What's going to happen on December 21st 2012?

Will the world end on 21st December 2012 because of the end of the Mayan calendar or because the winter solstice is "aligned" with the Milky Way?

It appears that Mayan ideas about time keeping and calendars were very cyclical. This is actually easy to understand because it's quite like our calendar which has cycles of various sizes very familiar to us. For example there is a:

• 1 Jan every year
• day 1 every month
• Monday every week
• 1am every day

The Mayan cycles were a bit more complex, such that every day in a 52 year period had a unique name from a combination of various different cycles (similar to the idea that there is a Monday 1st January only every 7 years or so). This 52 year cycle has is called a Calendar Round. To keep track of dates on longer time scales the Mayans then had what's known as the Long Count, which provides a unique numerical indicator for each day. Mayans did not count in base ten like we did, but usually instead in base 20 (although not always). The Mayan long count can be summarized as:

#days	Mayan count
1	1 kin
20	20 kin	1 uinal
360	360 kin	18 uinal	1 tun
7200	7200 kin	360 uinal	20 tun	1 kactun
144000	144000 kin	7200 uinal	400 tun	20 kactun	1 bactun

The name for a Mayan epoch apparently translates as 13 bactuns, which you can see is 13*144000 days or 5125.26 years (roughly). There is actually some minor disagreement over when the current Mayan long cycle started, but it was probably either August 11th or 13th 3114 BC, which means it comes to an end on either Dec 21st or 23rd 2012.

As I mentioned above the Mayan calendar was designed to be cyclical, so the fact that the long count comes to an end in Dec 2012, while having some significance for the Maya as the end of a great cycle (much like we celebrated the millennium (incorrectly as it happens) on Dec 31st 1999), does not mean that the "world will come to an end". It's actually true that there are Mayan names for periods of time longer than 13 bactuns, so that their calendar doesn't even end then, and even if it did there is no evidence to suggest that they (or anyone for that matter) have any special knowledge about the end of the world.

There are however a lot of theories knocking around the Internet which use the end of this calendar cycle to predict the end of the world. They often also mention the fact that Dec 21st is the winter solstice, and that the Sun on the solstice that year is "aligned" with the plane of the galaxy. On the winter solstice, the Sun always has a Declination of -23.5 degrees, and a Right Ascension of 18 hours, but exactly where this is on the sky relative to more distant stars changes very slowly due to the "precession of the equinoxes". We have a posted answer explaining this effect but how it's important in this answer (and how it was first noticed) is by the fact that it moves the position of the equinoxes, and solstices with a period of 26,000 years in a complete circle around the sky westward along the ecliptic. So the position of the winter solstice moves 360 degrees in 26,000 years. That means that it moves 360/26000 = 0.01 degrees a year. Defining an exact boundary for the plane of the Milky Way is tough, but it's at least 10-20 degrees wide across much of the sky, meaning that the solstice can be described as being "in the plane of the Milky Way" for 700-1400 years! To put it another way, the winter solstice that just past (2005) was only 0.1 degrees away from where it will be in 2012, a distance smaller than the size of the Sun itself (which is about 0.5 degrees in diameter). In any case the Sun crosses the plane of the Galaxy twice every year as we orbit around it, with no ill effect on Earth.

To conclude:

• The Mayan calendar does not predict the end of the world on Dec 12th 2012.
  1. The exact date of the end of the current Mayan Long Count is still a matter of debate amongst Mayan scholars, although it is likely to be around Dec 21 2012.
  2. The Mayan calendar is cyclical, and there are names for cycles longer than 13 bactuns of the Long Count which are coming to an end in 2012.
  Even if the Mayans did believe that the world would come to an end at the end of the Long Count (which I don't believe is true), there is no reason to assume that they have any special knowledge which would allow them to make this prediction correctly. You are free to believe the Sun won't come up tomorrow, but it will anyway....

• The fact that the winter solstice on 2012 is "aligned" with the plane of the Galaxy has no significance.
  1. It takes the winter solstice 700-1400 years to cross the plane of the Galaxy.
  2. The solstice last year (2005) was within 0.1 degrees (or 1/5th the size of the Sun) of where it will be on 2012.
  3. The Sun crosses the plane of the Milky Way twice every year with no ill effect.