Wobbleon

Wobbleon holds up a spinning globe, and its pole drifts in a small circle he cannot quite stop.

The Physics

Earth's rotation axis is not fixed to the crust; the pole wanders in a loop a few metres across. In 1891 S. C. Chandler found a component with a period of about 14 months, about 433 days. Wilson and Vicente (1990), using 86 years of optical astrometry, estimated 433.0 ± 1.1 days with a quality factor Q ≈ 179; later estimates fall near 431 days. A rigid Earth of the same shape would wobble freely with a period of about 305 days; the elastic mantle, the fluid core and the oceans lengthen it.

The wobble damps, so something must keep exciting it. Richard Gross showed in 2000 that during 1985–1996 it was driven mainly by changes in ocean-bottom pressure, with the atmosphere second.

T_Chandler ≈ 433 days vs T_rigid ≈ 305 days

• T_Chandler: observed free-wobble period
• T_rigid: Euler period for a rigid Earth of the same shape

S. C. Chandler, Astronomical Journal 11, 59 (1891); Gross, Geophys. Res. Lett. 27, 2329 (2000).

Honest caveat: Gross's result covers 1985–1996; the balance of excitation in other periods is still studied. The wobble is not precession, and its size in metres varies.

On the shirt

Front: Wobbleon, the pixel-art character, with “THE CHANDLER WOBBLE”. Back: the wandering pole, the 433-day period, the rigid-Earth comparison, the ocean-pressure driver and Chandler's 1891 paper. Printed on a unisex tee.