he following tool allows you to compute the excitation functions of the Earth rotation (according to the "Euler-Liouville" formalism) and to compare them to the geophysical excitation functions, as far as these later ones are available. Presently the geophysical excitation is restricted to the atmospheric forcing. Comparison is done through visual plot and computation of the correlation coefficients.

* The observed excitation functions (χx, χy, χz) are computed from the pole coordinates (x,y) and length of day changes ΔLOD of the IERS C04 series (sampling of 1 day, fluctuations > 6 days) according to the equations : χx + i χy=(x-i y) + i/σc d(x-iy)/dt where σc is the Chandler angular frequency : (σc = 2π/T ( 1 + i / 2 Q), T Chandler period, Q quality factor) and χz=ΔLOD/LOD (LOD=86400 s TAI)
* Effective Atmospheric Angular Momentum Functions provided by the Special Bureau for Atmosphere of the IERS (NCEP-NCAR reanalysis time series) (ftp.aer.com...) from 1962 to 2005/9/3 (see readme). From 2005/9/3 up to the current week these data are completed by those of the National Meteorological Center (NMC) (ftp.cpc.ncep.noaa.gov...).
* The atmospheric angular momentum functions are filtered and sampled before comparison. By default the pressure term is associated with oceans reacting as "Inverted Barometer" (IB) in front of the pressure variations : this is a realist approximation for variations larger than 10 days (for rough pressure term click the button "Non IB").
* Geodetic Excitation functions are derived from combined series C04 using an algorithm based upon trapezoidal integration of the geodetic excitation function.
* For the observed axial excitation the effect of zonal gravitational tides is removed.
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The equatorial excitation function is based upon the knowledge of the Chandler term period T and its quality factor Q. As these parameters are affected by large uncertainties, we let you the possibility to tune them within the allowed bands (426
Thanks to David Salstein (Atmospheric and Environmental Research, Boston) and Olivier de Viron (Royal Observatory of Belgium) for their respective advices. C. Bizouard

Metaphorically it is as if the "gear" of the revolving Earth is grinding ruggedly and perhaps breaking up at this "rough spot". Since the X position of the Spin Axis is moving into higher numbers, it means that it is moving closer to England down Greenwich Meridian. But actually it is moving at an angle of 45 degrees to the East, towards Northern Russia. OR, TO SAY THE SAME THING IN REAL MOTION OF THE CRUST, the crust of the Earth is moving UP Long. East 45 across the equator with more vector vigor than in any other direction.

am reading Sobel's Longitude , and she speaks of the Earth's rotation rate slowing as a result of tidal forces. At what rate is the Earth's rotation slowing down? What will be the result of this slowing, and how long will it take?

The Answer
The interaction of the Moon and the tides is pumping angular momentum out of Earth's spin and into the Moon's orbit.

Currently the day is lengthening by about 1.5-2 milliseconds per century. tycho.usno.navy.mil...

This is thought to be higher than normal due to resonance frequencies in the slosh time of the current configuration of oceans (which changes with continental drift). If this were to continue forever, the Earth and Moon would end up tidally locked so that they kept the same faces towards each other throughout each day=month which would be about 50 of our current days long. However, the Sun will expand and incinerate the Earth well before that happens.

David Palmer
for Ask an Astrophysicist