Talk:True polar wander

"Nearly equal" moments of inertia

The discussion of the principal moments of inertia of the Earth as causes of true polar wander seems a bit misleading to me, though correct. The article states that true polar wander can occur when the polar and one of the equatorial moments are "nearly equal." The problem is in interpretation/implication of the phrase "nearly equal." Namely, the Earth is spherical to a good approximation, so the principal moments are equal to a good approximation, or "nearly equal" in the usual sense of the phrase; that is, the differences between them are small compared to their magnitudes. The "nearly equal" which is the condition for true polar wander, on the other hand, is that the difference between the two moments is small compared to the difference between the third moment and these two moments. More simply, without losing much precision, the normal condition is that the unique (larger) moment of inertia is the polar one, and the two equatorial moment are smaller and similar in magnitude. True polar wander, then, occurs when instead an equatorial moment is unique and the polar moment and the other equatorial moment are similar in magnitude. (This needs to be a bit better written (possibly even with equations!) and I'll likely forget about it before I have time for that, so I wanted to get a note written down here.) Jefromi (talk) 20:58, 31 July 2008 (UTC)

Yes, that's correct.Eric Kvaalen (talk) 17:37, 2 August 2008 (UTC)

"Dzhanibekov effect"

Actually, it's not necessary that the difference between one of the equatorial axes and the polar axis be small, if the higher moment of inertia around an equatorial axis is greater than that around the polar axis! In that case the earth would "tumble" after a while, putting the north pole at the south and the south pole at the north, and this could repeat over and over. See Tennis racket theorem and the video to the right. Eric Kvaalen (talk) 16:59, 15 September 2019 (UTC)

You got it. I tried to make it a little more clear. In thinking about it (very late at night), I think that if all 3 moment of inertia axes are different, it will still align the greatest one with the spin axis, but correct me if I'm wrong. In this case, there will be only 1 solution for the eigenvectors, so the two equatorial ones are necessarily the smallest, and it's in agreement with my thought experiment of spinning an oblong rounded block. Anyway, I'm sure enough that I'm write that I put it in the article :). Awickert (talk) 08:20, 18 January 2011 (UTC)

Zanclean flood and floods of similar nature and magnitude as potential triggers for true polar wander

I contributed that segment. Is it relevant (both scientifically and with regard to whether it belongs on Wikipedia) or speculation on my part? User:Jay Yoon(talk) 02:19, 9 March 2012 (UTC) — Preceding unsigned comment added by Jay.yoon314 (talk • contribs)

Rugby/football analogy

This analogy is confusing. Where is the equator of a rugby ball? Audiosqueegee (talk) 14:10, 4 December 2013 (UTC)

I second this! Such a ball seems to have stitches on it, which make an arbitrary line more interesting than the non-marked equatorial plane. I never touched an american football in my life. I am from germany, do like sports but would not know a non-sperical ball that is no a shuttlecock.

For a better example: How about spinning a pen? Everyone reading wikipedia should have pencil-like objects in their vincinity.

Stand the pen upright. The equator will be a short ring around its middle. The tip and cap of the pen are its poles, furthest from the equator. 2001:4C80:40:488:92B1:1CFF:FE75:E2E3 (talk) 11:28, 21 August 2017 (UTC)

Earth's polar motion

I would submit (if anybody is reading this...) that the International Earth Rotation Service (the folks who've been bringing you sunrise every morning since 1987) have an excellent plot of the earth's pole's motion relative to the crust (ie. true wander) here: https://www.iers.org/IERS/EN/Science/EarthRotation/PolarMotionPlot.html?nn=12932 which may be even something which could be displayed here, if someone is fluent in navigating the legalities of such things, though the site says "all rights reserved", but it appears to be a non-profit of some sort. Anyway, at the least this is an excellent page to link to in the references section, yes?

Note, the diagram is given in seconds of arc, which at the earth's surface at the pole works out to 30.8 metres; the yearly circular wanders are about 0.2 arc-sec, or 6m radius. And on a longer time scale, the pole seems currently to be hell-bent for Toronto*, at about 10cm/year, so it should get there in about 51 million years (obviously extrapolation from one century's data is not warranted, but that's the current trend).

• that direction is almost exact - to 80deg west longitude

173.180.149.41 (talk) 07:33, 30 May 2017 (UTC) - just some wiki reader (and occasional grammar/spelling editor)

Well, since about AD 2000 the direction has changed and it's now headed for London! See Polar motion#Causes. Eric Kvaalen (talk) 16:59, 15 September 2019 (UTC)