The Earth's rotation axis is not fixed in space. Like a rotating toy top, the direction of the rotation axis executes a slow precession with period of 26,000 years for the entire ecliptic of our planetary bodies to travel around our sun, a trip of 360 degrees. Each one of the 12 signs of the zodiac takes about 2100 years for our solar system to pass through. Every 72 years we actually move backward 1 degree. After 2100 years we move out of one age and into another.

The precession is like a star clock that helps us date the rotations of earth in our solar system through our galaxy.

At the time of the birth of Christ we were moving out of the Age of Aries which was the Roman Empire into the Age of Pisces. That happened around 60 BC. The early Christians were aware of this and used as their symbol the 2 fishes going in opposite directions.

Since the rotation axis is precessing in space, the orientation of the Celestial Equator also precesses with the same period. This means that the position of the equinoxes is changing slowly with respect to the background stars. This precession of the equinoxes means that the right ascension and declination of objects changes very slowly over a 26,000 year period. This effect is negligibly small for casual observing, but is an important correction for precise observations.

History

The Babylonians possibly knew already that the rotation of the stellar constellations was subject to change, but Hipparchus was, in the 2nd century B.C., the first astronomer who gave a description of this phenomenon. It lasted until 1543, however, before Copernicus associated this change with a changing direction of the rotation axis of the Earth. Because of the gravitational forces of the sun and the moon on the equatorial bulge of the rotating earth, taking into account t the angle of 23.439 degrees between the rotation axis of the earth and the normal vector to the plane in which the earth orbits around the sun (the ecliptic), the rotation axis moves with respect to a space-fixed reference frame.

This motion is called precession and proceeds in about 25,800 years along a cone with a half apex angle of 23.439 degrees, which causes the vernal equinox to move along the ecliptic by 50.291 arcseconds per year. This precession of the equinoxes affects the length of a tropical year as well as the length of a sidereal day. Because the directions to the sun and the moon vary and because of gravitational forces from the planets, the true rotation axis wobbles around the precession cone. De largest of these nutations amounts to 9.2 arcseconds in 18.6 years and is caused by the moon.

Precession was the third-discovered motion of the Earth, after the far more obvious daily rotation and annual revolution. Precession is caused by the gravitational influence of the Sun and the Moon acting on the Earth's equatorial bulge. To a much lesser extent, the planets exert influence as well.

Polar Motion

Euler predicted in 1758 that the rotation axis would also show a motion with respect to an earth-fixed reference frame. Eventually, in 1891, it was Chandler who determined the periodf this free polar motion from some 50 years of observations of the geographical latitude of atronomical observatories. The Chandler period (435 days) deviates from the Euler period (304 days) because of the non-rigidity and the inhomogeneous mass distribution of the earth. The radius of the Chandler wobble of the rotation pole is about 6 metres.

In 1899 the ILS (International Latitude Service) was established to coordinate the observations of the rotation pole. Using these observations, the forced polar motions that had been predicted by Thompson in 1876 could be confirmed. These motions are caused by the gravitational forces of sun and moon as well as by geophysical processes within the atmosphere, the oceans and the interior of the earth. They amount to about 3 metres at the earth's poles.

Since the end of last century, it has been assumed that the rotation pole also shows a secular drift of about 10 cm per year in the direction towards Ellesmere Island, possibly caused by post-glacial and tectonic uplifts. In 1962 the ILS was superseded by the IPMS (International Polar Motion Service) and in 1988 the IPMS and the Earth Rotation Section of the BIH (Bureau International de l'Heure) were combined to form the IERS (International Earth Rotation Service; Central Bureau at the Paris Observatory).

The period of rotation (length-of-day) of the earth, however, was assumed to be constant until well into this century, apart from a secular change. In 1754 Kant predicted that friction with the tidal forces on earth would cause a deceleration of the earth's rotation, but it took more than a century before Ferrel and Delaunay could confirm this effect. The secular decrease of the rotation rate causes n increase of the length-of-day of about 2 milliseconds per century. This value can be determined by comparing the observations of eclipses of the sun and the moon by the Babylonians, Greeks, Arabs and Chinese with computed eclipses when using a constant rotation rate. At present also fossiles and paleomagnetic data are used to determine the increase in length-of-day.

Not until 1875 the surmise was raised by Newcomb that also the rate of rotation would be subject to irregularities. Only in 1936 was this confirmed by the determination of a seasonal.

Space-geodetic techniques:

Since the advent of the modern space-geodetic positioning techniques, VLBI (Very Long Baseline Interferometry), LLR (Lunar Laser Ranging) and SLR (Satellite Laser Ranging) in the sixties and GPS (Global Positioning System) in the past years, an ever-increasing number of variations in both nutation and polar motion and length-of-day have been found. From space-geodetic observations, the irregularities in the earth's rotation are now routinely determined at intervals of 1 day and even a few hours, with precisions which are less than 0.5 milliarcsecond for nutation and polar motion and less than 0.05 millisecond for length-of-day or UT, corresponding to 1.5 cm at the earth's surface.