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- Introduction
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.
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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.
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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.
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History
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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.
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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.
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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.
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Polar Motion
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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.
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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.
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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).
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Variations in length of
the day
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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.
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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.
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Space-geodetic techniques:
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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.
Kristen M. Neiling
Editor & Producer
CPLNews Agency ®
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