Earth's Spin Cycle

The solid inner core -- a mass of iron comparable to the size of the Moon -- spins faster than the outer portion of the iron core, which is liquid. A study in 1996 showed that over the previous century, the extra speed caused the inner core to gain a quarter-turn on the planet as a whole. So the inner core makes a complete revolution with respect to the rest of Earth in about 400 years. Immense pressure keeps it solid.