|Old Kingdom of Egypt astronomy|
- The Egyptians already had a lunar calendar
- to define the liturgical year.
- Their civil calendar based on the heliacal rising of Sirius
- gave them the ability to predict the flooding of the Nile.
- Three seasons changed with the behavior of the Nile,
- each with four months of thirty days.
- The season of Flood when the Nile floods.
- The season of Growth when the land emerges
- and crops are planted.
- The season of Low Water
- when crops are harvested.
- Each month with three weeks of ten days,
- each week marked by the rising of a decan,
- one of the thirty-six constellations that ring the sky.
- Because the solar year has about 365 days,
- the Egyptians added a thirteenth month of five days,
- each to celebrate the birthday of a god—
- Osiris, Horus, Set, Isis, Nephthys.
Drifting from a time
- The Egyptian calendar didn’t have a leap year,
- so it drifted by a day every four years. The rising of Sirius,
- initially on the first day of the first month of the season of flooding,
- was on the second day after four years, and so on,
- so having a record of Sirius appearing on 21 July 139
- Censorinus established when the calendar began.
- However, uncertainties abound, for example
- whether Censorinus fudged his the date to flatter his patron,
- and whether in the brief moment before sunrise Sirius could reliably be seen,
- and known inaccuracies, for example that a period of 1460 years
- doesn’t account for minor shifts of Sirius relative to the solar year,
- and that the return of Sirius varies by the latitude of the observer.
Flooding of the Nile
- Isis cries for Osiris.
- Her dead husband Osiris,
- the god of the afterlife,
- the green god of rebirth and regeneration,
- floats from the unknown world of the south
- and gives life to the people of Egypt.