The Concave Face Calendar Marker

The Great Pyramid's unique concave faces (creating an 8-sided structure) produce a shadow effect visible only during the equinoxes. This means: (1) the builders knew the exact dates of the equinoxes, (2) they engineered a sub-meter concavity over a 230-meter face to produce this effect, (3) it serves as a perpetual calendar that requires NO maintenance and will work for as long as the structure stands. This is arguably the most sophisticated calendar device ever built — it passively marks the equinoxes twice per year, visible from the air or nearby elevated positions. No other pyramid has this feature, suggesting it was deliberately engineered for the Great Pyramid alone.