Is it me, or does 644 kilometers wide seem kind of small for the inner core of the Earth? Does it even stay in place then, or does it kind of drift around slightly within the outer core over time?
That seems like a point of reference thing to me, presumably the iron core is denser than the molten layer around it, gravity is pulling that towards it, but it's all spinning, and there are other gravitational influences from the moon, the rest of the solar system, etc. which could cause some slight sloshing around, I'd bet.
