Chandler Wobble

The Chandler wobble is a small deviation in the Earth's axis of rotation relative to the fixed stars, which was discovered by American astronomer Seth Carlo Chandler in 1891. It amounts to change of about9 metres (30 ft) in the point at which the axis intersects the Earth's surface and has a period of 433 days. This wobble, or nutation, combines with another wobble with a period of one year, so that the total polar motion varies with a period of about 7 years.

The Chandler wobble is an example of the kind of motion that can occur for a spinning object that is not a sphere; this is called a free nutation. Somewhat confusingly, the direction of the Earth's spin axis relative to the stars also varies with different periods, and these motions (caused by the tidal attraction of the Moon and Sun) are also called nutations, except for the slowest, which is the precession of the equinoxes.

The existence of a free nutation of the Earth was predicted by Isaac Newton in Corollaries 20 to 22 of Proposition 66, Book 1 of the Philosophiæ Naturalis Principia Mathematica, and by Leonhard Euler in 1755 as part of his studies of the dynamics of rotating bodies. Based on the known ellipticity of the Earth, Euler predicted that it would have a period of 305 days. Several astronomers searched for motions with this period, but none were found. Chandler's contribution was to look for motions at any possible period; once the Chandler wobble was observed, the difference between its period and the one predicted by Euler was explained (by Simon Newcomb) as being caused by the non-rigidity of the Earth. The full explanation for the period also involves the fluid nature of the Earth's core and oceans: the wobble in fact produces a very small ocean tide with an amplitude of c. 6 mm, the pole tide, which is the only tide not caused by extraterrestrial bodies. Despite the small amplitude, the gravitational effect of the pole tide is easily detected by the superconducting gravimeter (see e.g. Fig. 2.3 in Virtanen 2006).

To measure the wobble, the International Latitude Observatories were established in 1899. (The wobble is also called the variation of latitude.) These provided data on the Chandler and annual wobble for most of the 20th century, though they were eventually superseded by other methods of measurement. Monitoring of the polar motion is now done by the International Earth Rotation Service.

The wobble's amplitude has varied since its discovery, reaching its largest size in 1910 and fluctuating noticeably from one decade to another. While it has to be maintained by changes in the mass distribution or angular momentum of the Earth's outer core, atmosphere, oceans, or crust (from earthquakes), for a long time the actual source was unclear, since no available motions seemed to be coherent with what was driving the wobble.

One promising theory for the source of the wobble was proposed by Richard Gross (2001) of the Jet Propulsion Laboratory, California. He used angular momentum models of the atmosphere and the oceans in computer simulations to show that during 1985.0–1996.0 the Chandler wobble was excited by a combination of atmospheric and oceanic processes, with the dominant excitation mechanism being ocean‐bottom pressure fluctuations. Gross found that two thirds of the 'wobble' was caused by fluctuating pressure on the sea bottom due to temperature and salinity changes and wind-driven changes in the circulation of the oceans. The remaining third is due to atmospheric fluctuations.