Expert: Tom Whiting - 10/8/2006

Question
what does the tilt of earths axis cause the north and south poles to do?

Answer
Hi  Sevryn,
Actually, the 23.5 degree tilt of the Earth's axis from the
vertical, simply causes our poles to point in a different
direction in space then they otherwise would,  if we were perfectly vertical. If there were no tilt, then, for instance,
our north pole would always point toward  the North Celestial Pole, a fixed point,  up in the constellation Draco....but due to the 23.5 degree tilt, our North pole thus points toward a point
offset by 23.5 degrees from the North Celestial pole, which is
currently, very close to the star Polaris, in Ursa Minor.

Now if we didn't have a large moon revolving around the
Earth, 'tugging on the Earth',  that point would always remain in the same position in space. And I think that gets to your REAL question....which should have been:

"What does the revolution of the moon around the Earth cause the Earth's axis (and thus), the north and south poles to do?"

The gravitational pull of the moon causes our axis (and
therefore the poles) to rotate slowly in a circle around that 23.5 degree offset, which is called 'precession' with a period of about 25,800 years.  Thus the polar point is continuously moving in a circular manner in the sky.  This changes the north star about every 4000 years because our pointing direction is slowly changing.  (By an amount equal to about
50 arc-seconds per year).  So it's not just the "tilt", it's also the moon that causes the precessional effect.

At the actual north pole of the Earth, it amounts to an
area about the size of a tennis court.  But when extended
out into space, it creates a large circle about 47 degrees
(23.5 doubled) in diameter in our night sky.  As far as astronomy goes, that means our north star is continuously changing over a long period of time as our axis rotates around the Celestial North Pole once every 25,800 years.  When the Pyramids were being built about 4000 years ago, the star Thuban (Alpha Draconis) was our "North Star"....currently we are within 3/4 of a degree from Polaris.
Closest approach to Polaris will be in 2105 AD when our
pole will be only 1/2 degree from Polaris.  Then it's all down
hill after that, for about 4000 years at which time our pole will point close to the star Gamma Cephei up in Cepheus.
In about 12,000 years, the bright star Vega will be our
North star, and it also was, about 14,000 years ago.

So it's just not the 23.5 degree tilt, it's the gravitational tug
of the moon that is the real cause of precession.  If there were
no moon, there would be no movement of our axis, and
no precessional movement, no matter how much the tilt.

Hope all this helps,
Tom Whiting
Erie, PA