Abandon all your possessions and run for the hills: It has been foretold that the world is coming to an end sooner than you think, in the year 2012. It seems that you can't pick up any newspaper or magazine without reading that the apocalypse is almost upon us.
What really is going to happen in 2012? Asteroid 433 Eros is going to pass within 17 million miles of the Earth in January; the United States will hand over control of the Korean military back to the Koreans in April; there will be an annular solar eclipse in May and a solar transit of Venus in June; the Summer Olympics will take place in London; the Earth's population will officially pass 7 billion people in October; the United States will elect a new President in November; construction of the new Freedom Tower will be complete in New York City; the sun will flip its magnetic poles as it does at the end of every 11-year sunspot cycle; and, as I'm sure you've heard by now, the Mayan calendar completes its 5,125 year cycle, presumably portending the End of Days.
Mayans had three calendars. They had a solar calendar that was 365 days long, and a ceremonial calendar that was 260 days long. These two calendars would synchronize every 52 years. To measure longer time periods, they developed the "long count" calendar, which expressed dates as a series of five numbers, each less than twenty; something like the way we measure minutes and seconds as a series of two numbers each less than sixty. And, just in case this might seem too simple, for some reason the second to last number was always less than eighteen. The first day in the Mayan long count calendar was expressed as 0.0.0.0.0, and by our calendar, this was August 11, 3114 BC. Every 144,000 days (or about every 395 years, which they called a baktun), the first number would increment, and a new baktun would start. Recall how we all got to enjoy the excitement on the millennium of watching the digital displays roll over from 12/31/1999 to 1/1/2000? Well, that's what's going to happen on December 21, 2012 to the Mayan calendar. It's going to roll over from 22.214.171.124.19 to 126.96.36.199.0, just as it has done each of the previous twelve baktuns. There's no archaeological or historical evidence that the Mayans themselves expected anything other than a New Year's Eve party to happen on this date: Claims that this rollover represents a Mayan prediction of the end of the world appear to be a modern pop-culture invention. It's true that the Mayan carvings of their calendar only depicted 13 baktuns, but what did you expect them to do? Carve an infinitely long calendar every time they wanted to express a date? The explanation could be as simple as they didn't expect people in the 21st century to still be obsessed with their archaic calendar.
Another story predicting doom in 2012 says that a new planet, variously described as Planet X, a planet/comet (which makes no sense), or the planet "Nibiru" is going to pass so close to the Earth as to cause earthquakes and tidal waves and all kinds of destruction, possibly even flipping the Earth completely upside down. This is an urban legend that's been around for a long time, but for most of the story's history, this was supposed to happen in May of 2003, as any Internet search for "Planet X" will reveal. Apparently what happened is that the Planet X advocates, perhaps embarrassed or disappointed that 2003 passed without incident, heard about the much more popular Mayan calendar story, and decided that 2012 is close enough to 2003 that it must be the correct date and that the Planet X destruction is probably what the Mayans were foretelling. The Planet X legend got started by misinterpretations of astronomical observations combined with an ancient Sumerian carving that has been erroneously interpreted to depict a solar system with ten planets. Why the craftsmen who made carvings in ancient Sumeria should be presumed to have planetary knowledge superior to that of modern astronomy is not convincingly argued. If you're interested in all of the actual science behind the Planet X story, there's no better source than Phil Plait's "Bad Astronomy" blog, which goes into all the facts, rumors, and sources in detail.
Here's one more reason people are frightened about 2012. About 500 years ago, Copernicus confirmed what Hipparchus had observed in 2200 BC: that the axis of the Earth, which leans over at 23.5°, completes one full rotation every 25,765 years. This means that in 12,000 years, Christmas will come to Australia in winter and the northern hemisphere will depict Santa in Bermuda shorts. Astrologers call this period a Great Year, and they divide it into 12 Great Months or astrological "ages", each about 2,147 years long. Each age corresponds to one of the signs of the zodiac. We are currently in the Age of Pisces, and like the song says, we're soon going to enter the Age of Aquarius. According to modern official delineations of the edges of the constellations, we'll move into the new age in the year 2600. But there's some disagreement, and some astrologers place it at 2595, 2654, or 2638. A few put it much earlier, as soon as 2150 or even 2062. However, once the news of the Mayan calendar broke, a large segment of the astrological community abandoned the official constellation definitions and stated that the Age of Aquarius will begin in 2012. So, you can call this a third major reason why the world will end in 2012, but you have to be awful loose with your astrology, and you also have to think of some reason why the dawning of the Age of Aquarius might bring on the end of the world. I have not found any plausible claims for how it might have this effect.
So that's a lot of reasons, weak though they might be, to predict that the we're all going to die in 2012. However, there's one significant fact that the 2012 doomsayers all seem to forget: Despite all the various 2012-ish predictions for the end of the world, there are far more stories of apocalypse with different dates. For example, popular interpretations of Nostradamus found predictions for the end of the world in July of 1999, December of 1999, June of 2002, and October of 2005. It's also been said that his writings could mean the dead will rise from their graves in either 2000, 2007, or the year 7000. Nostradamus never said anything about 2012.
Many Protestant Christians believe that the end of the world will come in the form of what they call the Rapture, when the righteous will all be whisked away to heaven. Shakers believed the Rapture would come in 1792. Seventh Day Adventists first calculated it would happen in 1843, then when nothing happened, they found an error in their calculations and corrected it to 1844. The Jehovah's Witnesses made firm predictions for 1918, 1925, 1941, 1975, 1984, and 1994. A book was published in 1988 called 88 Reasons the Rapture is in 1988. A number of Bible scholars found firm scriptural evidence that the Rapture would happen in October of 2005. Thousands of Koreans gave away all their money and possessions in preparation for the Rapture on October 28, 1992. Even Sir Isaac Newton made a calculation based on scripture that showed the Rapture could not happen before 2060. Some Jewish scholars place the "end of days" via Armageddon in the year 2240. I couldn't find 2012 mentioned in any of these stories.
In fact, James Randi's magnum opus publication An Encyclopedia of Claims, Frauds, and Hoaxes of the Occult and Supernatural lists 44 distinct end of the world predictions that all came and went unfulfilled. Why should we think that the 2012 legends are any different? Any examination of the science behind any of the stories, even a glib examination, reveals a complete absence of plausible foundation. Only the Planet X story, which is the most easily falsified as it depends on concrete astronomical observations that are demonstrably false, offers a proposed mechanism for exactly how this "end of the world" is to be accomplished, the alleged gravitational destruction. Neither the Mayan calendar people, nor the Age of Aquarius people, have offered any claims for how or why the world will end, only that their particular legend points to a rollover in some ancient calendar. My calendar rolls over every time the ball drops in New York, and I've yet to see this cause any planetary cataclysm, except for the guy who has to mop out the drunk tank at the NYPD.
Many people tend to place more trust in ancient neolithic traditions than in the observations of modern science. There's nothing wrong with studying and respecting our predecessors' history for what it was, but when you turn things over and start believing that scientific knowledge of the natural world has only decreased over time, you're not doing anyone any favors.