Skip to Content.
Sympa Menu

homestead - Re: [Homestead] Question re days and nights

homestead AT lists.ibiblio.org

Subject: Homestead mailing list

List archive

Chronological Thread  
  • From: tvoivozhd <tvoivozd AT infionline.net>
  • To: homestead AT lists.ibiblio.org
  • Subject: Re: [Homestead] Question re days and nights
  • Date: Sun, 09 Jan 2005 10:29:10 -0500

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






Archive powered by MHonArc 2.6.24.

Top of Page