Every spring and autumn equinox, the plumed serpent of light named Kukulkan makes an appearance at Chichen Itza, the ancient Mayan site in Mexico. For about half an hour, sunlight strikes the staircase bannister of El Castillo pyramid in such a way as to create an undulating body for the snake deity whose carved head is found at the base of the stairs. Kukulkan appears only twice a year, when day and night, light and dark, are equal and in balance.
My Rare One and I visited Chichen Itza in 2004 (but not on an equinox, alas). I climbed El Castillo's sacred staircase, a difficult task because of its extremely steep and shallow steps. I descended the stairs in the traditional manner -- sitting and bumming my way down each step! It's the safest method because it is so very easy to trip and fall all the way down if you walk upright. That's why an ambulance was tucked away in a nearby grove of trees, as we later noticed. In fact, I read recently that tourists are now no longer allowed to climb the pyramid, precisely because an elderly lady did have a fatal fall on the staircase.