The super-rotation of the inner core explains the details in differential arrival times of seismic waves from very distant earthquakes. The defining characteristic of an inverse problem is the challenge of determining properties of an unobserved cause based on observed properties of the effect. This provides an opportunity to raise some worthwhile methodological questions. The explanation, the account of what is changing and thereby causing the increase in arrival-time difference, starts with the idea that the inner core is grainy, like wood, with the grain running more or less parallel to the axis of Earth's rotation. The logical challenge of an inverse problem is characteristic of the challenge of most scientific reasoning. The geo-dynamic modeling of Earth's magnetic field suggests the faster rotation. No particular case is the confirmation of the rotation by virtue of being more direct. The credibility of the hypothesis derives from the agreement among the different kinds of data.
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