Chandler's Wobble 2000-2001 wobtrac_2000-2001_large.gif (13351 bytes)

Grid:  each square in the grid is .05 arcseconds, about 5 feet or 1.5 meters
+X = Greenwich Meridian (through England)
+Y = Long. West 90 (through the Great Lakes)
Geographical (Map) North Pole = where X and Y cross
Center of Chandler's Wobble was very close to Geographical North Pole in 1890


The right-hand side tip of the spiraling line is January 1998

The left-hand side tip of the spiraling line is March 31, 2001

The "DENT" in the bottom of the spiral is December 1998.  It looks as if the wobble went cockeyed from that point. It is not necessarily unusual. But it is not normal either. The inner circle is much smaller than the spiral "eyes" (X Min) of the previous 30 years. The recovery to March 31, 2001 covers less distance than during previous 6.5 year spirals. 

There is enough variation in the wobble to make  predicting what it is doing fairly difficult.  But comparisons with previous wobble spirals and key transistion periods suggests the following:

  • another major change in the wobble is underway

  • the change appears to have commenced with a "dent" in the wobble in December, 1998

  • since then, the wobble has or is   tightening up to become suddenly smaller as it was before just before highly irregular periods.

  • it is highly probable that the X or Y Max point in the current wobble cycle will be smaller than the previous two or three wobble cycles..

The overall spiral motion seems offset to the left since 1996 by about nearly 3 feet (one meter), implying a rate of absolute "shifting" of the location of Chandler's Wobble (literally the average location of the pole) at a rate which is accelerating over previous wobble cycles.  Here is how the past 40 years of wobble shifting computes out by using caliper and ruler calculations of Wobble Tracker charts:

Average Average
Years Inch/Yr Cent/Yr










Potentially more important is the change in the diameter of the wobble.  The figures below suggest that the size of the wobble in slowly increasing.  Any increase in the size of the wobble betrays an increase in the instability of the Earth's axis.  If such a  the trend continues, a more radical rate of shifting of the wobble will commence.

Wobble Ave. Dia Ave. Dia
ArcSec/Yr Meter/Yr


73-83 0.485 14.972
84-95 0.570 17.596
96-97 0.856 26.425

The last figure for the diameter of the wobble may overstate the average.  The previous periods were for 11 years and include the measurements for two spiral maximums (X Max).  The last period does not include the rebound of the spiral back to its maximum size and so the "average" figure is not fully comparable as an average.  It could become bigger, it may turn out smaller.

As things look at the moment, the wobble began to contact in size in December 1998 for unknown reasons.

For Wobble Tracker Charts for the 20th century, click here.

This chart can be refreshed with the latest data from the world database in about 10 minutes flat using Wobble Tracker and PhotoImpact Screen Capture software to turn the scree image into a gif image.  The literal process is to connect to the internet, boot up Wobble Tracker, click on update the data, wait a few seconds, set the grid and color and line params with six clicks or so, then click on the two years, click click.  Then use mouse to size the screen to this size, then click on F7 key which turns the Wobble Window into an image file, click on PhotoImpact, then click on save to Web gif file, select the right directory, then boot FTP software and click to the webhost account, set directories at both ends, then select the gif file, then click the arrow send key and wait for the completion bell. 

Earth Systems Polar Motion Monitor HomepagePolar Motion Monitor