Autumnal Equinox (today)
Pyramid of Kukulkan at Chichén Itzá
"...The pyramid is unique among all known pyramids, worldwide, for its central role in a dramatic shadow and light display during the equinoxes. At the appointed hour, the setting sun casts a shadow of a serpent writhing down the northern steps of the pyramid. The sunlight bathes the western balustrade of the pyramid's main stairway and causes seven isosceles triangles to form, imitating the body of a serpent 37 yards long that creeps downwards until it joins the huge serpent's head carved in stone at the bottom of the stairway."