An estimate of motion between the spin axis and the hotspots over the past century

Estimates of the motion of the spin axis over the past century either neglect plate motion or are relative to the mean lithosphere. To understand the response of the spin axis to mass changes, it is more useful to estimate spin axis motion relative to the hotspots because the hotspots well represent the solid earth. The hotspots originate in the lower mantle, which comprises 2/3 of the earth's volume. Relative to the lower mantle the plates and the upper mantle move in opposite directions, to a large extent canceling each other out. The mean lithosphere moves quickly relative to the hotspots and poorly represents the solid earth. The spin axis has moved relative to the hotspots over the past century along the 68.4 degreesW meridian at 4.03 milliarcsecond/ yr. This motion is 11degrees counterclockwise of, 0.5 milliarcsecond/ yr faster than, and 25% different than spin axis motion relative to the mean lithosphere.