The astronomical orientation of the pyramids of Egypt
Numerical simulations make it possible to quickly and easily visualize a number of archaeoastronomical issues: the orientation of monuments, the succession of star risings and settings on the local horizon, the culmination of stars in the local meridian, etc. The resulting simulations were carried out using three software programs designed during my doctoral thesis, as well as a brand new five-dimensional Sky/Earth interface: three dimensions represent the Earth's relief; stars visible to the naked eye are positioned on the surface of a sphere, the celestial sphere; finally, time (past, present, and future) represents the fifth dimension. Thanks to this Sky/Earth interface, it is now possible to move around the surface of our Earth, at the foot of the pyramids of Egypt for example, and visualize the celestial vault as it was at the time of their construction.
Below, the issue of the astronomical orientation of the pyramids of Egypt is summarized in three short video sequences, created using digital simulations. This question has already been addressed in a book published in 1998, an article published in Cahiers Caribéens d'Egyptologie No. 15 (2011) and a TV documentary (2023).
Sunset on the Giza pyramids in the year 2500 BC
The three pyramids of Giza, erected under the reigns of the pharaohs Cheops, Khafre and Mycerinus between the years 2550 and 2500 BC, are perfectly aligned in the direction of the cardinal points - as shown in this animation depicting the setting of the Sun behind the pyramid of Khafre on the day of the spring equinox. To achieve such a result, did Pharaoh's architects observe the rising or setting positions of a star, of the Sun? Or the upper culmination, in the northern sky, of one or more circumpolar stars? Did they use one or several instruments of observation and projection on the ground? These are the questions which have been posed to the Egyptologists and the Astronomers for several decades...
Heliacal setting of Procyon on the plateau of Giza in 2500 BC
The deviations from the east-west alignment could be measured on eleven pyramids of the Old Kingdom: those of the pharaohs Djoser, Huni, Snefru (2), Cheops, Djedefre, Chephren, Mykerinus, Sahure, Neferirkare and Unas. The insertion of these archaeological data within a numerical model allowing to recreate the conditions of observation of the celestial vault at the places and historical periods considered, has highlighted the possibility that the repeated observation, over more than 500 years, of the setting position (in the night or twilight sky) of one particularly bright star, alpha Canis Minoris (Procyon), could account for the orientation of each of the eleven pyramids of the Old Kingdom. The animation on the left shows the heliacal setting of Procyon in the axis of the pyramid of Chephren in 2500 BC.
The « Stretching of the Cord » ceremony
At the historical period considered, the heliacal setting of the star alpha Canis Minoris - that is to say, its disappearance in the western sky in the glow of the setting Sun - coincided with the upper culmination, in the northern sky, of the star eta Ursae Majoris materializing the extremity of the constellation of the Big Dipper - on the animation on the right, this star is circled in red. This conjunction evokes the content of texts describing the course of the Stretching of the Cord ceremony, of which several extracts have been found among the remains of monuments dating from the Old Kingdom to the Ptolemaic Period - a sign of the antiquity and persistence of this ritual preceding the laying of any foundation stone...
