Celestial Ephemeris Origin
Acronym: CEO
The CEO is the point on the true equator used to measure the rotation of the Earth and the origin of intermediate right ascension. The CEO replaces the true equinox.
The CEO was located on the true (CIP) equator of J2000.0 at a point 2.012 mas from the ICRS prime meridian (consistent the conventional definition of the Earth Rotation Angle), at right ascension 00h 00m 00s.00013416 in the ICRS.
As the true equator moves in space, the path of the CEO in space is such that the point has no instantaneous east-west velocity along the true equator. In contrast, the equinox has instantaneous velocity along the equator. After 1 century, the CEO is still within 70 mas of the ICRS prime meridian, whereas the equinox has moved nearly 1.4 degrees.
The CEO is sometimes also called the Non-Rotating Origin (NRO).
Related terms:
Celestial Ephemeris Pole (CEP)
used from 1984 to 2003 with the IAU 1980 Theory of Nutation as the reference pole for nutation and polar motion; the axis of figure for the mean surface of a model Earth in which the free motion has zero amplitude. This pole was originally defined as having no nearly-diurnal nutation with respect to a space-fixed or Earth-fixed coordinate system and being realized by the IAU 1980 nutation. It was afterwards determined by using VLBI observations of celestial pole offsets. It is now replaced by the CIP, which is defined by IAU 2000 Resolution B1.7.
Source:
IERS Technical Note 36: Glossary (PDF, 244KB, Not barrier-free file.) of the IERS Conventions (2010)
Original Source:
IAU 2006 NFA Glossary
Celestial Intermediate Origin (CIO)
origin for right ascension on the intermediate equator in the Celestial Intermediate Reference System. It is the non-rotating origin in the GCRS that is recommended by the IAU 2000 Resolution B 1.8, where it was designated the Celestial Ephemeris Origin. The name Celestial Intermediate Origin was adopted by IAU 2006 Resolution B2. The CIO was originally set close to the GCRS meridian and throughout 1900-2100 stays within 0.1 arcseconds of this alignment.
Source:
IERS Technical Note 36: Glossary (PDF, 244KB, Not barrier-free file.) of the IERS Conventions (2010)
Original Source:
IAU 2006 NFA Glossary
Celestial Intermediate Pole
Acronym: CIP
The CIP is an intermediate pole separating the motion of the pole of the ITRS in the GCRS (Geocentric Celestial Reference System) by dividing nutation from polar motion explicitly at the 2-day nutation period.
Nutations with periods less than two days are modeled by their equivalent polar motion.
Related terms:
Celestial Intermediate Reference System (CIRS)
geocentric reference system related to the GCRS by a time-dependent rotation taking into account precession-nutation. It is defined by the intermediate equator (of the CIP) and CIO on a specific date (IAU 2006 Resolution B2). It is similar to the system based on the true equator and equinox of date, but the equatorial origin is at the CIO. Since the acronym for this system is close to another acronym (namely ICRS), it is suggested that wherever possible the complete name is used.
Source:
IERS Technical Note 36: Glossary (PDF, 244KB, Not barrier-free file.) of the IERS Conventions (2010)
Original Source:
IAU 2006 NFA Glossary
celestial pole offsets
time-dependent corrections to the precession-nutation model, determined by observations. The IERS provides the celestial pole offsets in the form of the differences, dX and dY, of the CIP coordinates in the GCRS with respect to the IAU 2000A precession-nutation model (i.e. the CIP is realized by the IAU 2000A precession-nutation plus these celestial pole offsets). In parallel the IERS also provides the offsets, dψ and dε, in longitude and obliquity with respect to the IAU 1976/1980 precession/nutation model.
Source:
IERS Technical Note 36: Glossary (PDF, 244KB, Not barrier-free file.) of the IERS Conventions (2010)
Original Source:
IAU 2006 NFA Glossary
Chandler wobble
coordinate time
In the framework of General Relativity, a 4-dimensional space-time reference system is described by four coordinates among which three are the spatial coordinates and the other is x0 = ct, where c is the velocity of light in vacuum and t is the "coordinate time" in this reference system.
Coordinate time provides an unambiguous way of dating events in a specified reference system. It is the time basis (or coordinate) to be used in the theory of motions referred to this system. The relation of the observed (proper) time of an observer to the coordinate time t depends on the velocity, mass and energy.
Related terms:
Coordinated Universal Time
Acronym: UTC
In French: Temps universel coordonné
UTC differs from TAI by the total number of leap seconds, so that UT1-UTC stays smaller than 0.9 s in absolute value.
(i.e. UTC = TAI - leap seconds)
Related terms:
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