The ancient Egyptians measured the passage of time in relative terms: Day 10 of the first month of winter, Year 4 of King X. With the coronation of a new king, the year returned to 0. However, Akhenaten and several other pharaohs ruled for a time as co-regents, meaning the reign of two kings overlapped. In addition, we simply don't have enough information about the lengths of some kings' reigns to place them on a timelinein some cases the priests who wrote the ancient chronologies and kinglists deliberately omitted rulers they would rather forget. How can we then assign absolute dates to such remote events? When the ancient historians fail us, we look to the ancient astronomers.
The Sothic Cycle
The ancient Egyptian civil year was supposed to begin the day that the brilliant star Sirius ("Sothis" to the Egyptians) could just be seen on the eastern horizon before the rising of the sun, July 19 or 20. The Egyptian calendar consisted of 12 months of 30 days, with 5 "epagomenal" days to round out the year. Because the extra quarter day was not included, the Egyptian calendar advanced by one day every four years, and so the coincidence of New Year's Day and this heliacal rising of Sothis occurred only once every 1460 years. This period is known as the Sothic cycle. Thus dated documents that refer to the rising of Sothis can be converted into the modern calendar by multiplying the number of days elapsed since the first of the year by four, and then subtracting this result from the date of the beginning of that Sothic cycle. Thanks to the Roman historian Censorinus, we know that the civil New Year's Day and the heliacal rising of Sothis coincided in 139 CE. Accounting for a slight difference between a Sothic year and a solar year, the other Sothic cycles began in 1322, 2782, and 4242 BCE.
There are six documents from Egypt fixing the date of the rising of Sothis, but only three can be used reliably. The first is a letter to a priest from Kahun, warning him that Sothis will rise on the 16th day of the 8th month of Year 7. The king involved is Sesostris III of the 12th Dynasty. This day corresponds to 1866 BCE. The second is a medical papyrus from the beginning of the Eighteenth Dynasty. In a calendar added to the text (perhaps to ensure a correct conversion of dates given in the prescriptions), the heliacal rising of Sothis was supposed to have occured on the 9th day of the 11th month of Year 9 of Amenhotep I. Unfortunately it is not clear whether this observation was taken at Memphis, the old capital, or Thebes, the new capital of the Eighteenth Dynasty. If at Memphis, this gives a date of 1538 BCE, but if at Thebes, twenty years must be added, bringing the date to 1518 BCE. The third date shows that sometime during the reign of Thutmose III, who reigned for some 54 years, Sothis rose on the 28th day of the 11th month, which occurred in 1458 BCE (observed from Memphis) or 1438 BCE (observed from Thebes). Hence this year fell during his reign.
The Lunar Cycle
Based on the data we obtain from Sothic observations, it is possible to fairly accurately reconstruct the calendar of the Twelfth Dynasty, since this section of the ancient list of kings known as the Turin papyrus (which gives the lengths of kings' reigns) is nearly complete. However, the dating of the Eighteenth Dynasty is complicated by the different dates given by different observation points. It is possible to correct these using the so-called lunar cycle: sometimes Egyptian observers noted a certain day was "exactly new moon", and this cycle has a length of only 25 years. However, the ancient observers were not as "exact" as they claimed, especially since discrepencies can appear even within the same document.
Sometimes confirmation of dates can be found in Assyrian, Babylonian, or other ancient sources and king lists, especially later in history. In Memphis, the birth, coronation, and death dates of the Apis bulls were scrupulously noted as well, providing a point of comparison with political events. However, predictably, the accuracy of dates diminishes, even when astronomical data is available, as one looks further back into history. For now, the beginning of the Old Kingdom is dated with reasonable certainty to about 3090 BCE, the beginning of the Eleventh Dynasty to 2133 BCE, and the beginning of the Middle Kingdom and Twelfth Dynasty to 1991 BCE. The New Kingdom began either in 1567 or 1552 BCE, depending on whether Ramses II took the throne in 1290 or 1304 BCE (a difference of one lunar cycle).
Aurelien Joly is a Tunahead.