1 Dec
2006
1 Dec
'06
7:37 p.m.
At 22:18 -0600 11/15/06, Sridhar wrote:
90 degrees south always points to the South Pole. It
doesn't always
point to the same place, though. The precession of the Earth's
rotational axis causes the location of all coordinates on the Earth's
surface to shift by a small amount all the time. It's most easily
noticed at the poles. Barring smaller corrections of much shorter
period and magnitude, the Earth's axial precession occurs on a period of
about 26,000 years.
All true, but partially unconnected and somewhat
oversimplified. Earth's angular momentum vector precesses about the
normal to the ecliptic (the plane of Earth's orbit) in a cone with
half-angle 23.5 (or so) degrees and with a period of 26,000 years.
This is directly driven by the lunar and solar tidal torque on the
equatorial bulge of the Earth, which is in turn caused by the Earth's
rotation. For a rigid body, this could happen without any change in
the location of the "pole" as seen on the surface of the body.
Independent of that (or of any external torque), the point at
which the Earth's angular velocity (not momentum) vector intersects
the surface, which is the instantaneous pole, wanders around on the
surface. This is due to several things. One is the fact that the
angular momentum and angular velocity may not be perfectly aligned.
(Even a rigid body will do this if it is not spun along either its
major or minor principle axis of inertia. This is the "polhode"
motion of a freely rotating body as seen from the body.) Another is
the fact that the Earth is continually rearranging its mass
distribution by rainfall, earthquakes, ice-sheet melting, etc. etc.,
thus moving its principle axes of inertia around and causing the
first condition to apply. (These mass redistributions are in turn
driven primarily by the effects of incoming solar radiation, but this
is an indirect effect of the sun on the system.) Another cause is due
to the fact that the "solid" Earth is really a composite system, with
many different solid and liquid components, many of which may be
rotating with respect to one another and causing various interactions.
I think I will stop there, acknowledging that I'm still
oversimplifying.
I will however add that I at one point in the early 1980's
wrote a program in Fortran on a PDP-11 to display the current and
predicted (by the IERS) Earth instantaneous pole coordinates on a
Tektronix 4025 graphics terminal, erasing and re-drawing the
predicted pole with every new update of the current pole. It was
interesting to see how consistently off the predictions were.
Sorry for the delayed-gratification nature of this post; I'm
just getting back from 2 weeks' vacation.
--
Mark Tapley, Dwarf Engineer
(I haven't cleared my neighborhood)
210-379-4635 Dwarf Phone, 210-522-6025 Office Phone