The Short Life of the Average Fiat (Paper) Currency
Most Americans are so accustomed to spending U.S. government paper currency today that they probably assume it has always been that way in this country. Even many numismatists don’t realize...
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Seismic waves from powerful repeating earthquakes do show up differently at different times. The best interpretation of this is that the rotation of the core is changing over time, relative to the rotation of the mantle. That is... the core is always rotating in the same direction that the entire planet rotates (eastward), but sometimes it rotates faster than the outer layers, other times it rotates slower, and there are times when the rotations match up.
To someone standing in one spot on Earth's surface, these three states can appear, respectively, as though the core is spinning forwards, backwards, or that the spin has stopped.
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... With the variations in the rotation of the inner core revealed here, this means that it could also have a six to seven decade oscillation. ...