Final answer:
Chichen Itza tracked the alignment of the sun with the stairways of El Castillo to mark significant celestial events.
Step-by-step explanation:
Chichen Itza was an ancient Mayan city that flourished from the 9th to 13th centuries CE in present-day Mexico. One of the most recognizable structures at Chichen Itza is El Castillo, a nine-level pyramid with a square temple on top. At the spring and fall equinoxes, the setting sun casts a shadow on the stairways of El Castillo, creating a serpent-like shape that aligns with the carved serpent heads at the base. This phenomenon was likely tracked to mark important celestial events and possibly for religious or ceremonial purposes.