The length of an astronomical year is determined by the time it takes our planet to complete one full orbit around the sun, which is just about 365.25 days. Most modern calendars, such as the nearly ubiquitous Gregorian calendar, are based on the astronomical year.

The Mayan people of Mesoamerica did indeed have a calendar corresponding to one astronomical year (the "Haab" calendar), but they also had another calendar, known as the Tzolk'in, or the sacred calendar, which was in no way tethered to the astronomical year, consisting instead of a revolving series of 260 unique days. So why would someone use a 260-day calendar? What purpose would it serve?

To understand what the calendar means, we first have to understand how it works. To create 260 unique days, the creators of the Tzolk'in calendar had two major building blocks. One was a cycle of 20 named days, much like the Gregorian calendar's named days of the week (Sunday, Monday, etc.). These 20 days begin with "Imix" and end with "Ajaw." The calendar also includes a cycle of 13 cardinal numbers -- just as you would normally count, 1 to 13. These two cycles fit together like two gears in a machine, though the "numbers gear" is smaller than the "named days gear." As the gear bearing 20 named days progresses through its cycle, it connects successively with the 13 numbered spokes on the smaller gear. But when the 14th named day arrives, the smaller gear has already made a full revolution, and the numbers begin again at 1, so that the 15th named day connects with a '2,' and so forth [source: Carrasco]. It turns out, if you continue this process, the gears don't return to where they started -- that is to say, the number 1 doesn't fall on the first named day again -- until 260 unique pairings of named days and numbers have occurred. These pairings are the 260 days of the Tzolk'in calendar.

Whereas calendars based on the astronomical year are largely functional, giving us reliable predictions of seasonal changes, which farmers can use to schedule the planting and harvesting of crops, the Tzolk'in calendar was primarily used for ceremonial and religious reasons, creating cycles of temporal significance.

Each of the 20 named days in the Tzolk'in calendar was imbued with a unique cultural and religious significance that charged the calendar with meaning. For instance, the named day known as "Kimi" was associated with the Mayan deity of death and with morbidity in general, while the named day known as "K'an" represented corn specifically and foodstuffs broadly. Also, specific pairings of named days and numbered days triggered religious ceremonies or suggested portents. For example, when the named day "Chuwen" coincided with the 8th day of the 13-day cycle, it signaled the beginning of the "sacred year" [source: Pitts]. Each sacred named day was believed to execute its functional significance in many ways, such as by determining the personality traits or future characteristics of babies born under its sign, or by unleashing fateful events that related to the religious significance of the day.