During the fourth dynasty of Egypt, the Great Pyramids of Giza were built for pharaohs Khufu, Khafre, and Menkaure. The Sun god Ra – thought by Ancient Egyptians to be the father of all pharaohs – was believed to have created himself from a pyramid-shaped mound, while the shape of pyramids is believed to represent rays of light from the Sun.

“Khufu will indeed build his pyramid on the Giza plateau, in plain view from Heliopolis (the main theological centre of the cult of Ra), and will make an explicit reference to the Sun cult with the spectacular hierophany occurring at Giza at the summer solstice re-creating the ‘solarized’ version of the double mountain sign, Akhet,” archaeoastronomer Giulio Magli explains in a paper which has not yet been peer-reviewed.

“The successors of Khufu (Djedefra, Khafra, Menkaura) will all adopt the suffix -Ra in the their name and construct their pyramids in view of Heliopolis as well.”

And yet, once these pyramids were completed and pharaohs interred, there was a sudden and abrupt change to this practice. When Shepseskaf – likely the son of Menkaure – died, his body was placed in a construction on a plateau to the south of Saqqara, out of view of Heliopolis, and away from any other pyramid structures.

The structure itself, while still grand inside and out, was a large rectangle 99.6 meters (327 feet) long and 74.4 meters (244 feet) wide.

“All in all, the unprecedented and unequalled shape of the Shepseskaf tomb is really a mystery,” Magli continues. “A number of proposed solutions for this mystery is available, most of them being frankly very weak, or even untenable.”

These include the idea that it was merely a backup tomb, or that Shepseskaf – who ruled for up to seven years – lacked legitimacy. This, according to Magli, conflicts with the deliberate and grand construction of his eventual tomb.

“More close to the reality, Hassan (1936) has put forward the idea that Shepseskaf may have deliberately chosen to build a original monument to differentiate it from the pyramids, because these were too much associated with the Sun cult.”

Magli believes that this may have occurred due to an astronomical event. An archaeoastronomer, Magli attempted to calculate the path of totality of solar eclipses from the time. The orbits of the Moon and the Earth around the Sun are well-known, allowing us to calculate when they occurred thousands of years in the past or future (thank you, time-reversible laws of physics). But the path of totality is a lot trickier, as the Earth’s rotation can change in response to factors like changing sea levels, earthquakes, and interactions between the Earth and the Moon. 

Though these are approximations, they have gotten better in recent years, and Magli calculated that on April 1, 2471 BCE at around 7:59 AM, totality would have occurred in the area. In fact, it was likely a spectacular one.

“The Pleiades were so close to the eclipsed Sun that I would venture to suppose that they might have been seen, although their magnitude is low (as an asterism it is between 3.5 and 4),” Magli explains. “Impressively visible were the two planets Venus and Mercury, in a sort of symmetric configuration along the ecliptic, with the coupled Sun/Moon in the middle. The eclipse totality lasted a terrifying amount of time: almost seven minutes (eclipses cannot last more than about 7 min 30 sec).”

It is unclear whether the Ancient Egyptians would have seen the eclipse as a good omen, a bad omen, or something else entirely. Nevertheless, Magli told Space.com that the odd shape and structure of Shepseskaf’s tomb “resembles a building which was in the most sacred place inside the part of totality”, with the eclipse beginning a (temporary) shift away from the solar cult that dominated Ancient Egypt.

The study is posted to preprint server arXiv.