Algol as Horus in the Cairo Calendar: The Possible Means and the Motives of the Observations

Sebastian Porceddu 1 , Lauri Jetsu 2 , Tapio Markkanen 1 , Joonas Lyytinen 2 , Perttu Kajatkari 1 , Jyri Lehtinen 3 ,  and Jaana Toivari-Viitala 4
  • 1 Department of Physics, University of Helsinki,, Helsinki, Finland
  • 2 Department of Physics, University of Helsinki,, Helsinki, Finland
  • 3 Max-Planck-Institut für Sonnensystemforschung,, Göttingen, Germany
  • 4 Department of World Cultures, University of Helsinki,, Helsinki, Finland


An ancient Egyptian Calendar of Lucky and Unlucky Days, the Cairo Calendar (CC), assigns luck with the period of 2.850 days. Previous astronomical, astrophysical and statistical analyses of CC support the idea that this was the period of the eclipsing binary Algol three millennia ago. However, next to nothing is known about who recorded Algol’s period into CC and especially how. Here, we show that the ancient Egyptian scribes had the possible means and the motives for such astronomical observations. Their principles of describing celestial phenomena as activity of gods reveal why Algol received the title of Horus.

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