From 2012 Wiki
The 'Galactic Alignment' is an astronomical phenomenon that refers to an alignment between the December solstice sun with the equator of our galaxy, the Milky Way. It is usually associated with the end of the ancient Mayan calendar in 2012. It was originally popularised by John Major Jenkins, and is now a integral part of many new age proposals of how 2012 might affect us.
The Galactic Alignment is the alignment of the December solstice sun with the Galactic equator. This alignment occurs as a result of the precession of the equinoxes. Precession is caused by the earth wobbling very slowly on its axis and shifts the position of the equinoxes and solstices one degree every 71.5 years. Because the sun is one-half of a degree wide, it will take the December solstice sun 36 years to precess through the Galactic equator.
The Galactic Equator is a virtual line that describes 0 degrees longitude and 0 degrees latitude, and acts as a divisionary line between the northern and southern hemispheres of the Milky Way galaxy. http://en.wikipedia.org/wiki/Galactic_equator
The precise alignment of the solstice point with the Galactic equator was calculated to occur in 1998 (Jean Meeus, Mathematical Astronomy Morsels, 1997 - mentioned by Jenkins at http://alignment2012.com/whatisGA.htm ). This date was further refined by Smelyakov to May 7 1998 (http://tech.groups.yahoo.com/group/TIMEWAVEZERO2012/message/15675). Therefore, if the significance of 2012 is related to a precise intersecting of the Galactic Equator, whatever that significance would have meant would already have occurred.
Due the width of Sun causing an inprecise alignment to still be a visual alignment, the Galactic Alignment "zone" is 1998 +/- 18 years = 1980 - 2016. This is, according to Jenkins, "era-2012."
This Galactic Alignment occurs only once every 25,800 years, and is possibly what the ancient Maya were pointing to with the 2012 end-date of their Long Count calendar.
 Quotes from John Major Jenkins
The "Galactic Alignment" of 2012 was first popularized by author John Major Jenkins, in Maya Cosmogenesis 2012 (Bear & Co, 1998). He spends a lot of time discussing why the Galactic Alignment is meaningful in a 2012 context, but it is only in Appendix 5 of his book, titled Reponse to CounterArguments, that he says:
In the interest of clarity, I will mention that it would be more accurate to say that the alignment occurs in the era of A.D. 2012; because precession is such a slow phenomenon, fifty years on either side of 2012 might be appropriate. http://2stroker.net/app5.htm
This is despite saying earlier in the book, relatively misleading statements like:
Amazingly, the center of this cosmic cross, that is, right where the ecliptic crosses over the Milky Way, is exactly where the December solstice sun will be in A.D 2012. This alignment occurs only once every 25,800 years. [page XXXIX, italics are his]
The bottom line of my theory is that the ancient Maya chose the 2012 end-date because this is the date on which occurs a rare alignment of the solstice sun with the Galactic Center [page XLI, italics are his]
The Long Count calendar is a galactic calendar because it pinpoints a rare alignment with our Milky Way Galaxy, due to occur in A.D. 2012 - a date written as 18.104.22.168.0 in the Long Count [page 105]
He fixes this misconception in Galactic Alignment (Bear & Co, 2002):
Early on in my research, I recognised that the solstice-galaxy alignment would occur around 1998 or 1999. This was based on precise astronomical concepts (e.g. galactic equator and solstice colure) and a rough estimate I made with EZCosmos astronomy software, confirmed by the calculations of European astronomer Jean Meeus and the US Naval Observatory. I'll have more to say about this precise timing in chapter 21, but suffice it to say two things here. First, the fourteen-year error between 1998 and 2012 amounts to less than one-fifth of a degree - a tiny sliver of the width of the full moon and thus negligible, considering it is a forward calculation in precession of at least 2,000 years. Second, the sun itself is one-half a degree wide, so an alignment zone between 1980 and 2016 must be allowed, thus embracing the 2012 end-date [page 12]
And henceforth he speaks of the alignment being in the era of 2012.
In chapter 21 he says the alignment with the galactic center isn't until 2220 AD! Technically speaking, the alignment of 1998 is with the "galactic equator", which is not the same as the "galactic center".
 Galactic Equator or Galactic Center?
The alignment of May 1998 was between the December solstice point and the galactic equator. The galactic center which is, according to astrophysicists, around 5 and a half degrees south of the ecliptic, will never have an alignment with the Sun (or solstice point, equinox point, or any other point on the ecliptic). However, if we ignore the physicists with their radio telescopes and their view that Sagittarius A* is at the center of the galaxy, and simply look at the Milky Way with the naked eye, it could be speculated that the Mayans considered the location of the solstice Sun of December 2012 to be in some way the most significant point on the ecliptic, and that they aligned their calendar accordingly.
Incidently, during the precessional cycle, the date when the Earth's south pole is most tilted towards the physicists' galactic center is around 2226 AD.
The Milky Way Galaxy is the inspiration for the symbol of the Ouroboros. Myth refers to a serpent of light residing in the heavens. The Milky Way is this serpent, and viewed at galactic central point near Sagittarius, this serpent eats its own tail. The Milky Way Galaxy keeps a great time cycle that ends in catastrophic change. The sign of the Suntelia Aion is the sun rising out of the mouth of the ouroboros, which allegedly will occur on the solstice, December 21, 2012. The Greeks called the End of the Age the Suntelia Aion. Ancient historians and especially Plato referred to a cycle of catastrophe at the End of the Age. The AION was symbolized by the Ouroboros.