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[tvoivozhd <tvoivozd AT infionline.net>]

sanrico AT highdesert.com wrot

> Richard Gross, a scientist at NASA's Jet Propulsion Lab (JPL) in
> Pasadena, California, has modeled the coseismic effect on the Earth's
> rotation of the December 26 earthquake in Indonesia by using the PREM
> model for the elastic properties of the Earth and the Harvard
> centroid-moment tensor solution for the source properties of the
> earthquake. The result of this modeling is:
>
> Change in length of day: -2.676 microseconds (about one second in a 
> millennium)
> X-axis polar motion excitation: -0.670 milliarcseconds (approximately
> 2 centimeters or 0.8 inches)
> Y-axis polar motion excitation:  +0.475 milliarcseconds
> (approximately 1.5 centimeters or 0.6 inches)
>
> Since the length of the day can only be measured to an accuracy of
> about 20 microseconds (20 millionths of a second), this model
> predicts that the change in the length-of-day caused by the
> earthquake is too small to be observed. And, since the location of
> the earthquake was near the equator, this model predicts that the
> change in polar motion excitation is also rather small, being about
> 0.82 milliarcsecond in amplitude. Such a small change in polar motion
> excitation will also be difficult to detect.
>
> Nevertheless, at a science conference in Utah last week, Professor
> Ulrich Schreiber of the Munich Technical University in Germany
> reported that his Earth-based, ultra-precise
> "G" ring laser gyroscope was able to detect perturbations in the
> Earthís axis as a result of the Indonesian earthquake. This
> gyroscope--the largest of its kind in the world--contains a giant
> glass ceramic disc, 4.25 meters in diameter, 25 centimeters thick and
> weighing 10 tons. It is located in a sealed and pressurized chamber,
> eight meters below the surface of the Earth, at the Wettzell
> Fundamental Research Station in New Zealand. This instrument was
> specifically designed to be able to detect changes in the Earth's
> rotation within a day. For more information on the "G" ring laser
> gyroscope, see
> http://www.spie.org/app/publications/magazines/oerarchive/september/sep96/gyro.html
> or http://www.wettzell.ifag.de/publ/publ/IUGG_2003_RinglaserG.pdf
>
> In contrast, the GP-B gyroscopes have not detected any measurable
> effect as a result of the Indonesian earthquake and tsunami. GP-B is
> measuring the curvature and twisting of space-time around the Earth,
> not the planet itself. Our spacecraft is in a circular orbit, 640
> kilometers (400 miles) above the earth, always passing over the north
> and south poles, but varying in its longitudinal path with each
> orbit. The extremely small change in the earth's polar motion
> resulting from the Indonesian earthquake/tsunami is simply not
> significant enough to appear in our measurements. Also, any small
> change in the Earth's polar motion will factor itself out, given the
> changing longitude of each orbit.
>
> Our measurements do depend on knowing our spacecraft's position
> relative to specific locations on Earth to a high precision, and for
> these measurements, we use the Global Positioning System (GPS).  It
> is possible, though highly unlikely, that if a re-calibration in the
> GPS satellites is required, we may need to incorporate some
> compensating factors.
>
> Gravity Probe B could conceivably register a change in Earth's polar
> orbit, if the change were of immense proportions. For example,
> suppose the earth's spin axis were to change position by 3 degrees.
> This would significantly affect local space-time, and this change
> would be detectable by the science instruments on-board the
> spacecraft. However, such a change would require a cataclysmic event,
> ten million times larger than the Indonesian earthquake and
> tsunami--an event that would mean the end of Planet Earth, as we know
>  

tvoivozhd---more moon data:  
http://www.brainyencyclopedia.com/encyclopedia/m/mo/moon.html