Pinning Down a Pole's Position
Weisburd, Stefi, Science News
Columbus and other mariners found their way around the globe by noting their latitude as measured by the height of the North Star above the horizon. Centuries later, scientists discovered that there are subtle variations in this height caused by the circular excursions of the earth's spin axis relative to its solid crust -- a motion called the Chandler wobble (SN: 10/24/81, p. 269). Measuring this wobble is important not only for surveying and navigation but also for understanding the planet's inner workings that cause it. Until recently, however, these measurements were too inexact to shed much light on the dyamics of the pole's peculiar dance.
Now the accuracies of two measurement techniques have reached new heights. In the Sept. 20 SCIENCE geophysicists report that estimates of the earth's pole position made by satellite laser ranging (SLR) and very long baseline interferometry (VLBI) differ by about 2 milliseconds (msec) of arc, or about 6 centimeters, indicating that this is the maximum total root-mean-square error for both techniques together. For comparison, the total displacement of the pole during the corresponding 14-month Chandler period was about 500 msec of arc, or about 15 meters.
"A lot of people were very startled by this accuracy," says Douglas Robertson at the National Geodetic Survey (NGS) in Rockville, Md., who co-authored the paper with William Carter at NGS and three researchers at the University of Texas at Austin. The errors of other methods currently in use range from about 3 to 10 times greater than those of SLR and VLBI.
The high degree of accuracy of SLR, which involves bouncing a laser beam from earth off a satellite, and of VLBI, which measures at different observatories the differences among arrival times of radio waves from very distant quasars, has been available for only a few years. When Robertson and Carter last compared the time series data from these two techniques, in 1983, they obtained a root-mean-square error of about 6 msec of arc. Both this and the more recent study are part of an internationally sponsored project called MERIT (Monitor Earth's Rotation and Intercompare Techniques of Observation and Analysis).
By improving the accuracy of these measurements scientists hope to resolve the debate over which forces drive the Chandler wobble. …