Adding up regnal years gives a chain of reigns, but the chain needs to be pinned to our calendar somewhere. A few Egyptian texts record events in the sky, and the sky runs like a clock. If an ancient observation can be matched to an exact astronomical date, a reign can be tied to a year BC.
Key idea
Why it works
Worked example: How fast the calendar drifts
- One day of drift takes 4 years.
- 30 days of drift take 30 × 4 years.
- 30 × 4 = 120.
Answer: About 120 years.
Worked example: A whole cycle
- One day takes 4 years, so 365 days take 365 × 4 years.
- 365 × 4 = 1,460.
Answer: About 1,460 years (the "Sothic cycle", from Sothis, the Greek name of Sirius).
A Roman writer, Censorinus, recorded that the Egyptian new year and the rising of Sirius fell together in AD 139. Counting back from that fixed point, scholars can date older records of Sirius: one from the reign of Senusret III (the Illahun papyri) and one from Amenhotep I (the Ebers papyrus).
Why the answer moves by decades
- Sirius rises a few days later in the south than in the north. Was the observation made at Memphis, Thebes or Elephantine? Each choice shifts the date by decades.
- Moon dates repeat every 11 or 25 years or so. For Ramesses II, the lunar data allow an accession in 1304, 1290 or 1279 BC; most scholars now prefer c. 1279 BC.
- Scholars disagree about details of how the calendar was kept.
Common mistake