An ancient Egyptian Calendar of Lucky and Unlucky Days assigns luck with the period of 2.85 days. Previous astronomical, astrophysical and statistical analyses of the calendar support the idea that this was the period of the eclipsing binary star Algol approximately 3,000 years ago. However, next to nothing is known about who recorded Algol’s period into the calendar and especially how. In a paper published in the journal Open Astronomy, University of Helsinki researcher Sebastian Porceddu and colleagues argue that the ancient Egyptian scribes had the possible means and the motives for such astronomical observations.

“The ancient Egyptian texts known as the Calendars of Lucky and Unlucky Days, or hemerologies, are literary works that assign prognoses to each day of the Egyptian year. These prognoses denote whether the day, or a part of the day, is considered ‘good’ or ‘bad’,” Dr. Porceddu and co-authors said.

“Only nine such texts are known. We studied the best preserved one of these texts, the Cairo Calendar, dated to 1271-1163 BC.”

Algol, also called Beta Persei, is an eclipsing binary stellar system located in the northern constellation Perseus, about 92.8 light-years from the Sun, but about 7.3 million years ago it passed within about 10 light-years of the Sun.

It consist of two stars, Algol A and Algol B, that orbit each other with a period of 2.867 days. Algol A is brighter than Algol B. However, Algol B has a larger radius than Algol A.

Our line of sight nearly coincides with the orbital plane of this binary system. Therefore, these stars eclipse each other during every orbital round.

In a primary eclipse, the dimmer Algol B partly eclipses the brighter Algol A. This primary eclipse can be observed with naked eye.

In a secondary eclipse, the brighter Algol A partly eclipses the dimmer Algol B, but the decrease in total brightness of this binary system is so small that this secondary eclipse event can not be observed with naked eye.

“The discovery of Algol’s variability would have to be dated to thousands of years earlier than has been previously known,” Dr. Porceddu said.

“The star would have been a part of ancient Egyptian mythology as a form of the god Horus.”

The first European astronomer to note Algol’s variability was the Italian Geminiano Montanari around 1670.

Dr. Porceddu’s team shows that the ancient Egyptian scribes, the ‘hour-watchers,’ had the means and the motives to do this around 3,000 years before Europeans.

“The ancient Egyptian scribes observed regularly about 70 bright stars, or most probably a lot more, in a region where there are about 300 clear nights every year. They practiced this tradition for the timing of religious rituals,” the researchers said.

“The scribes probably discovered Algol’s variability from the changes that its eclipses caused in its hour-star pattern.”

_____

Sebastian Porceddu et al. 2018. Algol as Horus in the Cairo Calendar: The Possible Means and the Motives of the Observations. Open Astronomy 27 (1); doi: 10.1515/astro-2018-0033