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A pole shift theory is a hypothesis based on geologic evidence that the physical north and south poles of Earth have not always been at their present-day locations; in other words, the axis of rotation had been "shifted". Pole shift theory is almost always discussed in the context of Earth, but other solar system bodies may have experienced axial reorientation during their existences.
    One early popular proponent of a pole shift theory was Charles Hapgood in his books The Earth's Shifting Crust (1958) (which includes a foreword by Albert Einstein) and Path of the Pole (1970). Hapgood speculated that the ice mass at one or both poles over-accumulates which destabilizes the Earth's rotational balance, causing slippage of all or much of earth's outer crust around the earth's core, which retains its axial orientation. This happens either slowly (conservative version) or quickly (radical version). The results of the shift occurring every 12,000 to 20,000 years or so results in dramatic climate changes for most of the earth's surface as areas that were formally equatorial become temperate, and areas that were temperate become either more equatorial or more arctic.
Other theories which are not dependent upon polar ice masses include:
  • A high-velocity asteroid or comet which hits Earth at such an angle that the lithosphere moves independent of the mantle
  • An unusually magnetic celestial object which passes close enough to Earth to temporarily reorient the magnetic field, which then "drags" the lithosphere about a new axis of rotation. Eventually, the sun's magnetic field again determines the Earth's, after the intruding celestial object "returns" to a location it cannot influence Earth.
These theories are currently not accepted by the scientific community, but debate is ongoing. Pole shifts are a cataclysmic inversion of the planet's axis of rotation, up to 180 degrees; a sudden slippage of the planet's solid crust around the molten core. There are several major scientists of this century that also expouse this concept as an event that has happened before in Earth's past.
Geographic axis: man-made arbitrary north-south fixed reference that determines lines of longitude and latitude, and the geographic North and South Poles.
 
Axis of instantaneous rotation: the true astronomical axis; the rotation axis. The line drawn through the Earth about which it is actually rotating at any point in time. The points where the line cuts through the earth's surface are called the "rotation poles"; the visualised extention of the north axis line in space currently points to the star Polaris, the North Star.

Axis of maximum moment of inertia = the axis of figure: because the Earth is not a perfect sphere, but is physically an oblate spheroid, the position of the axis of figure is not a precise constant, but is affected by the constant change in the earth's distribution of total mass: (ie, by ocean tides, atmospheric cond- itions, plate tectonic movements, etc.).
The combinations, and movements, of these masses with gravitational, centrifugal and orbital velocity vector forces, create what we call the equatorial bulge, and thus, the axis of figure. If the Earth were a perfect sphere, there would not be such an axis. The angular difference between the astronomical axis and the axis of figure, called a "nutation", causes an Earth orbital spirical oscillation, known as the famous Chandler "wobble". The equatorial bulge, the rotation axis' angle of inclination to the ecliptic plane, the gravitational tidal forces of the Sun, Moon and planets, have an affect on the Earths' orbit that produces the famous precession of the equinoxes.
 
Axial Tilt: the ecliptic is the plane of the Earth's ellipsoidal orbital path around the sun; the rotation axis is currently inclined to the ecliptic by an angle of 23.5 degrees: this is called the obliquity, or tilt, of the axis. It is this axial tilt that causes our annual seasons. Due to orbital dynamics, the obliquity varies between a minimum of 21 degs. 39 mins. and a maximum of 24 degs. 36 mins. over a 41,000 year cycle. The axial tilt's rate of change, (angular differentiation), is currently measured as .013 degs. per century.

Geomagnetic axis: not to be confused with the geographical axis, as it often times is. The Earth's magnetic field (whose real source is still an ongoing scientific mystery, but generally attributed to the interactions between the interior molten convection currents and the nickle-iron core, generating an electromagnetic force field, coupled with the rotational and orbital forces), itself has, by its own lines of force through space, the magnetosphere, a north- south axis. At present, the angular difference between geographic and geo- magnetic north poles is about 11 degrees.
 
Through paleomagnetism, (the study of the magnetic properties of rocks), scientists now have solid proof that the earth's magnetic field, and thus the geomagnetic north and south poles, have reversed itself namy times in the past. Magnetic field polarity reversals are magnetic poleshifts. The geologic record also shows that the strength of the magnetic field varies widely in time, and fluctuates wildly during field reversals, sometimes dropping to zero gauss strength; ie: the field vanishes, disappears! It is also a proven fact that the magnetic poles wander, literally zig-zagging around its axis.

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