We asked what caused the biannual solar event at the Abu Simbel temple complex in Egypt to shift dates. In simplest terms, the dates moved because the temple complex moved.
From the 13th century B.C. to 1964 A.D., sunlight shown upon the statues of pharaoh Ramses II and the sun gods Amon-Re and Re-Horakhte inside the temple's inner sanctuary every Feb. 21 and Oct. 21, believed to mark the anniversaries of the pharaoh's birth and coronation, respectively (though inconsistencies in the ancient Egyptian solar calendar make this a topic of scholarly debate).
In 1964, however, Egypt was in the midst of constructing the Aswan High Dam hydroelectric plant on the Nile River, thereby creating the artificial Lake Nasser, which would threaten the Abu Simbel complex. Without drastic measures, water would cover one of Egypt's national treasures, effectively drowning it in the desert.
The United Nations Educational, Scientific, and Cultural Organization (UNESCO) and the Egyptian government undertook the daunting task of cutting the Abu Simbel temples out of their rock-cliff housings and reassembling the monuments 690 feet away in a new man-made mountain situated on higher ground. Engineers reinforced the rebuilt temples to ensure that the "new" Abu Simbel monument would not only be safer, but stronger than before.
The process took four years, and it required the construction of a massive concrete and steel dome, the outside of which engineers designed to simulate the appearance of the original rock cliff.
While the groups made every effort to preserve the necessary orientation to facilitate the temple complex's famous solar event, the ambitious relocation project caused the event dates to shift by one day—from Feb. 21 and Oct. 21 to Feb. 22 and Oct. 22. By most accounts, this was a small price to pay to preserve one of the great monuments of Ancient Egypt.