I’ve tried finding a clear-cut, straightforward difference between the Chandler wobble and the so -called “axis wobble” or precession of the equinoxes.
Even the below explanation doesn’t quite satisfy me. I look forward to your feedback.
davidmich
Earth’s axis of rotation relative to the Earth’s surface vs. direction of the Earth’s spin axis relative to the stars
The Chandler wobble is a small motion in the Earth’s axis of rotation relative to the Earth’s surface, which was discovered by American astronomer Seth Carlo Chandler in 1891. It amounts to 0.7 arcseconds (about 15 meters on the Earth’s surface) and has a period of 433 days. This wobble combines with another wobble with a period of one year so that the total polar motion varies with a period of about 7 years. The Chandler wobble is an example of the kind of motion that can occur for a spinning object that is not a sphere; this is called a free nutation. Somewhat confusingly, the direction of the Earth’s spin axis relative to the stars also varies with different periods, and these motions (caused by the tidal attraction of the Moon and Sun) are also called nutations, except for the slowest, which is the precession of the equinoxes.
"The Chandler wobble of the earth is 6 different wobbles, not just one. This is cyclic, and not all at the same time. This wobble causes the El Nino cycles as well. "
The Chandler wobble is (as your quote above says) due to the fact that Earth isn’t a perfect sphere. Basically, any object that isn’t a perfect sphere has some preferred axes that it “likes” to spin around. If you set an object like this spinning about an axis that’s not quite aligned with one of these preferred axes, then the axis of rotation will actually change relative to the body, describing a circle around the preferred axis. This is what’s known as “torque-free precession.” It happens even in the absence of any outside forces acting on the object. You may have noticed this happening when you throw a frisbee poorly — the disc will seem to wobble for a bit before effects from the air damp out the wobbling.
The precession of the equinoxes is a different beast. It’s due to the gravitational effects of the Sun and Moon (mostly them, plus a tiny bit from the other planets) on the Earth. This also causes the Earth’s spin axis to change, albeit much more slowly.
Perhaps the best way to put it is this: Suppose that aliens teleported the Earth out of the Solar System tomorrow, leaving it spinning just as it was. The Chandler wobble would proceed as it had before, since it doesn’t occur because of outside forces — it’s just a property of rotating objects. However, the precession of the equinoxes would stop, since it’s due to external forces.
MikeS. Thank you for that clear feedback. Could you tell me what the 6 wobbles(that supposedly make up the Chandler wobble) are that I quoted up above. I haven’t been able to find any reference to those anywhere else.
davidmich
The confusion there might just be someone trying to differentiate between two slightly different things: what the spin axis of the Earth is doing according to someone on Earth’s surface, and what the spin axis of the Earth is doing according to an alien hovering nearby watching the Earth spin. Certain properties of the rotation (the angular momentum, if you know what that is) will appear to change according to observers on the Earth, but that’s because the observers on the Earth are themselves moving; if you ask the alien hovering nearby, it’ll say that the angular momentum of the Earth is constant.
As far as the six components of the wobble you mentioned above, I’m not familiar with that. I’m teaching a course currently that involves some of this stuff, and if I happen across any information about it, I’ll check back in. (It’ll be a couple months before this topic becomes relevant, though.)
