1461 years, the calculated time that it takes the 365-day year of the ancient Egyptians to lose enough time that it the start of its year would once again coincide with the rising of the star Sirius. This rising was believed to mark 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 Calendar was not only 365 days long, but did not have any intercalary days added to keep it in alignment with the Julian calendar of 365.25 days long. (The Tropical year and Sidereal year vary from this figure by 0.01 days, so this is accurate enough for the following exposition.) As a result, the first day of the year slowly arrived earlier as the years progressed, until after 1461 years it finally fell at the same point in time, as measured by the seasons or the stars.
This phenomena 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 is based, one of which is believed to date to the 7th year of Senusret III. From his calculations, scholars commonly believed that the Egyptian civil calendar was created in 4121 BC, a date that appears in a number of old books; research and discoveries have since shown that the First dynasty of Egypt did not begin before c.3100 BC, and the claim that this 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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