Ancient Egyptian Calendar

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Heliopolis (Iunu, biblical On), the ancient Egyptian city most dedicated to observing the heliacal rising of Sirius (Sopdet in Egyptian).

“Pharaoh gave Joseph … Asenath daughter of Potiphera, priest of On, as his wife … (Gen. 41:45).

Heliopolis was a major religious center serving the sun god Ra. It was the primary hub for priests and astronomers who tracked celestial events and kept records.

The heliacal rising of Sirius every 365.25 days, based on sightings, was tied to the Nile flood cycle. Many say Thebes or Memphis were observation hubs, usually based on their preference for high or low chronologies. The early chronology closely adheres to Heliopolis in the Delta — the sacred center since the Old Kingdom.

The Egyptian civil calendar is a "wandering calendar." New Year's Day occurs every 365 days, but the heliacal rising of Sirius, the bright Dog Star, occurs ostensibly every 365.25 days. Therefore, every four years, the event occurs one calendar day later. After about 1461 Egyptian years, Sirius' rising and flood season coincided with New Year's Day again. This is known as the Sothic Cycle.

Historical records reveal many Sothic dates, but it is challenging to assign exact absolute years to the risings. Even so, the cycle is another arrow in the quiver.

Censorinus wrote that an Egyptian New Year fell on July 20, 139 AD according to the Julian Calendar handed down to us. Therefore, when historical manuscripts, relics and artifacts using Egyptian dates are discovered, the equivalent Julian date can easily be determined.

Claudius Ptolemy used the Egyptian Calendar when dating the reigns of kings. Ptolemy's Cannon of Kings listed kings dating as far back as the first year of the Babylonian ruler, Nabonassar, which was Feb 26 (Thoth 1 by Egyptian reckoning.), 747 BC.

Khonu, the moon god was depicted as a mummy child or with a falcon head. He and Thoth were linked to time. Ra, the sun god, wore the solar disk, and Khonu wore the lunar disk.

The religious calendar was lunisolar. The disappearance of the waning crescent began the month at dawn rather than the visibility of the waxing crescent in the evening.

309 synodic months coincide with 25 civil years (309 x 29.5307 = 9125 = 25 x 365), which affects dating. This harmony is chiefly responsible for the high and low chonologies for the 18th Dynasty. Lesser harmonies, like 11 and 14 years, allow tangent thories.

For example: The battle of Har Megiddo occurred on the new moon in the 23rd year of Thutmose III, which was either Pachon 21 (May 9 1457 BC) or Pachon 21 (May 16, 1482 BC), 9125 days (25 years) earlier.