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If put enough weight on a particular point on Earth's surface disturbing the balance between hemispheres, is it possible that the Earth's spin could change like an unbalanced spinning top?

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  • $\begingroup$ Answered here: physics.stackexchange.com/questions/156744/… $\endgroup$ – called2voyage May 16 '16 at 21:28
  • $\begingroup$ By the way I think this is on topic here in Astronomy too, because true polar wander has been studied (modelled) on, for example, Io. $\endgroup$ – Andy May 17 '16 at 7:37
  • $\begingroup$ @Andy This has been addressed early on in the community. Just because a question is applicable to other worlds doesn't mean asking that question about Earth is on topic, unless the question is broadened to include other worlds as well. $\endgroup$ – called2voyage May 17 '16 at 14:12
  • $\begingroup$ @Andy The reason for that is that early on on this site we received a relatively large volume of questions about Earth that drew attention away from more astronomy-focused questions. $\endgroup$ – called2voyage May 17 '16 at 14:14
  • $\begingroup$ @called2voyage thanks, understood. I was confused by the blurb above "unless directly related to phenomena observable on other celestials", but of course that would mean most of Geology would become astronomy :) But I get it now. $\endgroup$ – Andy May 17 '16 at 14:39
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The Earth does spin like an unbalanced top. The Earth's rotation axis is not fixed. It instead moves in a complex manner due to a combination of external torques exerted by the Moon and Sun, a torque-free nutation due to the oblate shape of the Earth, and also due to changes on and in the Earth.

The torque-induced motions are called precession and nutation, distinguished by period. The largest and slowest of these motions is the axial precession. This causes the Earth's rotation axis to trace out a cone over the course of 26000 years.


(source: nasa.gov)

The torque-induced nutations are also cyclical motions induced by the Moon and the Sun. These are much smaller in magnitude and have a much shorter period. The largest of these has a magnitude of about 20 arc seconds and a period of 18.6 years. All other nutation terms have much smaller magnitude and have shorter period.

The torque-free nutation would have a period of about 305 days if the Earth was solid. The oceans, the atmosphere, and the outer core alter this. The Chandler wobble has a period of about 433 days and a magnitude of less than an arc second. Because the Chandler wobble isn't as predictable as are precession and nutation, it's lumped into a catch-all category called "polar motion." The redistribution of water over the course of a year (e.g., snow on Siberia in the winter but not in the summer) results in a yearly component of the polar motion.

There are lots and lots of other factors, all small. Polar motion is observed after the fact.

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  • $\begingroup$ How the Japan Earthquake Shortened Days on Earth: space.com/11115-japan-earthquake-shortened-earth-days.html "analysis of the 8.9-magnitude earthquake in Japan has found that the intense temblor has accelerated Earth's spin, shortening the length of the 24-hour day by 1.8 microseconds" $\endgroup$ – Wayfaring Stranger May 16 '16 at 21:17
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    $\begingroup$ @WayfaringStranger - That is a calculated result. The noise in the measurements are orders of magnitude higher than that. $\endgroup$ – David Hammen May 16 '16 at 21:53

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