The foundation of all the doomsday prophecies slated for December 21, 2012 (or December 23, depending on which calculations you use) are based on the assumption that the ancient mayans knew when the world was going to end, and structured their calendar accordingly. The final day of their calendar corresponds to 21 December (or 23rd) by the Gregorian calendar which we use in the modern western world.
The Gregorian calendar was introduced by Pope Gregory XIII on 24 February, 1582, as an update to the old Julian Calendar (introduced by Julius Caesar) to correct errors which had crept in. It is the modern calendar which we are all familiar with - twelve months of varying length making up a civil year of 365 days, with leap years every time the year number is divisible by four and not divisible by one hundred (unless it's also divisible by four hundred). The julian calendar was exactly the same, except that leap years were every fourth year with no exceptions, which led to the calendar date drifting out of alignment with the definition of a year (one orbit around the sun), with the count of years starting at the year 1AD, the traditional date of the birth of Jesus.
(Updated 13-jan-2010. Previous version only considered the Long Count, ignoring the various other calendars, and the significance the mayans attached to the numbers thirteen and twenty)
The ancient Mayans had a different concept of time altogether. For them, a calendar represented a mesh of spiritual and physical cycles, and they therefore had many different calendars which were used for different purposes. The calendars were based on such diverse things as astrological mythology, the menstrual cycle, astronomical observations, and mathematical calculations based on the numbers twenty and thirteen. Each day had a patron spirit, signifying that each day had specific use. The tzolk'in calendar was 260 days long, while the haab' was close to the solar year at 365 days. These two calendars were combined into a cycle lasting 52 haab's, called the Calendar Round. Within the Round were the trecena and the veintena cycles, consisting of thirteen and twenty days respectively. They also had a system called the Venus Cycle, based on the location of Venus in the night sky (they were keen and accurate astronomers), and possibly had even more cycles based on the other planets as well.
The Calendar Round was very efficient and practical for normal daily affairs, such as remembering religious ceremonies, harvest times, or your birthday, but since it only covered fifty two years, wasn't much use for recording history, or long term predictions. For this purpose, the Mayans devised the Long Count, and it it this Long Count on which the doomsday cultists are focusing.
The Long Count works as follows: Twenty days make one uinal, 18 uinals (360 days) make one tun, 20 tuns make one k'atun, and 20 k'atuns (144000 days) make up one b'ak'tun. The calender starts at the day they believed the universe was created, and is presumed to end after 13 b'ak'tuns. Notation works like this: The date of 126.96.36.199.15 represents 8 b'ak'tuns, 3 k'atuns, 2 tuns, 10 uinals and 15 days since creation.
The reasoning goes, then, that since the ancient Mayan's believed that the earth was created on date 0.0.0.0.0 and their calendar runs out of number on 188.8.131.52.0, then the world must surely end on 184.108.40.206.0, which corresponds to 21 or 23 December 2012.
The first problem with this theory, I think, is that 0.0.0.0.0 corresponds with either 11 or 13 august, 3114 BC. This conflicts with geological evidence (indicating that the earth is over four billion years old), young earth creationist ideas (that the earth was created in 4004 BC), and pretty much every other mythology from around the world. We have to ask what makes the ancient Mayans so trustworthy, especially when we have solid archaeological evidence of human activity from long before then? More likely the creation date was symbolic mythology, much like the Judeo-Christian creation account, and the date has no actual physical significance.
Secondly, the Long Count could just as easily run up to 220.127.116.11.0 - roughly the year 8000AD. And finally, there is no evidence to suggest that the ancient Mayan's themselves attached any significance to the end of their calendar. There's no reason to believe that the Long Count wouldn't just reset, as makes sense for a cyclical system and as even our own annual calendar does. After all, December 31 is usually followed by January 1, not the end of the world! In fact the traditional interpretation of the end of the Long Count is that it signifies the beginning of a new era, a spiritual transformation (not unlike the idea of moving into The Age Of Aquarius), with no bundled apocalypse!
In short, it seems that the proponents of various apocalyptic scenarios have latched onto New Age beliefs regarding the Mayan calendar, and are using 21 December 2012 as simply the latest in a very long line of Armageddon predictions. There's nothing new here to see, we can all just move along.
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