A Straight Dope Classic from Cecil's Storehouse of Human Knowledge

How come the Chandler wobble hasn't dampened out?

December 3, 1999

Dear Cecil:

Here is a question to test Cecil's mettle. This ain't "what caused the big bang?" but close enough. How come the Chandler wobble hasn't dampened out?

Cecil replies:

R.M., honestly. You're getting ahead of yourself. The average person, on hearing about the Chandler wobble, doesn't wonder why it hasn't dampened out. Rather he asks:

Q: The what?

A: The Chandler wobble, discovered by astronomer S. C. Chandler in 1891, is a variation in the earth's axis of rotation amounting to 0.7 seconds of arc over a period of 435 days, or about 14 months. To put it another way, the earth's poles wander a bit as the planet spins, describing an irregular circle ranging from 10 to 50 feet in diameter. (For a chart of polar wandering over the last decade, see http://www.usno.navy.mil/USNO/earth-orientation.)

The Chandler wobble is but one of several fluctuations of the earth's axis. The most important is precession, which has a cycle of 25,800 years due to variations in the pull of the moon and sun. Because of precession, the star Vega will replace Polaris as the North Star many millennia from now, a fact to keep in mind if you're planning to navigate by the stars in 14,000 AD. There's also nutation, an aggregation of sub-wobbles within the larger precessionary wobble, the most significant of which has a period of 18.6 years and results from variations in the distance of the moon.

Q: Why is the Chandler wobble important?

A: Who said it was important? It is, however, an unexplained geophysical phenomenon, and if a scientist's reaction to such things is "who cares?" it's a safe bet they're not going to put him on the tenure track.

Q: No, really.

A: If you're dependent on celestial navigation, variations in latitude due to the Chandler wobble can throw you off by as much as a fifth of a mile.

Q: Who's monitoring the Chandler wobble?

A: The International Earth Rotation Service, based in France (hpiers.obspm.fr). I guess I'm glad somebody is keeping an eye on this, but you have to wonder what they think they're going to do if things get out of whack. "Yo! Everybody in Argentina! Two steps to the left!"

Q: What causes the Chandler wobble?

A: Scientists aren't sure. Absent any external force, you'd expect the CW to dampen after a while, but that hasn't happened. The wobble's diameter has varied quite a bit over the decades, having reached a peak in 1910. Why this occurs is one of the nagging questions of geophysics, and to hear some tell it, fame, wealth, and groupies await whoever figures it out. To date, however, most attempts at explanation have been couched in language such as the following:

"We find that the similarity of the degree 2 Love number inferred at a nine-day period to that at seismic frequencies, combined with the significant amount of relaxation at the Chandler period, together imply that Q models operating within the seismic band cannot extend all the way to long tidal periods." Probable translation: "I don't know."

Q: Take a stab in English.

A: The Chandler wobble is due to variation in the forces acting on the earth. The question is, what forces? Many claim to see two factors at work. The first is seasonal variation due to the weather, e.g., ice and snow accumulation, changes in atmospheric mass distribution, etc. The second factor is, well, something else. Some blame earthquakes, volcanoes, movements of material inside the earth, and so on. But maybe it's just the Teeming Millions moving stuff around.

Q: So what's your theory, Cecil?

A: Come now. Any knucklehead can dream up theories. What we need is a practical system of research. You'll remember my efforts years ago to persuade the People's Republic of China to have its one billion-plus citizens get up on chairs and jump off, to see if the earth would be thrown out of its orbit. They wouldn't cooperate, the slackers. But the Teeming Millions are more civic-minded. My idea is, on New Year's Eve we all get in our Airstreams and drive to Alaska with a hundred pounds of lead shot, the better to increase our angular velocity. Best case: the planet tips over, which surely proves something. Worst case: we party. How can we lose?

A little off

You described the Chandler wobble accurately (a glitch in the Earth's rotation) but with a tone that suggested it didn't matter very much. All that would happen if the Chandler wobble were neglected is that someone stupid enough to navigate by the stars would be off by a mile or so upon reaching their destination, implying that nobody does that anymore. Only problem is, U.S. Trident nuclear missiles orient themselves after launch with a system called StarTracker, so in WWIII any missile that doesn't know about the Chandler wobble will miss by, well, a mile or so.

Cecil replies:

Not that I want to encourage slovenly work by our nuclear forces.  But you're telling me if I drop a city-killing nuclear warhead on, say, Beijing, it matters if I'm off by a mile?

Dear Cecil:

The point is that the Tridents are not aimed at cities but at hardened military targets that must be hit with great precision, otherwise why would they bother to put StarTracker on them in the first place? Sure,  it's an anachronistic way of thinking about nuclear weapons, but the democratically elected commander in chief wants it that way. And so will his successor by the looks of it.

Cecil replies:

Precisely. Just trying to elicit that crucial detail.

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