From How the Mayan Calendar Works:

The Mayans were the first Mesoamericans to keep any sort of historical record, and the beginnings of the calendar were born. The Mayans used stelae, or stone monuments, to carve their civil events, calendars and astronomy knowledge. They also inscribed their religious beliefs and mythology on pottery.

While the Mayans weren’t the first civilization to ever use a calendar, they did devise four separate calendars that lasted for discrete time periods. Depending on their needs, the Mayans used different calendars or some combination of two calendars to record each event. Their Long Count calendar expires in 2012, leading some to believe that it coincides with an apocalyptic event.

But in order to decipher these different calendars, you’ll first need a brief lesson in Mayan math. They used a series of dots and bars to signify numbers. One dot equaled one unit, while one bar equaled five units. A shell symbol signified zero.

In a system similar to the one we use now, the Mayans used place values to designate large numbers. However, the similarities between math systems end there.

Mayans’ place values are vertical, whereas ours are horizontal. For instance, we write the number 27 horizontally — the number two, then the number seven to the right of it. The Mayans, however, would write 27 vertical­ly — their symbol for seven (a line representing five units with two dots over it) would be on the bottom, and the symbol for 20 (a dot on the line above) would be directly over it. The same applies for other numbers, like 29.

Like many Mesoamerican calendars, the Tzolk’in, or Sacred Round, calendar operated on a 260-day cycle. One theory for the significance of its cycle length is that the 260 days correlate to pregnancy [source: Maya Mystery School]. Another proposes that the calendar represented the length of time to cultivate corn. However, it’s more likely that it was based on the Mayans’ reverence for the numbers 13 and 20.

The Tzolk’in calendar is made up of a set of 20 day names, symbolized by images called glyphs, and 13 numerals called tones. The days are numbered one through 13, and the names are also given in sequence.

The beginning of the Tzolk’in calendar begins with the first day name, Imix’, and the number one. The days continue in sequence, with the second day being a combination of Ik’ and the number two; the day names and numbers combine in sequence until all 13 tones are used.

Once the calendar reaches the day 13, denoted by B’en and the number 13, the numbers begin again with one, but the day names move forward with the 14th glyph, Ix. By rotating like this, the two sets form 260 unique combinations of a day name and a number. For instance, once you reach the end of the cycle of day names with 7 Ajaw, the day names begin anew at Imix, and the numerals continue: 8 Imix’, 9 Ik’, 10 Ak’b’al and so on.

The Mayans designed the Long Count calendar to last approximately 5,125.36 years, a time period they referred to as the Great Cycle [source: Jenkins]. The Long Count calendar is divided into five distinct units:

  • one day - kin
  • 20 days - uinal
  • 360 days - tun
  • 7,200 days - katun
  • 144,000 days - baktun

English anthropologist Sir Eric Thompson looked to the Spanish Inquisition to calculate the Mayan-to-Gregorian date conversion, known as the Thompson Correlation. Events that occurred during the Inquisition were recorded on both the Mayan Long Count calendar and the Gregorian calendar. Scholars then gathered dates that matched on both calendars and compared them to the Dresden Codex, one of four Mayan documents that survived the Inquisition. This codex confirmed the date long thought by Thompson to be the beginning of the current Great Cycle — Aug. 13, 3114 B.C. [source: Mayan Long Count].

In recent years, as the conclusion of the Long Count calendar approaches on Dec. 12, 2012, doomsday theorists have predicted the worst. That Gregorian date is denoted as on the Long Count, signaling the end of the current Great Cycle.

However, Mayan scholars and natives dismiss the apocalyptic theories, noting that end of the calendar would be regarded as a time of celebration, much like modern-day New Year festivities [source: Stevenson]. There are also no Mayan inscriptions or writings that predict the end of the world when the Great Cycle concludes [source: MacDonald].

The most notable event slated for the 2012 winter solstice will happen in the sky. For the first time in around 25,800 years, the sun will align with the center of the Milky Way galaxy [source: Stevenson]. Although the event sounds impressive, astronomers claim that it won’t have any effect on the Earth. And with that, the next Great Cycle will quietly begin anew.

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