The Mayayns were excellent astronomers, excellent enough to observe (correctly) that there are basically two XY coordinate systems in the night sky as seen from Earth:

1.) The coordinate system of the "fixed stars", with the X axis defined by the plane of the Milky Way galaxy, and the Y axis defined as a perpendicular axis running through the galactic central bulge, which is essentially the axis around which the stars in our galaxy revolve. and yes, there is a super massive black hole at the origin of this coordinate system, although that fact is scientifically irrelevant.

2.) The coordinate system of the "moving stars", or planets, with the X axis defined by the plane of our solar system and a Y axis perpendicular to it, passing through our sun, which is essentially the axis around which the local planetary system revolves.

What was truly astute of them, was that they noticed (again, correctly) that the local solar coordinate system precesses over time relative to the galactic coordinate system, and at some time in their past, and some time in their future, these two coordinate systems would coincide, or basically align one overtop of the other, during these two times. The chose the amount of time between these alignments to define their "long count" calendar, and they understood that their civilization only occupied a small sliver of time somewhere between these two alignments. But nonetheless, through careful observations of the planets motions and the seasons, they were able to very accurately predict the dates when these alignment would occur. Kudos to them for some pretty astute, but not impossible, stone-age astronomy.

However, at no time did the Mayans claim that these alignments had any greater significance than the Mayan New Year. Scientifically speaking, there is no quantifiable difference in gravity, electric, or magnetic field here on on near Earth between last week, last century, today, or tomorrow. The galactic central bulge is just too far away, and the minuscule difference in angle of precession over the past few hundred years means there is effectively no change over that timescale, impaired to the enormous universal timescales involved in the entire cycle.

Moral of the story? The Mayans likely planned to do the same thing on 12/22/2012 as we usually plan to do on January first of any gregorian year, they planned to face the new year with a sense of renewal and hope for the future, just like we do every New Year. Possibly with. Hangover, but much the same as we do, nonetheless. so sit back, relax, and realize that you have nothing to fear from Nature or the ever-fear inducing, yet never occurring Apocalypse, and the biggest thing any of us have to fear are the actions of mentally unstable people who actually believe this apocalyptic garbage, and set about causing chaos to validate their own self-fulfilling prophecy.

Crazy universe? not scary at all. Crazy people? Be afraid. Be very afraid.