What is Precession?
Everybody knows (or should know) that the earth spins on its axis once per day. Also the earth orbits the sun, completing a full orbit once a year. In addition to these motions, the axis of the earth 'wobbles' slightly, and very slowly, completing a circle every 26,000 years or so. (See the video, below.) This additional motion is called precession. Other terms that you'll hear in connection with this phenomenon are "precession of the equinoxes" and "Milankovitch Precession".
|A precessing gyroscope|
The precession you've just seen is the same kind of motion you see in a spinning top or gyroscope. Gravity pulls downward on the gyroscope, which reacts by wobbling slightly. The point of its rotational axis describes a circle. This is a normal motion of spinning celestial bodies since they are, in effect, giant gyroscopes.
What is the cause of precession?
The major cause of precession is gravity. Because the Earth rotates, it isn't a perfect sphere: it has a bulge around the equator. (See the video What Causes Precession?) The gravitational forces of the Moon and Sun pull on the equatorial bulge, causing the Earth to wobble.1
What are the effects of precession?
Precession is a slow, steady motion. It's not something that you can see or feel.
However, it has several effects that can be detected through meticulous naked-eye astronomy over long periods of time. Hipparchus discovered it by examining several centuries of Greek and Babylonian records. The ancient Maya were certainly capable of discovering it, but the extent of the Maya's knowledge of precession is uncertain.2
So: what effects of precession would have been detectable by ancient astronomers? One effect is that "we get a different North star" over time. This occurs because the "North star" is just the star at which the North end of the Earth's axis points. As the axis slowly wobbles, it points to different stars. The graphic in the top right corner of this page shows this phenomenon on a standard star chart. You'll see that the north celestial pole (the imaginary point in space that the earth's north pole points to) moves in a circle. It takes about 26,000 years to complete the circle.
A second observable effect of precession is a change in the range of dates during which a given star or constellation will be visible each year. Over time, our 'Winter' constellations become our 'Summer' constellations, and vice-versa.
In other words, precession causes the constellations to ever so slowly drift from their historical positions. That’s why the dawn rising of Sirius no longer signals the ‘Dog Days of Summer’, and one of the reasons why Astrology is bunk.
What does precession have to do with 2012?
There's much confusion surrounding precession in claims about 2012.
Some 2012 proponents have claimed that 'the earth's axis will change in 2012', implying that a sudden change will happen. As you can tell from the above discussion, such a sudden change is not precession. Most importantly, it cannot be brought about by anything short of a direct impact from a Mars-sized object.3 Since claims of a sudden change in 2012 are common, we should emphasize here that there is no such object on a collision course with earth. If there were, hundreds of thousands of amateur astronomers would have seen it by now. There is no way to hide such an object.
Where precession is relevant to "2012" is in bringing about (or supposedly bringing about) some type of Galactic Alignment. However, the Galactic Alignment claims have their own fallacies.
For more information
For more information on precession and other motions of the earth, see http://www.opencourse.info/astronomy/introduction/03.motion_earth/