Definition
of the Earth Orientation 
The
Earth's orientation is defined as the rotation beween a rotating geocentric
set of axes linked to the Earth Gxyz (the terrestrial system materialized
by the coordinates of observing stations) and a nonrotating geocentric
set of axes linked to inertial space GXYZ (the celestial system materialized
by coordinates of stars, quasars, objects of the solar system). The
common way for describing the Earth rotation is to specify the rotation
matrix between these two systems.

If
the Earth would rotate uniformly... 
If
the Earth was rotating uniformly around a fixed rotation axis (with
respect to the Earth crust and the celestial system), the Earth rotation
changes would be described through one parameter only : the rotation
angle varying linearly with time, or the time scale which can be derived
from this rotation angle (the universal time).

Irregularities
of the Earth rotation 
Actually
the rotation axis is neither fixed with respect to the crust nor with
respect to a celestial system, and the Earth's rotation rate undergoes
slight variations. The changes of the Earth rotation vector are caused
by the gravitational torque exerted by the Moon, Sun and planets, displacements
of matter in different parts of the planet and other excitation mechanisms.
The observed oscillations can be interpreted in terms of mantle elasticity,
earth flattening, structure and properties of the coremantle boundary,
rheology of the core, underground water, oceanic variability, and atmospheric
variability on time scales of weather or climate. The understanding
of the coupling between the various layers of our planet is also a key
aspect of the research on the Earth's rotation.

Monitoring
Earth orientation is done through 5 parameters 
In principle the Earth's orientation can be described through three
independant angles (for instance the Euler angles). However the classical
monitoring of the Earth's rotation considers separately the motion of
the rotation axis in the Earth and in space. For routine determination
of the Earth orientation, five Earth Orientation Parameters (EOP)
are generally assessed. Technically, they are the parameters which
provide the rotation of the International
Terrestrial Reference Frame (ITRF) to the
International Celestial Reference Frame (ICRF) as a function
of time.

Celestial
Ephemeris Pole (CEP) 
These
parameters are refered to the Celestial Ephemeris Pole (CEP),
which is close to the rotation pole. Its motion in space is the precessionnutation
motion. Pratically its motion is equivalent to that of the geographic
pole (ITRF pole) for periods greater than 2 days. The spatial position
of the CEP is well modeled at a 0.001" accuracy by the lunisolar
(and to a lessextent the planetary) torque on the Earth equatorial
bulge and small additional impacts of atmospheric and oceanic effects.

The
Celestial pole
offsets(dPsi, dEps)

However
a precessionnutation
model
(such
a model is kept as a part of the
IERS Conventions)
cannot take into account the
variable components due to atmospheric, oceanic, and earth internal
processes. The actual
departure of the motion with the model is observed by VLBI
and LLR. The observed differences with respect to the conventional celestial
pole position defined by the model are monitored and reported by the
IERS thanks to two Celestial
pole offsets (dPsi, dEps).

Universal
time (UT1) 
The
CEP remains very close to the instantaneous rotation axis (offset under
0.02"), and therefore is appropriate for reckoning the rotation angle
of the Earth in space. The Earth's rotation is no more considered uniform,
but still used for the common life. Therefore the IERS does not provide
the Earth rotation angle, but the associated time scale UT1,
that is the time which is required for the rotation angle if the Earth
would rotate uniformly at the mean rotation rate (360°/86164.09891s).
The tables provided to the users report the offset with respect uniform
time scale TAI and UTC : UT1  TAI / UT1  UTC. The excess of
the rotation period with respect to the mean period is called excess
of length of day (LOD).

2
Coordinates
of the pole (x,y) 
After
applying the precessionnutation
matrix corrected by the celestial pole offsets and the rotation angle
around the CEP, a residual rotation remains to get into the International
Reference Frame. It is associated with the 2
small angles (x,y), which are the
coordinates
of the pole (x,y) of the CEP, relative to the Gz axis
of the International Terrestrial Reference Frame (the geographic pole
axis). The coordinates of the pole undergoes variations mostly because
of atmospheric and oceanic mass redistributions as seasonal time scale.
The terrestrial coordinates of the CEP correspond to the circular celestial
motions of Gz axis having period smaller than 2 days, in turn their
variations range from several years to a few minutes except the retrograde
diurnal band.
N.B.: The xaxis is in the direction of the IERS Reference Meridian
(the Greenwich Meridian) ; the yaxis is in the direction 90 degrees
Est longitude.

Towards
the Celestial Intermediate Pole 
Actually
the original definition of the CEP, based upon a conventional precessionnutation
model, does not take account of diurnal and higher frequency variations
in the Earth's orientation. The XXIVth General Assembly of the IAU,
held in Manchester in August 2000, has extended the concept of CEP to
that one of CIP.
excerpt
of resolution B1. 7
"The XXIVth IAU General Assembly [...] recommends [...] that
1.
the Celestial Intermediate Pole (CIP) be the pole, the motion of which
is specified in the Geocentric Celestial Reference System (GCRS)[...]
by motion of the Tisserand mean axis of the Earth with periods freater
than two days.
[...]
3.
that the motion of the CIP in the GCRS be realised by the IAU 2000 A
model for precession and forced nutation for periods greatr than two
days plus additional timedependant corrections provided by the International
Earth Rotation Service (IERS) through appropriate astrogeodetic observations
4. that the motion of the CIP in the International Terrestrial Reference
System (ITRS) be provided by the IERS through appropriate astrogeodetic
observations and models including highfrequency variations
[..]
6.
that implementation of the CIP be on 1 January 2003"
The
implementation of the CIP will not modify the deep nature of the EOP,
only brings some new constraints :
The
celestial pole offsets will be refered to the new precessionnutation
model of the IAU (instead of the IAU 1980 model)
The celestial pole offsets do not contain any highfrequency variations
(as it is already realised, but not stated)
