At 22:18 -0600 11/15/06, Sridhar wrote: 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