# Q & A: Earth’s rotation changing

Q:
Will the Earth’s rotation or direction ever change?
- Elaine (age 13)
A:
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)

## Follow-Up #1: Herodotus and the sun

Q:
Just read Herodotus on Egypt (written plus minus 500BC) - he speaks to the priests of Egypt who advise him that in their recorded history the Sun has risen in the West and set in the East twice - doesn't state if the twice was a period of time each or just a day. What would cause this to happen please?
- Avril Browne (age 66)
Port Elizabeth, South Africa, 6065
A:

I'm afraid I can't give any reason at all for the sun to behave the way Herodotus said the priests said it behaved. It didn't happen, because it would violate basic physical laws. Who knows at what point some story of a real event (if any) might have been scrambled?

Mike W.

(published on 11/20/2016)