The Causes of Precession Image: www.aquarian-age.net Most planets and satellites are not perfectly spherical and much like a spinning top, they rotate with a wobble. Earth as a multi-layered aggregate is about 40 km wider across the equator (tidal bulge) where the gravitational pull of Sun and Moon is strongest (Kaler). With more mass across its center, the planet's rotational speed differs within the layers according to distance from the rotational axis. For the most part of its history, Earth's axis has been inclined at a relatively steady angle--currently it is 23.5 degrees with regard to the ecliptic--and this angle determines the orbital plane. Although the gravitational interactions with the Moon, Sun and to a lesser part the passing planets push Earth's axis perpendicular to the vector direction of the torques (Hecht), Earth manages to keep the orbital plane steady. Similar to the way a rapidly spinning gyroscope precesses and keeps its original plane of rotation, Earth responds to the torques with an axial relocation rather than a change in the axis' tilt. Precessional motion therefore compensates for the forces that are trying to straighten the axis (Kaler; Hecht). As Earth precesses, the top and the bottom of the projected axis describe a conical circle with a frequency of 19000 to 26000 years. The change in the axis' direction is followed by a shift of the equator and a relocation of the tidal bulge. To see the effects of precessional motion, look at Fig 1 or look at the wooden spinning top. Imagine a steady plane cutting horizontally across its center. Now you will notice how the spinning top's equator sways over the plane again and again. Of course Earth swivels only very slowly, but the movement proves significant in that the geographic poles ride a circle while the equator moves back and forth producing periodic changes in the amount of insolation over lower latitude regions. This is evident from the rock record where laminations signal cyclic climate changes that are indicative of orbital cycles (Schwarzacher). Image: www.space.com As precession is a reflection of the Earth, Sun, and Moon dynamics, the planet's rotational motion responds with a variety of high-frequency movements. Superimposed on the long-term precessional wobble are small-scale swivels termed nutations. Varying in intensity and amplitude, these are considerably rapid nod-like wobbles thought to originate from the varying Moon-Earth distance with a period of 18.6 years. Other cyclic pulses are blamed on crustal shifts, post-glacial subsidence rebounds and also hydrological and meteorologic circulation patterns (Mörner). One of these nutations is the so-called Chandler Wobble, named after American hobby astronomer Seth Chandler, who discovered them in 1891 (ENN). The motion has a period of 433 days and changes the location of the rotational poles over an area up to 20 meters in diameter (Eubanks et. al). Evaluation of ocean bottom data analysis gave rise to correlating these excitations predominantly to fluctuating ocean bottom pressures, temperature and salinity gradients and only partly to changes in atmosperic pressures as was previously the case (ENN; Sullivant). Since minute deviations in polar motion can now be measured with great accuracy by monitoring changes to fixed distant stars with satellite laser ranging and VLBI (very long baseline interferometry), motions on the order of 0.002 to 0.02 milliseconds are detected (Eubanks et.al). These are too small and too fast to be explained by the regular tidal forces of Moon and Sun. Another scientist suggested that irregular iron particle accumulations at the boundary between the Earth's core and mantle may be the cause for more puzzling nutations recorded and termed magnetic jerks. Not only varies Earth's rotational speed at different internal layers, the axis through the core also has a slightly different direction which causes the magnetic field to emanate through the mantle. However, according to this study, the iron particle deposits which are conductive may perturb the free passing of the magnetic field creating enough electrical drag to affect the rotational stability of the planet's rotational movement(space.com staff). For more detailed information, visit the www.space.com site. Precession changes the arrival time of the equinoxes, the two days in the year, when night and day are equally long. Each year the moment the Sun crosses the celestial equator arrives a little earlier, so that the equinoxes and the stellar constellations ride a great circle during one precessional period. Rotational speed of Earth has slowed over the eons of time due to tidal friction and there is geologic evidence that precessional frequencies appear to be decreasing also (Davey). For a computed annual time change see the next page