The Egyptian Civil Calendar is based on 12 months of 30 days plus an intercalary month of 5 days making a total of 365 days. This calendar being out of synchronisation with a true solar year means that there is no easy conversion from an Egyptian month to a Julian or Gregorian calendar month.

This is important because we are told in the Westcar Papyrus the day that Ruddjedet will give birth:

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Nederhof, Westcar Papyrus
'When will Ruddjedet give birth?'
(And Djedi said:) 'She will give birth on the fifteenth day of the first month of the Season of Growing.'

The season of growing or Peret, There are four months in the Egyptian Calendar year prior to the first month of Peret - 120 days plus 15 days making this date 135 days into the calendar year for the given date. Important to note that it is accepted that 1 Thoth 1 (ie the first day of the first month) does not align to the heliacal rising of Sirius, so we cannot mark the 135 days from the Heliacal rise. This 135 days since 1 Thoth 1 allows us to provide a test of sorts for any proposed alignment date.

According to Wikipedia on the Egyptian Civil Calendar:

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Wikipedia - Sothic Cycle
However, because the civil calendar was exactly 365 days long and did not incorporate leap years until 22 BC, its months "wandered" backwards through the solar year at the rate of about one day in every four years.

In order to determine the date of 1 Thoth 1 for a given year, we need to establish a baseline - the Wikipedia article again offers:

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Wikipedia - Sothic Cycle
Most scholars either move the observation upon which he based this forward by one cycle of Sirius to 19 July 2781 BC or reject the assumption that the document in question indicates a rise of Sirius at all.

I assume this is a Julian date and not Gregorian. Schaefer calculates that in the epoch described the heliacal rise of Sirius would occur about three days earlier.
Testing the date for October 28, 2638 BCE, therefore we have:
101 days between 19 July and 28 October
The difference between 2781 BCE (the beginning of a Sothic Cycle according to Wikipedia) and 2638 BCE yields 143 years. Dividing this by 4 is 35.75. As the months move backwards over time by approximately 1 day every four years, this means the distance from 1 Thoth 1 for this date would be 101 + 35.75 = 136.75 days. Given potential for transcription errors, or poor weather such as clouds obscuring Sirius in the events recorded, this date being with two days of the expected date for 1 Thoth 1 is acceptable.

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Schaefer
What are the uncertainties in these dates? A variation in the extinction co-efficient by one sigma (0.09 magnitudes per air mass) results in a variation in the heliacal rise dates of typically 2 days. The year to year variation in the optimal longitude will result in variations of up to one day, with the typical variation being half a day. Any change in the observers latitude from 30 degrees North will result in close to 1 day change for every degree of difference in latitude (the heliacal rise date becomes later as the observer moves north. If the Egyptians were basing their calendar on the requirement of first sighting for a single year then clouds will provide a variable delay perhaps averaging a day or so. On top of this will be observational error, both with false alarms and missed sightings, which is a few percent of the time for similar observational tasks. In all, the observational task of recording the date of the heliacal rise of Sirius has a one-sigma scatter of a few days.

The other date that I have proposed is October 27, 2581 BC. Being 200 years from the start of the Sothic cycle, this year would see 1 Thoth 1 set 50 days prior so is 100 + 50 = 150 days into the year. Being outside the boundary of a few days, this date appears to be out quite substantially.

In conclusion, I would argue that the October 28, 2638 BCE date is a better fit for the birth of the children of Ruddjedet. It is not an exclusive date but remains the best candidate date that I have reviewed so far.

References:
Schaefer, Bradley, The Heliacal Rise of Sirius, JHA xxxi (2000) pp149-150
Wikipedia "Sothic Cycle" accessed from [en.wikipedia.org]



Edited 1 time(s). Last edit at 27-Jan-20 03:08 by engbren.