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Q & A: Spin of the center of the Earth

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Most recent answer: 01/09/2016
So, the earth spins at about 1,000 mph. My question is: is the very center of the earth still and unmoving? There has to be an axis, or axle, but if there were a dot or a small space at the very center, would it be still? Like when you draw a circumference in geometry, the anchor pin is stationary.
- Susan P. Blevins (age 70)
Houston, TX, USA

The core of the Earth spins. Both the outer core and the inner core spin in the same direction but at slightly different rates. Just in case you are not familiar with the structure of the Earth, if you dig down toward the center of the Earth, there are three layers called the mantle, the outer core and the inner core. The liqid outer core spins a little bit slower than the mantle, whose spin rate is one spin per a day (so the spin rate of the Earth that we usually think). On the other hand, the solid inner core spins a little bit faster than the mantle. But the difference in the rates is very small, so they spin at almost the same rate.

Now what about the "point" at the center of the Earth? I would say that, as you said, the point at the center "stays still" in the sense that it doesn't change position with respect to the average position of the Earth. Here I quote-and-quoted "stays still" because the spin of a mathematical point cannot be defined. A mathematical point does not have area nor volume. The length of its radius is zero. So we cannot define spin of a point.

So if you meant a mathematical point, it "stays still." However, if you meant a small volume at the center of the Earth, it spins at a little bit faster but almost the same rate as the one-spin-per-day rate of the Earth spin.



p.s. You may wonder why the different parts spin at slightly different rates. Shouldn't friction make them all spin at the same rate? It would except for some complications. One is that tidal friction caused by interaction with the Moon and Sun gradually slows the spin of the outer mantle. Then the magnetic fields in the core, driven by heat flow, cause some other torques. /Mike W.

(published on 01/09/2016)

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