Scientists have long puzzled over how the ancient Egyptians built the Great Pyramid of Giza (aka the Pyramid of Khufu) with such "extreme precision," per Live Science. This Wonder of the World is lined up with the compass points "with an accuracy of better than four minutes of arc, or 1/15th of one degree," archaeologist Glen Dash writes in the Journal of Ancient Egyptian Architecture—an accuracy ScienceAlert says one "wouldn't expect to see from an era without drones, blueprints, and computers." However, Dash notes a "straightforward" method that's been "largely ignored," theorizing the Egyptians may have used the equinoctial solar gnomon technique, which involves placing a vertical rod called a gnomon into the ground and using it as a sundial of sorts. Dash speculates the builders tracked the rod's shadows on the fall equinox, when the lengths of night and day are as close to equal as they get, to achieve nearly perfect north-south-east-west alignment.
After erecting a wooden platform complete with gnomon in his own Connecticut yard on Sept. 22, 2016, Dash found that the tip of the shadow cast by the rod fell in an almost perfect east-west line, with just a tiny counterclockwise adjustment. That matches up to the error not only of the Great Pyramid, but also of the nearby Khafre Pyramid and Dahshur's Red Pyramid, suggesting they were all built using the same method. Dash can't say for certain this is how the ancient Egyptians set the structures up—no records exist to confirm any building processes, and other theories have been suggested that use the stars or sun for alignment—but he says it's possible if they just had the equipment and a "clear, sunny day." "It is hard to imagine a method that could be simpler, either conceptually or in practice," he notes. (A huge find inside the Great Pyramid of Giza late last year.)
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