Atmosphere linked to Earth’s rotation

One of the annoying features of the Earth as a planet is that it engages in a kind of Saint Vitus’ dance.  The best known of its wandering are those involving variations in the eccentricity of its orbit, and the tilt and precession of its axis of rotation.  These follow from the gravitational influences of massive planets elsewhere in the Solar System, and are implicated in the modulation of climatic change through the last 2.5 Ma.  Rather less well-known, and even more aggravating are far more rapid, but geometrically quite small deviations from good behaviour.  One of these is the habit of the spin axis to wander around the geographic poles within a circle roughly 3 to 6 metres across.  It does this every 14 months.  It takes a certain degree of dedication to chart such a tiny planetary tic.  Chandler Wobble is the single claim to fame of its eponymous discoverer.  Seth Carlo Chandler Jr, an American businessman and amateur astronomer, discovered the quirk in 1891 by observing stars with a degree of single-mindedness that might have put a lesser mortal on the couch.  He set out to verify the famous Swiss mathematician Leonhard Euler’s prediction that the Earth ought to wobble every year, and he did.

So minuscule is Chandler Wobble, that keeping it going is something of a vexing problem, for a single jostle’s effect ought to fade away in a few years.  There are innumerable ways of nudging the Earth, and deciding which is sufficiently regular and just right to maintain the wobble is no easy task.  Following in the great tradition of Seth Chandler, Richard Gross of the Jet Propulsion Laboratory compared Wobbling between 1985 and 1996 with the continual but inconstant motions of atmosphere and oceans, as simulated by super-computer modelling of climate.  The forces of winds and currents are simply insufficient to induce the Wobble, but variations in atmospheric and deep-water pressure, together with their positional shifts are, in the manner of Goldilocks and the little bear’s porridge, just about right.  Because changes in water depth are wind-driven (as for instance with the wandering hump in the Pacific’s surface, linked with El Niño), ‘weather’ is the ultimate driving force for Chandler Wobble.

Why devote time to this picayune curiosity?  The answer is to chart more accurately the position of distant spacecraft; not easy when the measuring platform is behaving like a Womble.

Source:  Richard A. Kerr, 2000.  Atmosphere drives earth’s tipsiness.  Science, v. 289, p. 710.

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