Really Neat Graphic Number 2

To start the animation click the play button.  You can stop the earth's spin at any time by clicking directly on the earth, as in the last graphic.

In this animation, the red marker (again, remember the red marker is the earth's rotational axis and what you see is the North Pole) is in motion.  If you look closely, you can see that red marker (indicating the North Pole) is actually rotating in the opposite direction as the normal daily rotation.

For all those who still need help with the idea of the earth's wobble (or precession), I will now break down the Really Neat Graphic Number 2 into several still pictures and discuss them, thus illuminating you to my infinite wealth of wisdom, with regard to precession that is.

We can't wait, Mr. Brisban!

I'm sure.  We will start with the picture to the right labeled, 'Very Pretty Graphic Number 3.'

Really Pretty Graphic Number 3

Oh, it is pretty, Mr. Brisban.

Of course it is.  It's pretty just like me.

Now be quiet, I have work to do.  The red arrow around the globe is the direction of the normal daily rotation of the earth that takes 23 hours and 56 minutes.  For more information on the daily rotation, go back to the No Frills Help from Sub-unit 1, "Apparent Motions of the Sun". To get back to this page you will need to maximize the window.

The blue arrow on the top of our animation indicates the earth's wobble, which takes 26,000 years to complete, meaning it takes 26,000 years to come back to the original starting point.

In this next picture we are looking straight down on the earth (onto the North Pole) just as in the animations above.

Fairly nice  graphic Number 4

In this picture notice how the pole seems to rotate around the center of the globe when looking down.  This rotation is very subtle and it takes hundreds of years to detect it unless you use highly accurate measuring devices, which experts, such as myself, know how to use.

Other experts and I now know what causes precession (the earth's wobble). Let us now discuss what exactly precession is and how it affects astronomy.

I'm ignoring you two.  The definition of precession is the movement of the celestial poles with respect to the stars.  Here is a real life situation to help you understand. 

If you look to our North Celestial Pole at night, what do you see?

A star?

Why yes, Dick and Jane.  Specifically, you should see the North Star, which is Polaris in the Ursa Minor constellation.  Now, 12,000 years from now (around 14,000AD), if we take precession into account, we will see that Polaris will no longer be the North Star.  In fact, Vega, in the constellation Lyra, will become the North Star.  Vega will be the North Star because the North Celestial Pole will no longer point to Polaris as it does now. 

Because of precession?

Exactly!  The following really nice graphic may help you understand this concept.

Really Nice Graphic Number 5

In this picture you see the celestial sphere surrounding the earth and it is important to know that this sphere does not move.  In the picture, there is a solid line that points to Polaris.  This is earth's orientation today.  But given time, the solid line will point to the star Vega (as shown by the dotted line on top).  This change occurs as the North Celestial Pole line (the solid line that points to Polaris) follows the circular path as the picture shows.