Very similar to the Mayan Calendar. The Aztecs didn't add much to the system except treating it even more seriously regarding sacrifices to the Gods.

The two systems are:
  • The 13/20 day naming system that repeats itself every 260 days
  • The 18 months of 20 days each system that with an additional 5 unlucky days to make up the year
Every 52 years the systems synchronize, and the calendar starts over which made the Aztecs go absolutely crazy sacrificing to make sure that the sun would rise again the first day of a new period. Cortes and his troops found 132,000 skulls under the main temple of Tenochtitlan when tearing down the main temple of the city.

The system was also one of the reasons for the easy downfall of the Aztec Empire: Just as Cortes entered the city - November 8, 1519 - , the periods were turning - and an old legend had it that a god (and old king) - Quetzacoatl - would return that excact year. Rumour has it that Cortes knew this (from his mistress Malinche) and dressed a bit up for it.

The calendar also provide us with an insight with evidence of the Aztec's knowledge of astronomy and mathematics. The calendar contained the pictographs for their days, months and suns (cosmic cycles). The stone is 3.6 meters (12 feet) in diameter and weighs about 24 metric tons. It took 52 years to complete, from 1427-1479, it is believed to be made through the use of only stone tools. This calendar is 103 years older than the Gregorian calendar which is used worldwide today.

Originally the stone was placed on top of the main temple in Tenochtitlan, the capital of the Aztec empire.. The Aztec calendar faced south in a vertical position and was painted red, blue, yellow and white. The stone was later buried by the Spaniards when they conquered Tenochtitlan. and was lost for over 250 years until December of 1790 when it was found by accident during repair work on the cathedral. Today it is located in the National Museum of Anthropology in Mexico City.

The face of Tonatiuh is in the center circle of the stone. Around the face are four squares called Nahui- Ollin, or Four Movements. According to Aztec legend, these squares represented the different ways that the four previous suns (or worlds) had come to an end: first by wild animals, then by wind, by fire, and by floods. The Aztecs believed they were living in the fifth and last world.

Continuing outward from the center, the next concentric circle shows twenty squares, each naming one of the twenty different days of the Aztec month. Clockwise these days are as follows:

Twenty Days of the Aztec Month Snake – Coatl
Lizard – Cuetzpallin
House – Calli
Wind – Ehecatl
Crocodile – Cipactli
Flower – Xochitl
Rain – Quiahuitl
Flint – Tecpatl
Movement – Ollin
Vulture – Cozcacuauhtli
Eagle – Cuauhtle
Jaguar – Ocelotl
Cane – Acatl
Herb – Malinalli
Monkey – Ozomatli
Hairless Dog – Itzquintli
Water – Atl
Rabbit – Tochtli

Deer – Mazatl
Skull – Miquiztli

The Aztec year consisted of eighteen months, each having 20 days. Each month was given a specific name. This arrangement took care of 360 days (18x20), to which five dots were added inside the circle. These dots, known as Nemontemi, were sacrificial days, or , as mentioned in the previous node “unlucky days.”

The next concentric circle is composed of square sections with five dots in each section, probably representing weeks of five days. Next are eight angles dividing the stone in eight parts. These represent the suns rays placed according to the cardinal points.

On the lower portion of the stone, two snakes encircle the stone and face each other. Their bodies are divided into sections containing the symbols for flames, elephant-like trunks, and jaguar-like forelegs. It is believed that these sections are also records of fifty-two year cycles. A square is carved at the top of the calendar between the tails fo the snakes. Inside the square the date 13 Acatl is carved. This corresponds to 1479, the year the calendar was finished. Eight equally spaced holes appear on the very edge of the calendar. The Aztec placed horizontal sticks here and the shadows of the stick would fall on the figures of the calendar; thus the stone also served as a sundial. (Information taken from a book titled Multicultural Mathematics Materials by Marina C. Krause and published by the National Council of Teachers of Mathematics.)

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