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According to “The Accidental Superpower” by Peter Zeihan:

“The mind-set of eternal stability was so deeply entrenched that when ancient Egyptian scholars discovered that they had failed to account for the extra day in leap years, instead of adjusting their calendars they decided it would be less disruptive to wait until their calendar—too short by 0.25 days annually—simply cycled all the way around again, a process that took 1,461 years. When that day arrived, the Egyptian leadership declined to make the adjustment, since from their point of view the inaccurate calendar had triggered no deleterious events in the past millennia and a half. It wasn’t until the Greeks occupied Egypt that they forced the adoption of an accurate calendar.”

As far as I can tell this is exact story is not substantiated by historical records. But do we have any clue as to why the Egyptians didn’t introduce the leap year concept (or some other method for correcting for the extra 1/4 of a day each year) until very late into their civilization?

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  • If you’re talking about the Decree of Canopus for the calendar reform, it was not even followed, and really, it wasn’t until Augustus that Egypt had and used an intercalary calendar.
    – Pierre Paquette
    Commented Jan 27 at 23:13

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Prediction wasn't Required

The main reason to link the solar calendar with the secular calendar is to predict food production conditions.

When is the salmon run? When is the first/last frost?

If you predict the frosts wrong, you will lose your crop. If you predict the salmon run wrong, you might arrive too early and exhaust local resources, or too late, and miss part of the salmon harvest.

In contrast, ancient Egyptian agriculture didn't require this type of prediction.

Ancient Egyptians waited until the annual floodwaters receded, and then planted. They could just observe the flooding, and see that it was complete - no farmer's almanac required.

Frosts were likewise not an issue given the local climate.

Note that over a ~60 year lifetime, the lack of a leap day shifts the calendar by about 2 weeks, so if June was mid-summer as a child, it's still summer at the end of your life.

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    Precise calendaring is also useful for astronomy. Did Ancient Egyptians not practice it?
    – SPavel ♦
    Commented Jan 25 at 18:56
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    @SPavel - I think its the other way around - astronomy was primarily useful because it helped make a more accurate calendar (via tracking constellations against equinox / solstice timing). People died if you screwed up the timing on the first frost, whereas accurately predicting when Venus was at the horizon was... less important.
    – codeMonkey
    Commented Jan 25 at 19:31
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    An inaccurate calendar is only a problem when you keep the same calendrical rules in effect for a very long period of time. Like ~1600 years for the Julian calendar before Pope Gregory reformed it. Or the similar length of time that the Hebrew calendar has been fixed. In the ancient world, the people in charge just weren't thinking that long-term, so they'd either just live with the calendar drift, or make ad hoc adjustments to keep the calendar in sync with the solar year.
    – dan04
    Commented Jan 25 at 22:33
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    @SPavel : that's right, but for religious purposes astronomy didn't really need an ultra-accurate calendar, the margins for interpretation were beyond a 0.25 per 365 precision range anyway.
    – Evargalo
    Commented Jan 26 at 10:36
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    @rcgldr: Yeah, before the fixed 19-year cycle was introduced, the Sanhedrin would have to evaluate the late-winter weather conditions, and add a leap month if necessary so that Passover wouldn't start before the prerequisite barley harvest. The Romans had a similar system of having pontiffs declare as-needed leap months, but in practice, the calendar was politically manipulated to hasten or postpone elections, and so had drifted 80 days by the time Julius Caesar decided to replace the highly-inaccurate lunisolar calendar with a solar one.
    – dan04
    Commented Jan 26 at 14:53

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