Chandler wobble

The Chandler wobble is a free rigid-body wobble of the Earth's rotation axis with a period of approximately 433 days and an amplitude of a few metres at the surface. It was discovered in 1891 by the American actuary-astronomer Seth Carlo Chandler, who found the 433-day periodic signal in pooled historical latitude observations from observatories worldwide.

Its physical origin is Euler's prediction (1758) that a rigid body rotating about an axis not exactly aligned with its principal axis of inertia undergoes a free nutation. For a perfectly rigid Earth the predicted period would be 305 days (Euler's period). The observed 433-day period reflects the Earth's elastic response: the equatorial bulge yields slightly to the wobble-induced stresses, extending the period by about 40%. Simon Newcomb worked out this physical explanation within a year of Chandler's discovery.

The wobble should damp out in about 70 years through internal friction, but it has not; it is continually re-excited, apparently by ocean-bottom pressure fluctuations and atmospheric variability acting as a stochastic forcing on the Earth-as-rigid-body. Modern space-geodetic techniques track the wobble to sub-millimetre precision, and it is a standard input to every high-accuracy Earth-orientation model. Its ongoing amplitude — the equilibrium between random forcing and internal damping — is still the subject of active geophysical research.