The Maya strongly believed in the influence of the cosmos on daily life. Consequently, Mayan knowledge and understanding of celestial bodies was advanced for their time: For example, they knew how to predict solar eclipses. They also used astrological cycles to aid in planting and harvesting and developed two calendars that are as precise as those we use today.
The first, known as the Calendar Round, was based on two overlapping annual cycles: a 260-day sacred year and a 365-day secular year. Under this system, each day was assigned four pieces of identifying information: a day number and day name in the sacred calendar and a day number and month name in the secular calendar. Every 52 years counted as a single interval, or Calendar Round. After each interval the calendar would reset itself like a clock.
Because the Calendar Round measured time in an endless loop, it was a poor way to fix events in an absolute chronology or in relationship to one another over a long period. For this job, a priest working in about 236 BC devised another system: a calendar that he called the Long Count. The Long Count system identified each day by counting forward from a fixed date in the distant past. (In the early 20th century, scholars found that this “base date” was August 11 or August 13, 3114 BC.) It grouped days into sets, or cycles, as follows: baktun (144,000 days), k’atun (7,200 days), tun (360 days), uinal or winal (20 days) and kin (one day).
The Long Count calendar worked the same way that the Calendar Round did–it cycled through one interval after another–but its interval, known as a “Grand Cycle,” was much longer. One Grand Cycle was equal to 13 baktuns, or about 5,139 solar years.