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Re: GPS ERROR ? Kep's



OK -- let me weigh in here with the definitive answer. The effects of the
earthquake on the earth are really hard to see!

The "official" source for UT1 (the position of the earth relative to the
fixed stars) is Very Long Baseline Interferometry (VLBI -- see
http://ivscc.gsfc.nasa.gov/). Some of you may know that I developed the VLBI
network that makes the observations before I retired from NASA (My group can
be seen at http://lupus.gsfc.nasa.gov/). It was my colleagues who published
the material cited in the NASA press release seen at
http://www.nasa.gov/home/hqnews/2005/jan/HQ_05011_earthquake.html. The real
publication was in last week's American Geophysical Journal's weekly
newspaper called EOS (I can supply a PDF version for anyone who is seriously
interested).

If you want to see the actual quick-response data, it is published by the US
Naval Observatory in Bulletin A at http://maia.usno.navy.mil/. In
particular, take a look at the plot in
http://maia.usno.navy.mil/bullaprobt.gif and note that the earthquake,
Dec.26 was on MJD 53365. You might also be interested to note the changes in
X- and Y-pole position in http://maia.usno.navy.mil/bullaprobx.gif and
http://maia.usno.navy.mil/bullaproby.gif (On these two plots, 1
milliarcsecond is about 3 cm at the surface of the earth. The +X axis passes
thru the Greenwich meridian and the Indian Ocean earthquake lay right on the
+Y axis at 90d East).

The reason UT1 varies with time is that the rotating earth must conserve
angular momentum. On time scales of seasons, the principal driver is
variations in atmospheric angular momentum (AAM) due to global wind fields.
On the plot, you will see the result plotted as AAM of integrating global
climate models. On a longer time scale we have effects of oceanic
circulation changes, and on even a longer time scale (like 10's to 1000s of
years) we have changes in the circulation of the molten rock under our feet.
These very long scale effects are undoubtedly coupled to the dynamo that
makes the earth's magnetic field, and these flows are known to change enough
to reverse the earth's magnetic field on time scales of hundreds of
thousands of years. Also on long time scales, we have effects from the tidal
dragging of the moon on the earth. Aeons ago, the day was only 18 hours and
the moon has dragged it down to 24. To compensate for this in the earth-moon
system, the moon is slowly receding from the earth (at a speed, if I recall
correctly ~ 1 cm/year). A brief description of UT1 variability can be found
at http://www.iers.org/iers/earth/rotation/ut1lod/ut1lod.html. To see that
UT1 (i.e. the time read out by your sundial) has varied over a ~3 year
period, see http://www.iers.org/iers/earth/rotation/ut1lod/figure1.html;
corresponding to this plot, the actual variability of the length of each day
is seen in http://www.iers.org/iers/earth/rotation/ut1lod/figure2.html.

Except for effects of precession and nutation, the earth's spin axis is
fixed in space. However the crust of the earth shifts with respect to the
inertial axis. The X & Y pole (and hence your apparent latitude) wander by a
few meters, typically over time scales of ~13 months (called the Chandler
wobble, see http://science.howstuffworks.com/question442.htm and listen to
this July 200 report at http://www.npr.org/ramfiles/me/20000718.me.04.ram).

Hope this helped -- 73 de Tom, W3IWI
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