The displacement of the Chandler wobble is measurable -- Imagine a gigantic ballpoint pen poked through the center of the earth, entering at the South Pole and exiting at the North Pole. Imagine the pen is drawing on a scratch pad-equipped space station directly over the North Pole. After a day (one full rotation of the earth on its axis) the ballpoint pen draws a circular path, and not a dot, because of the "wobble" in the earth's rotation on its axis. (A Doodletop, a toy drawing tool that is a pen attached to a top, draws the same sort of path.) Over 14 months the pen draws a spiral path similar to this drawing.

The American astronomer Seth Carlo Chandler discovered the wobble in the late 1800s. The exact cause of the wobble in Earth's polar motion has stumped scientists with few agreeing on the actual cause, other than the fact that the earth is not a perfect sphere.

There are theories about the cause and the effect of the wobble. Some think tides and the liquid interior of the earth could play a part. As others have reported recently, some clues have been discovered. Some of the more recent conjectures include the constant winds over the oceans pushing varying amounts of water on the earth at one time or even the effects of a major earthquake. This article talks about one of the most recent theories, which attributes most of the wobble to pressure changes in the ocean.

The chandler wobble doesn't really have any effect on most people. The people who live with it on a daily basis are astronomers using earth-based telescopes and people using various navigation systems. With telescopes, the wobble affects the ability to point at a star accurately. The Chandler wobble also affects celestial navigation, since the latitude does change over a period of 14 months. Global Positioning Systems, (GPS), can overcome the effect of the wobble on navigation. Navigators' star charts, however, still have to be updated to show the new reference point for the geographic North and South Poles. The magnetic North Pole, used by a compass, is not affected.

Here are some interesting links:

• What is Chandler Wobble caused by and how well can it be predicted?
• Chandler biography
• Scientists no longer waver on Earth's wobble - ENN News - Environmental News Network
• National Public Radio Report: As The Earth Wobbles 3:16
• Geophysical Research Letters Online mentioned in the above radio broadcast
• Representation of the Chandler Wobble
• Encyclopaedia Britannica entry on Seth Carlo Chandler
• How a Global Positioning System Receiver Works