The Aztec Calendar

The Aztecs didn't just have one calendar - they had two that worked together. One was similar to our own calendar and was used for counting years, the other was used for religious purposes.

Xiuhpohualli - Counting the Years

The Xiuhpohualli calendar was solar in nature and resembled our own, consisting of 365 days. It was used for civil and agricultural purposes and related ceremonial activities.

The solar calendar was divided into 18 "months" (veintenas) of 20 days each. Since 18 * 20 = 360 this left five days left over in the year. These were nemontemi and were festival days when people would party and the priests make ceremonial sacrifices.

The Xiuhpohualli is famously represented in the Aztec Sun Stone or "stone of the sun". This is a 12 foot diameter stone slab from the 15th century that depicts the solar year.

Tonalpohualli - Counting the Days

The Tonalpohualli calendar was used for religious purposes and consisted of 260 days. It was divided into 13 groups - trecenas - of 20 days each. The twenty days in each period were each allocated to one of the cardinal compass directions and also to a particular symbol. These day symbols included the alligator, the eagle and the earthquake. Each period was also attributed to a god, hence the combination of symbols could be used to determine the appropriate days for particular religious activities.

A particular day was identified by a symbol and a number from one to thirteen. But unlike our current calendar this was not a simple case of each symbol in turn being associated with each number in turn. Instead the first symbol (Cipactli) was associated with 1 to make "Cipactli 1" the first day. The second day was then the second symbol associated with the number 2 - so "Ehecatl 2" was the second day 2. Day 14 of the calendar was thus the 14th symbol with the number 1 - "Ocelotl 1" was day 14 of the year. Similarly the 21st day of the Aztec religious year was "Cipactli 2".

One way of thinking of this complex system is to imagine two gears, one with 13 numbered cogs and one with 20. They begin with cog 1 on each gear lined up then the smaller rotates around the larger. The day number is the number on the smaller gear together with symbol relating to the larger gear number.

Calendar Round

Taken together these two calendars produced a 52 (calendar) year cycle known as the calendar round. (52 * 365 = 18,980 which is the Least Common Multiple of 265 and 260). This period of 52 years is sometimes called a Mesoamerican century.

The two Aztec calendars would thus return to alignment every 52 years. This was marked by a ceremony where the city fires were extinguished then relit to mark the new "century".