Deep in the northern Yucatan jungle lies one of the Northern Hemisphere's oldest solar observatories. At the ruins of an ancient city called Chichen Itza ('the mouth of the well'), a well-preserved collection of buildings and plazas reveals the sophisticated astronomical and architectural knowledge possessed by the Maya more than a thousand years ago.
The 75-foot pyramid in the center of the site is the Pyramid of Kulkulkan, the Feathered Serpent God of the Maya. The Maya were obviously careful observers of the apparent change in the location of the Sun in the sky throughout the year; different parts of the pyramid are aligned with different seasonal solar occurrences, including the rising and setting of the Sun on the longest and shortest days of the year (solstices).
But the pyramid's special show is reserved for the equinoxes. The Maya positioned and designed the structure of the pyramid so that on the spring and autumn equinoxes, the setting Sun falls precisely on the edges of the pyramid's stepped terraces, casting a series of interlocking, triangular shadows that slither down the pyramid's sacred staircase northern of the pyramid like a giant snake. As the Sun sets, the shadow connects with a stone serpent head at the base of the sacred stairway, and then vanishes until the autumn equinox.