Ancient Egyptians wrote Calendars of Lucky and Unlucky Days that assigned astronomically influenced prognoses for each day of the year. The best preserved of these calendars is the Cairo Calendar dated to 1244 – 1163 BC (Ramesside Period). According to scientists at the University of Helsinki, this papyrus is the oldest preserved historical document of naked eye observations of a variable star, the eclipsing binary star Algol.

The Egyptian Museum of Cairo purchased this unique hieratic papyrus from an antiquities dealer in 1943. Twenty three years later, Egyptian scientist Abd el-Mohsen Bakir published it as the Cairo Calendar No. 86637.

The document is divided into three sections (Books I, II and III). Its largest part, Book II, consists of 365 passages, one for each day of the 360-day Egyptian year plus five epagomenal days. The passages seem to concern religious feasts, mythological incidents, favorable or adverse days, forecasts, and warnings.

University of Helsinki researchers Lauri Jetsu and Sebastian Porceddu have now performed a statistical analysis of the texts of this document.

“Our statistical analysis leads us to argue that the mythological texts of the Cairo Calendar contain astrophysical information about Algol,” the scientists said.

The analysis revealed that the periods of the variable star Algol (2.85 days) and the Moon (29.6 days) strongly regulate the actions of deities in this calendar.

“We show that Algol was represented as Horus and thus signified both divinity and kingship,” the researchers said.

“The texts describing the actions of Horus are consistent with the course of events witnessed by any naked eye observer of Algol.”

“The period of the Moon, 29.6 days, has also been discovered in the Cairo Calendar,” they said. “We show that the actions of Seth were connected to this period, which also strongly regulated the times described as lucky for Heaven and for Earth.”

This research, published in the journal PLoS ONE online on December 17, confirms that the brightest phases of Algol and the Moon had particularly positive meanings for the Ancient Egyptians.

It also confirms that the first variable star, as well as its period, were discovered much earlier than previously thought.

“In 1596, Fabricius discovered the first variable star, Mira. Holwarda determined its eleven month period 44 years later. In 1669, Montanari discovered the second variable star, Algol. Goodricke determined the 2.867 days period of Algol in 1783,” Jetsu and Porceddu said.

“All these astronomical discoveries were made with naked eye. Since then, they have become milestones of natural sciences.”

“Our statistical analysis of the Cairo Calendar confirms that all these milestones should be shifted about three millennia backwards in time,” the scientists concluded.

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Jetsu L. & Porceddu S. 2015. Shifting Milestones of Natural Sciences: The Ancient Egyptian Discovery of Algol’s Period Confirmed. PLoS ONE 10 (12): e0144140; doi: 10.1371/journal.pone.0144140