Precession

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. This additional motion is called precession.

What is the cause of precession?

A precessing gyroscope

Precession is the same kind of motion you see in a spinning top or gyroscope. The axis of the top or gyroscope will wobble slightly, with the point describing a circle. This is a normal motion of spinning celestial bodies.

The major cause of precession is gravity. The Earth is not a perfect sphere. It has a bulge around the equator. The tidal forces of the Moon and Sun pull on the equatorial bulge. This is like pushing on the gyroscope, which causes it to move at a 90 degree angle from the direction of the force.

What are the effects of precession?

If you look at the graphic in the top right corner, you will see that precession causes the north celestial pole (the imaginary point in space that the earth's north pole points to) to move in a circle. It takes a very long time for it to complete the circle. The primary effect of this is a change in the positions of the constellations in the sky over time. Eventually our 'Winter' constellations will be 'Summer' constellations.

The precession is what 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?

Absolutely nothing.

Some 2012 proponents have claimed that 'the earth's axis will change in 2012' implying that a sudden change will happen. This is not precession. Short of a direct impact from a mars-sized object there is nothing that will suddenly change the direction of the earth's axis. Let me reassure you again 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.

Precession is a slow, steady motion. It is not something that you can see or feel. Hipparchus discovered precession by examining several centuries of Greek and Babylonian records.

For more information on precession and other motions of the earth, see http://www.opencourse.info/astronomy/introduction/03.motion_earth/