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VLBI (Very Long Baseline Interferometry), the process of using extragalactic
radio sources, such as quasars, to pinpoint changes in the Earth's
orientation to inertial space is a tremendously helpful tool. Like
LLR (Lunar Laser Ranging), SLR (Satellite Laser Ranging) and GPS (Global
Positioning Systems) VLBI and other space-geodetic technologies have
helped us better understand subtle changes in our Earth and the Earth's
orientation.
As precise as VLBI is in recording and documenting the Earths orientation
to space it does not determine the "causes" for the change
in orientation. Rather the official NASA VLBI website recognizes two
historically accepted categories of causes for such a change and then
relies on the geoscientists to model the data.
Here is a quote from the NASA VLBI website:
"Changes
in the Earth's orientation in inertial space have two causes: the
gravitational forces of the Sun and Moon and the redistribution of
total angular momentum among the solid Earth, ocean, and atmosphere.
VLBI makes a direct measurement of the Earth's orientation in space
from which geoscientists then model such phenomena as atmospheric
angular momentum, ocean tides and currents, and the elastic response
of the solid Earth."
Consequently, the earth's orientation is measured to a high degree
of accuracy but the exact forces that cause that orientation to occur
are still being identified and weighed. Asteroids are one area of
force that is suspected of causing a change in earth's orientation.
Attempts have been made to quantify the effects of the larger asteroids
but the number of such objects is so large their exact cumulative
effect is difficult to determine. Another area apparently unaccounted
for is movement of the Sun around the galaxy. Note there is no mention
of movement of the Sun and solar system through space in the quote
from the VLBI NASA website or related sites NEOS (National Earth Orientation
Service) and IERS (International Earth Orientation Service. Yet obviously
if the Sun and solar system were moving through space it would cause
a change in the Earth's orientation (as well as all the other planets)
independent of lunisolar forces, tides or any geophysical activity.
This is because the Earth and all the planets are effectively attached
to the Sun via gravity and if the Sun is moving either around the
galactic center or in any unknown intermediate motions it takes the
earth and planets along with it. Because current technology and lunisolar
precession equation assumptions do not account for any such long term
or unknown causes, any measured changes in the earths orientation
to inertial space are attributed to known sources. Indeed, because
we know the total of precession (50.29 arc seconds annually), but
not the proportional contribution from all causes, including unknown
causes, it is logical to assume we overweight the known causes.
"Unknown" intermediate motions of the Sun could include
the Sun being pulled periodically by another star sometime during
its 200 million year journey around the perimeter of the galaxy, or
it might even include a very long cycle binary or multiple relationship
between our Sun and other stars. If such a motion took thousands and
thousands of years to complete one orbit it would still be undetected
based on today's technology and the very long observation cycle required.
The fact of the matter is, unless we had radio telescopes on the surface
of the Sun, that could simultaneously coordinate with such devices
on the surface of the Earth, we cannot be sure that all the causes
of the Earth's changing orientation to inertial space are strictly
local.
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