Well, I won't say "no" because the Earth's rotation axis does change
and so does its rotational speed. But these effects are quite tiny.
For the most part, the Earth's rotation speed and the direction of
the axis are (nearly) constant because of the conservation of angular
momentum. You need to apply some external torque (an off-center push or
pull) to change the angular momentum of an object, and to a very good
approximation, there aren't any of these in space. So the Earth's
rotation axis stays pointed at the North Star and each day lasts 24
hours and the sun always rises in the East.
BUT! The Earth's rotation axis doesn't point exactly along the
axis of its angular momentum because it does have a tiny amount of
lopsidedness to its distribution of mass. And the moon creates an
asymmetrical tug on the lopsidedness of mass, and also causes tides in
the water, which have to slosh around and are a little offset from the
direct pull of the moon. The Sun contributes to the tidal bulges which
the moon also pulls on. There are thus externally applied torques, and
therefore some precession of the axis of rotation.
You can get the axis to precess even without the external torques
if the Earth isn't rotating around one of its principal axes -- think
of a football thrown up in the air, spinning mostly on its long axis
but with a little wobble to it. The spin axis of the football will move
around in a circular path. This is what the Earth's axis of rotation's
doing all the time. Every 26,000 years or so, the Earth's rotation axis
makes a complete circle in the sky. In 3000 BC, the North Star was not
Polaris, but Thuban, a star in the constealltion Draco. In 14,000 AD,
Vega a star in the constellatin Lyra, will be the new North Star. In
26,000 AD it'll point back to Polaris.
The rate at which the Earth rotates is also slowing down, ever so
slowly. This happens because of the action of the tides from the moon.
The change in strength of the moon's gravity from one side of the Earth
to the other causes water to bulge on the side of the Earth towards the
moon, and also on the side away. The Earth is turning all the time,
also pushing the tidal bulges around. It turns out that some of the
energy of the Earth's rotation goes into pushing the moon into a higher
orbit, and some of its energy goes into frictional heating of the
sloshing ocean water. The Earth's rotation rate slows at a rate of
0.005 seconds per year per year. This is enough to cause havoc with
calculations of when and where solar eclipses were visible in ancient
times. If the day is constant in length, then a different part of the
Earth would be under the moon's shadow than if it were slowing down.
The accumulated time difference is a few hours over the course of a
couple of thousand years. Over the 4.6 billion year life of the Earth,
this means that the day started out about 14 hours long.
Tom
(published on 10/22/2007)
