English: A demonstration by artificial light of how a plumed serpent can be seen along the edge of the staircase leading up to the top of the Chichen Itza El Castillo pyramid. The same effect can be seen on the Spring and Autumn equinoxes, at the rising and setting of the sun along the west side of the north staircase. On these two annual occasions, the shadows from the corner tiers slither down the northern side of the pyramid with the sun's movement to the serpent's head at the base.