Ancient calendar reveals two startling discoveries about Egyptian astronomy

Close analysis of an ancient calendar has revealed that the ancient Egyptians made an astronomical discovery thousands of years earlier than the previous 1638 CE record—and tied it closely to their mythological calendar.

This papyrus document, known as the Cairo 86637 Calendar, is the oldest preserved historical text of naked eye observations of a variable star—a star whose brightness seems to fluctuate over time. The star in question, Algol, is actually composed of two stars; it’s part of a binary star system whose brightness dips when one star eclipses the other.

The Cairo Calendar lists the luckiness of each day of the year, ranging from very favorable to very adverse, tying them to actions of ancient Egyptian deities.

Another startling discovery

Published in PLOS One, the study found the amount of time it takes for Algol’s two stars to complete an orbit around themselves (2.85 days) and the amount of time it takes for the Moon to complete an orbit around the Earth (29.6 days) strongly correlates to the listed actions of the deities.

In fact, the actions of Horus, the sky god, tied strongly to the phases of Algol. The Moon, however, correlated strongly with Seth, the god of chaos. It appears that the brightest phases of Algol and the Moon were especially lucky.

“Until now, there were only conjectures that many of the mythological texts of the Cairo Calendar describe astronomical phenomena. We can now unambiguously ascertain that throughout the whole year the actions of many deities in the Cairo Calendar are connected to the regular changes of Algol and the Moon,” said co-author Sebastian Porceddu, of the University of Helsinki, in a statement.

Since this document dates to between 1244 and 1163 BCE, this means that both the first documentation of a variable star and its orbital period came nearly 3000 years before “classical” natural scientists did in 1638.

Not that everyone is convinced, though.

“I would have serious doubts if someone claimed, for example, that the Bible contains information about water in Mars. We claimed that Ancient Egyptian religious texts contain astrophysical information about Algol. It was no surprise to us that there were, and there still are, sceptics,” said co-author Lauri Jetsu.


Feature Image: One page of the Cairo Calendar. Inside the superimposed rectangle is the hieratic writing for the word ‘Horus’. (Credit: Lauri Jetsu)

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