# Earth's stationary iron core

Why is the earth's iron core stationary, while the liquid matal circles around it creating the magnetic shield. Don't understand how can the entire planet rotate where as the planet's center is stationary. Anyone could explain this?

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i dont believe thats the case – propaganda Jan 22 '12 at 22:39
In physics when we say "stationary" we must always ask "stationary relative to what?" – Mark Beadles Jan 23 '12 at 2:12
@Mark: No we don't. For rotations we never ask "stationary relative to what?". – Ron Maimon Jan 23 '12 at 4:36
@RonMaimon Of course you are right and I spoke loosely. Sorry. – Mark Beadles Jan 23 '12 at 5:13

Research shows that Earth's solid inner core is rotating slightly faster than the rest of the planet.

Rotation of the inner core has been suggested based on large set of seismographic data showing that the time that seismic waves generated by an earthquake at one location need to reach a seismographic equipment at another location steadily varied over the period of a few decades. This is currently explained by slow super-rotation of the inner core together with variations in inner core's structure which causes the speed of the waves to vary depending on the path through the inner core.

Relevant articles:

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Great links, Adam - in particular I'm fascinated by the discovery that the core is asymmetrical. – Mark Beadles Jan 23 '12 at 2:11

The earth's core is solid but it is only stationery relative to the surface - it rotates at (approximately) the same rate as the earth.

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To answer the main misunderstanding, since the inner core includes the central axis of rotation of the earth, in the rest frame of the center of mass of the earth, the axis is stationary. This is true of all rotations of bodies, any shape.

As soon as one examines matter even fractions of a mm away from the axis of rotation there exists a velocity, very small, but there, since angular velocity is constant for a whole rigid body rotating about an axis.

Earth is not a rigid body so the inner core could be moving at a different rotation rate then the outer, and it seems such a small difference has been discovered, as other answers have said.

The misunderanding come from giving the attributes of the axis of rotation to the whole core.

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