Tenayuca

It is thought that the serpent pyramid, which was dedicated to the sun cult, used to be given a new casing every 52 years, in keeping with the Aztec calendar cycle. The last occasions on which this happened under predominantly Aztec influence were in 1351, 1403, 1455 and 1507. The two previous occasions, which makes a total of seven in all, could well have been in 1247 and 1299 under Chichimec-Toltec influence.

The core of the building measures 31 3 12 m (102 3 39 ft) and has a height of 8 m (26 ft), the final casing structure measuring 66 3 62 m (217 3 203 ft) with a height of 19 m (62 ft). Although no actual observatory-type buildings have been found, it is assumed, on the basis of various indicators, that this site was also used for star-gazing and that the building was aligned with heavenly bodies. A pointer to this is the existence of posts in the pyramid walls at the exact spot where the sun goes down on March 21st and September 23rd, in other words at the vernal and autumnal equinoxes. As at Chichén-Itzá and Teotihuacán the north-south axis here at Tenayuca has also as a result been shifted 17 degrees to the east.

The main part of what is visible today of the pyramid derives from the last three casing structures to be added. The final appearance of the site is in its essential features very similar to the main temple (Templo Mayor) of Tenochtitlán, which was almost completely destroyed, its remains having only been discovered in the last few years. The pyramid is surrounded on three sides by a platform which is decorated by the bodies of recumbent serpents. This "serpent wall" (Coatepantli) today comprises 138 serpent sculptures. Originally the pyramid is thought to have been embellished with a total of 800 stone serpents. The rolled-up fire or turquoise serpents (Xiuhcóatl) next to the small platforms (two on the north and one on the south side), which presumably served as symbols of the sun's authority, date from the time that the fifth casing structure was added. A vault decorated with skulls and skeletons, to the right of the pyramid, probably symbolises the setting of the sun. Two wide parallel flights of steps used to lead across the four-storey pyramid to the double temple.

About 3km/2mi north of the serpent pyramid is the 10 m (33 ft) high temple pyramid of Santa Cecilia Acatitlán, which has been completely restored. This is thought to have been used for worshipping the sun and the rain-god Tláloc. Of special interest is the temple building on the pyramid; this is still the original one, although restored. The museum adjoining the pyramid is in a house with an attractive patio.