Yes, yes, the Great One, I, Brisban, have returned to help guide you and illuminate your path on our journey to immense wisdom.  Well, maybe not to immense wisdom, but at least to explain how is it that the vernal equinox can move with respect to the stars.

We're here too, Mr. Brisban, ready for you to teach us!

Oh, man, I thought I managed to leave you two at the last gas station!

We caught a ride with a trucker.  He was nice.

I don't think I will ask any further questions.  On to our topic.  This movement we see the vernal equinox exhibit is actually called precession.   Hipparchus, an early astronomer who lived around 150 BC was the first to discover precession.  In 134 BC, Hipparchus created a catalog that contained the position and brightness of just under 1000 stars.  When he finished, he compared his catalog with one that was over 150 years old and he found that some stars had shifted in the sky up to 2 degrees. The fact is that the precession shifts the vernal equinox 1 degree every 78 years.

That was a good history lesson, Mr. Brisban.  But what does Hippy Chorus have to do with us understanding precession.

That's 'Hipparchus' you dolts!  And if you were listening you would have heard that the vernal equinox shifts by 1 degree every 78 years.

Oh, we're sorry, Mr. Brisban.  We see how Hip Parka helped.  Please don't be mad, Mr. Brisban.

If I wasn't a fixed icon I'd slap myself on the forehead.  But back on topic, what does it all mean?  And what causes precession?  Precession is cause by the wobble of the earth. 

The following two animations will try to give you an idea of what we mean by the earth's wobble.

Really Neat Graphic Number 1

To start the animation click the play button.  You can stop the earth's spin at any time by clicking directly on the earth.  This animation shows the earth's axis (the red marker) always pointing in the same direction.  But this is not what happens with our planet.

Oooh! That's neat.

That's why it's called the 'Really Neat Graphic Number 1'

It is really neat, so that that makes sense that it's called 'really neat.'

Of course it makes sense.