Figure of Ramesses II at the entrance to one of the temples at Abu Simbel. Image by Nick Peretti via Wikimedia Commons.
The society I’ve discussed most extensively so far is Rome. After all, that’s where the Gregorian calendar, which we all use, comes from.
Rome was heavily influenced by Greece, the birthplace of Western civilization. The Greeks, in turn, were well aware that there was a land far more ancient than their own, with more magnificent works of architecture.
That land was Egypt.
Egypt was under Greek rule from the time Alexander the Great invaded in 332 BCE until the Romans took over in 30 BCE. The Greeks in Egypt marveled at the towering monuments all along the Nile. The temple at Karnak was the largest in the world. But the top attractions were, and still are, the pyramids and sphinx in Giza. When the Parthenon was built in Athens nearly 2500 years ago, the Giza pyramids were already 2000 years old.
In 2500 BCE, the pharaohs of the Old Kingdom could get 10,000 people to toil for 30 years to build a pyramid. What they couldn’t do was command the people to switch to a new calendar. Matters like that had to be handled delicately.
Until then, the Egyptians, like the Mesopotamians, followed a lunar calendar. The months were based on observed lunar phases which meant they could have either 29 or 30 days. A thirteenth month was added to the religious calendar every few years to bring it in line with the seasons.
While the Mesopotamians used the shadows of a gnomon to measure the length of the year, the Egyptians appear to have used observations of Sirius, the brightest star in the sky after the Sun. Sirius is visible in the Egyptian sky for nearly 300 days of the year. The year began when Sirius reappeared after going missing for a couple of months.
And just like in Mesopotamia, Egypt appears to have originally also had a separate 360 day calendar with twelve 30-day months.
Here is where the two stories completely diverge. In Mesopotamia, the 360 day calendar was only used by the astronomer-priests to make their astrological calculations easier. The observation-based lunar calendar was far more important. Major religious festivals took place during a new moon or full moon. It was not that big a deal that twelve lunar months was less than one tropical year. When the new year had drifted too far from the spring equinox, the king would add an extra month to bring it back in sync.
In Egypt, on the other hand, the annual flooding of the Nile became even more important than lunar phases as agriculture depended on it. Having the length of the year vary became unacceptable. Having it be 360 days long was also unacceptable since observations of Sirius, which predicted the annual flooding with reasonable accuracy, showed that it was closer to 365 days. Therefore a 365 day calendar had to somehow be sold to the people.
This appears to have been accomplished through an ingenious myth which goes back at least to the Fifth Dynasty (25th-24th century BCE). Here is Hernst Zinner’s retelling of it, taken from Saha & Lahiri (1955):
“The Earth god Seb and the sky goddess Nut had once illicit union. The supreme god Ra, the Sun, thereupon cursed the sky goddess Nut that the children of the union would be born neither in any year not in any month. Nut turned to the god of wisdom, Thoth, for counsel. Thoth played a game of dice with the Moon-goddess, and won from her 1/72th part of her light out of which he made five extra days. To appease Ra the Sun-god, these five days were given to him, and his year gained by five days while the Moon-goddess’s year lost five days. The extra five days in the solar year were not attached to any month, which continued to have 30 days as before; but these days came at the end of the year, and were celebrated as the birthdays of the gods born of the union of Seb and Nut, viz., Osiris, Isis, Nephthys, Set and Anubis, five chief gods of the Egyptian pantheon.”
Note that the real loser in this myth was the Moon, who had nothing to do with the forbidden union between the Earth and the sky. The myth appears to have accomplished the extraordinary task of completely removing lunar phases from religious life. The important feasts that used to take place during specific lunar phases were transferred to specific dates on this new civil calendar. The people stopped using the Moon as a timekeeper.
As the Moon lost its privileged position, so did astronomers. The beginning of the month no longer had to be announced by the astronomer-priest. No complicated calculations had to be performed to bring the month in line with the year. Anyone can remember that there are twelve months with 30 days each and five extra days at the end of the year.
If astronomers had continued to have a voice, they might have pointed out that the year is actually slightly more than 365 days long. Within a few centuries, it was apparent that the new year kept drifting relative to the seasons. Adding six extra days, instead of five, every four years would have made the calendar nearly perfect. The Greeks who ruled Egypt in the 3rd century BCE did just that that but the people wouldn’t accept the change.
The Greek pharaohs failed to understand something the pharaohs of the Old Kingdom had known all too well. No mortal, not even a pharaoh, could fix the Egyptian calendar by decree. When you mess with the calendar, you mess with religion. The calendar had to adhere to the will of the gods.
Two hundred years later, the Egyptians finally warmed up to the addition of the leap day. Even with this revision, their calendar was the simplest in the world.
And here’s the great irony. While the simplicity of their calendar had rendered Egyptian astronomers redundant in the 3rd millenium BCE, that same simplicity made it the preferred choice of later astronomers. The most influential Greek astronomer in Egypt, Claudius Ptolemy, would use it as would his most famous challenger, Nicolaus Copernicus, 1500 years later.
Egyptian farmers continue to use their ancient calendar. So does the Coptic Orthodox Church, with the additional leap day. The Egyptian calendar also inspired the Ethiopian Calendar and the French Republican Calendar.