Like gum sticking to the bottom of a chair, iron-rich sediments appear to be sticking to the boundary between Earth's core and mantle, creating drag that alters the planet's already wobbly rotation.

The action is all 1,800 miles (2,900 kilometers) under your feet, where the hot liquid core meets the mostly solid mantle. The two regions rotate at different rates, creating friction and generating the magnetic field that emanates from Earth's poles and envelopes the planet.

A new theory proposes that iron-rich sediments are floating to the top of the core and sticking to the bottom of the mantle, creating drag that throws Earth's rotation about its axis off by 0.04 to 0.08 inch (a millimeter or two) over a period of about 18.6 years.

"The wobble is explained by metal patches attached to the core-mantle boundary," said Raymond Jeanloz, professor of geology and planetary science at the University of California, Berkeley. "As the outer core turns, its magnetic field lines are deflected by the patches and the core fluid gets slowed down, just like mountains rubbing against the atmosphere slows the Earth down."

The theory, first proposed by Bruce Buffett of the University of British Columbia, might also explain a peculiar slowing of seismic waves that ripple along the core-mantle boundary after an earthquake.

The wobble values that the theory explains have been adopted by the International Astronomical Union as its standard for calculating the position of Earth's axis into the past as well as the future.

But Earth resists this pull. The result is that the axis moves in a cone-shaped pattern, called a precession, with the celestial North Pole describing a full circle every 26,000 years or so. Right now, the north celestial pole points towards Polaris, the Northern Star, but this has not always been the case, and it will change in the future.

During this precession, the planet's tilt remains at 23.5 degrees.

The Earth's precession alters the time of the annual vernal and autumnal equinoxes, making them arrive earlier each year. Each equinox is the day between seasons when there are the same number of daylight and dark hours. They occur when the Sun's light shines directly on the equator.

An annual deviation that lagged behind the tidal pull of the Sun first suggested to Buffett 10 years ago that strange processes may be going on at the boundary between the mantle, made up of viscous rock that extends 1,800 miles below the crust, and the outer core, which is thought to be liquid iron with the consistency of water. The inner core, made of very pure, solid iron, along with the outer core rotate, dragging the Earth's magnetic field with them.

"The Earth is getting pulled and tugged at regular periods, but we observe a difference in the way the Earth responds to these tugs and pulls and what we predict," Buffett said. "One of the ways you could explain that is by having some dissipation in the vicinity of the core-mantle boundary as the fluid moves back and forth relative to the mantle. But the viscosity of the fluid core is comparable to water, and having water slosh back and forth relative to a rigid mantle wasn't going to produce the kinds of dissipation we needed to see."

He hit on another way the rotating core could dissipate energy -- via electrical drag.

Based on experiments Jeanloz, the Berkeley researcher, had performed on the chemistry of rocks at the high temperatures and pressures characteristic of the core-mantle boundary, Buffett suggested that silicon-containing minerals would float to the top of the liquid outer core, carrying iron with it. Together they would form an iron-rich, porous sediment at the mantle boundary that would stick to the mantle, settling into depressions.

Because Earth's core rotates about a slightly different axis than the mantle (due to the tug of the Sun and Moon), the core's magnetic field is dragged through the mantle, passing unhindered because the mantle does not conduct electricity. The porous, iron-containing sediment stuck to the mantle, however, would resist the rotation of the magnetic field, creating just enough tug to perturb Earth's rotation.

"As the core rotates it sweeps the magnetic field with it, which easily slips through the mantle with no resistance," said Buffett. "But if the bottom of the mantle has conductivity, then it's not so easy to slip the magnetic field lines through the mantle."

The magnetic field tends to stretch and shear or pull out across the interface, generating currents that damp out the motion. Buffet said this dampening creates the kind of dissipation needed to explain the lag.

The work was supported by the National Science Foundation and other institutions.