El Castillo / Temple of Kukulkan

20.6829°N, 88.5686°W.

El Castillo — the Temple of Kukulkan — at Chichén Itzá, Yucatán, Mexico, the central pyramid of the Maya–Toltec city built between approximately 800 CE and 1200 CE. Original base 55 metres per side; original height 30 metres on nine terraces with a temple of two chambers on the summit. The pyramid has 91 steps on each of its four staircases plus the temple platform — totalling 365, the days of the solar year. On the spring and autumn equinoxes, the late-afternoon sun strikes the northwest balustrade and casts a shadow against the pyramid's tiered face that produces the appearance of a giant feathered serpent descending the staircase to join the carved serpent head at its base — an effect that lasts approximately 45 minutes and was documented as deliberate by the original surveyors of the site in the 1920s. An interior earlier pyramid was discovered inside the structure in 1937, containing a red-painted jaguar throne studded with jade.

A pyramid that is also a calendar that is also a serpent that descends twice a year exactly when the day matches the night.