Evidence exists that the poles have changed position during the recent past in a very rapid way - in a matter of days. This possibility, however, so far has been disregarded on the basis that such a phenomenon is thought to be physically impossible: no mechanism is known and no energy capable of provoking such a phenomenon.

The hypothesis that the inclination of the terrestrial axis in relation to the ecliptic and that the position of the poles might change has been taken into consideration since last century. Some of the greatest scientists of the time, including J.C.Maxwell and Sir George Darwin, considered this problem and decided that the stabilising effect of the equatorial bulge was so great that no conceivable force originating within the Earth could make it shifting on its axis, except for the collision with another planet.

They did not take into account, however, any phenomenon of instability which could occur to an Earth-like gyroscope, made by a plastic shell, easily deformed by centrifugal forces, covered by freely-moving liquids - the only body of this kind in the solar system.

This work shows that the impact of astronomical objects as small as an Apollo class asteroid could "trigger" a process which in a matter of days provokes a "reshaping" of the equatorial bulge around a different axis, and therefore a shift of the poles and a change of the tilt.

In order to trigger the process, the peak value of the torque developed by the impact has to overtake a "threshold" value, equal to the stabilizing torque developed by the equatorial bulge. A rough calculation shows that the threshold value can be reached during an impact of a body much smaller than a one-km-wide asteroid.

A shift of the poles of this kind would provoke world-wide destructive phenomena such as: earthquakes and volcanic activity in all areas interested by adjustments of the crust; violent winds and torrential rains, with unprecedented floods all over the world; wide fluctuations of the oceans’ level, with subsequent temporary submersion of large parts of the continents; perturbation of the magnetic field. On top of that, there would be permanent changes of the climate, due on one side to the shift of the poles - and therefore change of latitude of many regions, deviation of oceanic and aerial currents and so on -; on the other side to the change of the tilt, with subsequent change of the seasonal pattern.