Those definitions corresponding to the IAU 2000 resolutions are given in italic type(1)
. The new
definitions proposed by the NFA WG (including those formally endorsed by the IAU in 2006 or
by the IUGG in 2007) are underlined while superseded definitions are given in a smaller font(2)
(1) Some of the definitions prior to the IAU 2000 resolutions may not be compliant
(2) Although the following definitions are provided with capitals this does not mean that they must be used with
capitals. The policy adopted throughout this document is to capitalize those terms that are defined in IAU or IUGG
resolutions. This includes the words that are represented by the abbreviations BCRS, CIO, CIP, CIRS, ERA,
GCRS, GTRS, HCRF, ICRF, ICRS, ITRF, ITRS, TAI, TCB, TCG TDB, TDT, TIO, TIRS, and TT.
the apparent angular displacement of the observed position of a celestial object from
its geometric position, caused by the finite velocity of light in combination with the motions of the
observer and of the observed object. Annual aberration is due to the motion of the Earth around
the Sun, while diurnal aberration is due to the Earth’s rotation. In star catalogs of the pre-1984.0
era, it was common to include the secular part of stellar aberration (the so-called e-terms) in the
published star positions.
a geocentric position (e.g. apparent right ascension and declination) in the true
equinox and equator of date reference system at a specified date.
apparent right ascension and declination:
angular coordinates in the true equator and equinox of
date reference system at a specified date. They are geocentric positions differing from the ICRS
positions by annual parallax, gravitational light deflection due to the solar system bodies except the
Earth, annual aberration, and the time-dependent rotation describing the transformation from the
GCRS to the Celestial Intermediate Reference System (they are similar to intermediate positions in
the CIO based system but the apparent right ascension origin is at the equinox). Note that apparent
declination is identical to intermediate declination.
direction of a solar system body formed by applying the correction for the
barycentric motion of this body during the light time to the geometric geocentric position referred
to the ICRS. Such a position is then directly comparable with the astrometric position of a star
formed by applying the corrections for proper motion and annual parallax to the catalog direction
at J2000. The gravitational deflection of light is ignored. For high-accuracy applications,
gravitational light deflection effects need to be considered, and the adopted policy declared.
centered at the solar system barycenter.
Barycentric Celestial Reference System (BCRS): a system of barycentric space-time
coordinates for the solar system within the framework of General Relativity with metric tensor
specified by the IAU 2000 Resolution B1.3. Formally, the metric tensor of the BCRS does not fix
the coordinates completely, leaving the final orientation of the spatial axes undefined. However,
according to IAU 2006 Resolution B2, for all practical applications, unless otherwise stated, the
BCRS is assumed to be oriented according to the ICRS axes.
Barycentric Coordinate Time (TCB):
the coordinate time of the BCRS; it is related to
Geocentric Coordinate Time (TCG) and Terrestrial Time (TT) by relativistic transformations that
include secular terms.
Barycentric Dynamical Time (TDB):
a time scale originally intended to serve as an independent
time argument of barycentric ephemerides and equations of motion. In the IAU 1976 resolutions,
the difference between TDB and TDT was stipulated to consist of only periodic terms, a condition
that cannot be satisfied rigorously. The IAU 1991 resolutions introducing barycentric coordinate
time (TCB) noted that TDB is a linear function of TCB, but without explicitly fixing the rate ratio
and zero point, leading to multiple realizations of TDB. In 2006 TDB was re-defined through the
following linear transformation of TCB (IAU 2006 Resolution B3):
TDB = TCB − LB
x ( JDTCB
) x 86400 + TDB0
= 2443144.5003725, and LB
= − 6.55x10-5
s are defining
see Barycentric Celestial Reference System.
the intersection of the hour circle of zero right ascension of a star catalog with
the celestial equator.
a star catalog position.
Celestial Ephemeris Origin (CEO): the original name for the Celestial Intermediate Origin (CIO) given in
the IAU 2000 resolutions
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.
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.
Celestial Intermediate Pole (CIP): geocentric equatorial pole defined by IAU 2000 Resolution
B1.7 as being the intermediate pole, in the transformation from the GCRS to the ITRS, separating
nutation from polar motion. It replaced the CEP on 1 January 2003. Its GCRS position results
from (i) the part of precession-nutation with periods greater than 2 days, and (ii) the retrograde
diurnal part of polar motion (including the free core nutation, FCN) and (iii) the frame bias. Its
ITRS position results from (i) the part of polar motion which is outside the retrograde diurnal band
in the ITRS and (ii) the motion in the ITRS corresponding to nutations with periods less than 2
days. The motion of the CIP is realized by the IAU precession-nutation plus time-dependent
corrections provided by the IERS.
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.
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
, 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
, in longitude and obliquity with respect to
the IAU 1976/1980 precession/nutation model.
celestial pole offsets at J2000.0:
offset of the direction of the mean pole at J2000.0, provided by
the current model, with respect to the GCRS. These offsets are part of what is often called frame
CEO: see Celestial Ephemeris Origin.
CEP: see Celestial Ephemeris Pole.
see Celestial Intermediate Origin.
CIO locator (denoted s )
: the difference between the GCRS right ascension and the intermediate
right ascension of the intersection of the GCRS and intermediate equators. The CIO was originally
set close to the mean equinox at J2000.0. As a consequence of precession-nutation the CIO moves
according to the kinematical property of the non-rotating origin. The CIO is currently located by
using the quantity s
CIO right ascension and declination
: see intermediate right ascension and declination.
see Celestial Intermediate Pole.
see Celestial Intermediate Reference System.
CTRS: see Conventional Terrestrial Reference System.
Conventional International Origin: the international origin of polar motion adopted for use by the former
International Latitude Service (ILS). It was defined in 1967 by an adopted set of astronomical latitudes of the 5
stations of the ILS. It approximately coincided with the mean pole of 1903.0 as determined by the ILS. To avoid
ambiguity, this origin should be designated by its full name. This designation should be avoided for the current origin
(the ITRF pole) of the polar motion, which no longer coincides with the conventional international origin.
Conventional Terrestrial Reference System (CTRS): term used in the 1991 IUGG Resolution 2 for
designating the ideal terrestrial system to be defined from a geocentric non-rotating system by a spatial rotation. It is
proposed to replace this designation by GTRS, which was endorsed by IUGG 2007 Resolution 2.
a reading of a time scale.
an interval of 86400 seconds. The day, and related conventional units such as the Julian year
and Julian century, may be used with any continuous time scale (TCG, TCB, TT, TDB, ...). In
precise work the time scale should be specified (e.g. 1 TT day). This also applies to UT1 when
used as a time scale. Note that for UTC, on the days that leap seconds are added, the day can be
86401 (or 86399) seconds.
angular distance north or south of the celestial equator. It is measured along the hour
circle passing through the celestial object. Declination is usually given in combination with right
ascension or hour angle.
dynamical mean equinox:
the ascending node of the ecliptic on the mean equator. The mean
equinox of epoch (to which the recent analytical and numerical solutions for the Moon and planets
refer) corresponds to the definition of the ecliptic in its “inertial” sense. It differs by 93.66 mas
from the “rotational dynamical mean equinox of J2000.0”, which was intended to coincide with the
Earth Rotation Angle (ERA): angle measured along the intermediate equator of the Celestial
Intermediate Pole (CIP) between the Terrestrial Intermediate Origin (TIO) and the Celestial
Intermediate Origin (CIO), positively in the retrograde direction. It is related to UT1 by a
conventionally adopted expression in which ERA is a linear function of UT1 (see IAU 2000
Resolution B1.8). Its time derivative is the Earth’s angular velocity. Previously, it has been
referred to as the stellar angle.
the plane perpendicular to the mean heliocentric orbital angular momentum vector of the
Earth-Moon barycentre in the BCRS (IAU 2006 Resolution B1). In the past, there was no unique
interpretation; an ecliptic was defined by means of the angles of the precession theory.
Ephemeris time (ET): the time scale used prior to 1984 as the independent variable in gravitational theories of
the solar system, with its unit and origin conventionally defined. It was superseded by TT and TDB.
a fixed date used to reckon time for expressing time varying quantities. It is often
expressed in the system of Julian date, marked by the prefix J (e.g. J2000.0), with the Julian year of
365.25 days as unit. The term is also used to designate the date and time of an observation, e.g.
“epoch of observation”, which would be better expressed by ‘date of observation’.
equation of the equinoxes:
the right ascension of the mean equinox referred to the true equator
and equinox; alternatively the difference between apparent sidereal time and mean sidereal time
(GAST − GMST).
equation of the origins
: distance between the CIO and the equinox along the intermediate
equator; it is the CIO right ascension of the equinox; alternatively the difference between the Earth
Rotation Angle and Greenwich apparent sidereal time (ERA − GAST).
either of the two points at which the ecliptic intersects the celestial equator; also the time
at which the Sun passes through either of these intersection points; i.e., when the apparent
longitude of the Sun is 0° or 180°. When required, the equinox can be designated by the ephemeris
of the Earth from which it is obtained (e.g. vernal equinox of DE 405). By 2100 the equinox will
have moved 1.4° from the ICRS meridian, due to the precession of the equinoxes.
equinox right ascension
: right ascension that is measured from the equinox; also simply called
see Earth Rotation Angle.
ET: see Ephemeris time.
see free core nutation.
The ecliptic of a given ephemeris at an adopted epoch. Such a fixed ecliptic has a
specified obliquity and crosses the ICRS equator at a specified offset from the ICRS origin.
the three offsets of the mean equator and (dynamical) mean equinox of J2000.0,
provided by the current model, with respect to the GCRS; the first two offsets are the mean pole
offsets at J2000.0 and the third is the offset in right ascension of the mean dynamical equinox of
free core nutation (FCN):
free retrograde diurnal mode in the motion of the Earth’s rotation axis
with respect to the Earth, due to non-alignment of the rotation axis of the core and of the mantle; it
is a long period (of 432 days) free nutation of the CIP in the GCRS.
see Greenwich sidereal time.
see Geocentric Celestial Reference System.
GCRS CIP coordinates
: direction cosines X
, of the CIP in the GCRS; these quantities are often
multiplied by 1296000″/2π
in order to represent the approximate values in arcseconds of the
corresponding angles with respect to the polar axis of the GCRS.
see Greenwich mean sidereal time.
referring to the center of the Earth.
Geocentric Celestial Reference System (GCRS): a system of geocentric space-time coordinates
within the framework of General Relativity with metric tensor specified by the IAU 2000
Resolution B1.3. The GCRS is defined such that the transformation between BCRS and GCRS
spatial coordinates contains no rotation component, so that GCRS is kinematically non-rotating
with respect to BCRS. The equations of motion of, for example, an Earth satellite, with respect to
the GCRS will contain relativistic Coriolis forces that come mainly from geodesic precession. The
spatial orientation of the GCRS is derived from that of the BCRS, that is (c.f. IAU 2006 Resolution
B2), unless otherwise stated, by the orientation of the ICRS.
Geocentric Coordinate Time (TCG):
coordinate time of the GCRS based on the SI second. It is
related to Terrestrial Time (TT) by a conventional linear transformation provided by IAU 2000
Geocentric Terrestrial Reference System (GTRS)
: a system of geocentric space-time
coordinates within the framework of General Relativity, co-rotating with the Earth, and related to
the GCRS by a spatial rotation which takes into account the Earth orientation parameters. It was
adopted by IUGG 2007 Resolution 2. It replaces the previously defined Conventional Terrestrial
geodesic precession and nutation:
the largest components (in fact the only non-negligible ones)
of the relativistic rotation of the GCRS with respect to a dynamically non-rotating geocentric
reference system in the framework of General Relativity. Geodesic precession is the secular part
of the rotation and geodesic nutation is the periodic part. Geodesic precession and nutation are
included in the IAU 2000 precession-nutation model.
Greenwich mean sidereal time (GMST):
Greenwich hour angle of the mean equinox defined by
a conventional relationship to Earth Rotation Angle or equivalently to UT1.
Greenwich sidereal time (GST):
Greenwich apparent sidereal time (GAST), the hour angle of the
true equinox from the Terrestrial Intermediate Origin (TIO) meridian (Greenwich or International
GST (or GAST):
see Greenwich sidereal time
see Geocentric Terrestrial Reference System
IAU 2000 precession-nutation: IAU 2000 Resolution B1.6 recommends the IAU 2000A
precession-nutation model for those who need a model at 0.2 mas level. It represents the CIP. An
abridged model, designated IAU 2000B, is available for those who require a model at the 1 mas
see International Celestial Reference Frame.
see International Celestial Reference System.
: a direction in ICRS coordinates, (e.g. ICRS right ascension, the right ascension
measured from the ICRS origin on the ICRS equator, and ICRS declination, the declination
measured from the ICRS equator).
: equatorial plane through the center of the Earth and perpendicular to the
direction of the Celestial Intermediate Pole (CIP) at some epoch (it is synonymous with the
instantaneous equator or true equator of date, or equator of the CIP).
: direction of an object in the Celestial Intermediate Reference System (e.g.
intermediate right ascension and declination), analogous to an apparent place in the equinox based
system, but the origin for intermediate right ascension is at the CIO.
intermediate right ascension and declination
: angular coordinates measured in the Celestial
Intermediate Reference System at a specified date. They specify a geocentric direction that differs
from the ICRS direction by annual parallax, gravitational light deflection due to the solar system
bodies, except the Earth, annual aberration, and the time-dependent rotation describing the
transformation from the GCRS to the Celestial Intermediate Reference System. They are similar to
apparent right ascension and declination when referring to the equinox based system. Note that
intermediate declination is identical to apparent declination.
International Atomic Time (TAI):
a widely used practical realization of TT with a fixed shift
from the latter due to historical reasons (see TT); it is a continuous time scale, now calculated at
the Bureau International des Poids et Mesures (BIPM), using data from some three hundred atomic
clocks in over fifty national laboratories in accordance with the definition of the SI second.
International Celestial Reference Frame (ICRF):
a set of extragalactic objects whose adopted
positions and uncertainties realize the ICRS axes and give the uncertainties of the axes. It is also
the name of the radio catalog whose 212 defining sources is currently the most accurate realization
of the ICRS. Note that the orientation of the ICRF catalog was carried over from earlier IERS
radio catalogs and was within the errors of the standard stellar and dynamic frames at the time of
adoption. Successive revisions of the ICRF are intended to minimize rotation from its original
orientation. Other realizations of the ICRS have specific names (e.g. Hipparcos Celestial
International Celestial Reference System (ICRS):
the idealized barycentric coordinate system to
which celestial positions are referred. It is kinematically non-rotating with respect to the ensemble
of distant extragalactic objects. It has no intrinsic orientation but was aligned close to the mean
equator and dynamical equinox of J2000.0 for continuity with previous fundamental reference
systems. Its orientation is independent of epoch, ecliptic or equator and is realized by a list of
adopted coordinates of extragalactic sources.
International Terrestrial Reference Frame (ITRF):
a realization of ITRS by a set of
instantaneous coordinates (and velocities) of reference points distributed on the topographic
surface of the Earth (mainly space geodetic stations and related markers). Currently the ITRF
provides a model for estimating, to high accuracy, the instantaneous positions of these points,
which is the sum of conventional corrections provided by the IERS Convention center (solid Earth
tides, pole tides, ...) and of a “regularized” position. At present, the latter is modeled by a
piecewise linear function, the linear part accounting for such effects as tectonic plate motion, postglacial
rebound, and the piecewise aspect representing discontinuities such as seismic
displacements. The initial orientation of the ITRF is that of the BIH Terrestrial System at epoch
International Terrestrial Reference System (ITRS):
according to IUGG 2007 Resolution 2, the
ITRS is the specific GTRS for which the orientation is operationally maintained in continuity with
past international agreements (BIH orientation). The co-rotation condition is defined as no residual
rotation with regard to the Earth’s surface, and the geocenter is understood as the center of mass of
the whole Earth system, including oceans and atmosphere (IUGG 1991 Resolution 2). For
continuity with previous terrestrial reference systems, the first alignment was close to the mean
equator of 1900 and the Greenwich meridian. The ITRS was adopted (IUGG 2007 Resolution 2)
as the preferred GTRS for scientific and technical applications and is the recommended system to
express positions on the Earth.
see International Terrestrial Reference Frame.
the plane passing through the geocenter, ITRF pole and ITRF x
see International Terrestrial Reference System.
ITRS CIP coordinates
: direction cosines of the CIP in the ITRS, also called pole coordinates.
They are currently expressed in the form of x
coordinates, in arcseconds, the values of which
represent the corresponding angles with respect to the polar axis of the ITRS. The sign convention
is such that x
is positive towards the x
-origin of the ITRS and y
is in the
direction 90° to the west of x
defined in the framework of General Relativity by IAU 1994 Resolution C7 as being the
event (epoch) at the geocenter and at the date 2000 January 1.5 TT = Julian Date 245 1545.0 TT.
Note that this event has different dates in different time scales.
a period of 100 Julian years (36525 days). The Julian century as a unit of time
may be used with any continuous time scale (TCG, TCB, TT, TDB). In precise work the time
scale should be specified (e.g. 1 TT Julian century). Note that this extends the IAU 1994
Resolution C7 where Julian century was defined as 36525 TT days.
the interval of time in days and fractions of a day since 4713 B.C. January 1,
Greenwich noon, approximately. The Julian date can be used with any time scale (TCG, TCB, TT,
TDB). In precise work, the Julian date in TT, TCG and TCB, has its origin fixed according to the
IAU 1991 Resolutions by the condition that on 1977 January 1, 00h
TAI at the geocenter,
the readings of TT, TCG and TCB are 1977 January 1, 00h
.184 (JD 244 3144.5003725).
The equivalent TDB reading depends on the adopted ephemeris (for example, the same reading for
TDB(DE405) is JD 244 3144.5003725 − 65.564518 µs). The Modified Julian date is JD − 240
a period of 365.25 days. The Julian year as a unit of time may be used with any
continuous time scale (TCG, TCB, TT, TDB). In precise work the time scale should be specified
(e.g. 1 TT Julian year).
equator associated with a celestial pole whose direction is determined only by the
precession portion of the precession-nutation transformation.
equinox associated with the mean equator.
position of an object on the celestial sphere referred to the mean equator and equinox
at a standard epoch.
in the context of the GCRS or the ITRS, the point on the intermediate
equator such that its instantaneous motion with respect to the system (GCRS or ITRS as
appropriate) has no component along the intermediate equator (i.e. its instantaneous motion is
perpendicular to the intermediate equator). It is called the CIO and TIO in the GCRS and ITRS,
forced periodic part of the motion of the pole of rotation of a freely rotating body that is
undergoing torque from external gravitational forces.
a topocentric place that includes the effect of refraction.
offset in right ascension:
expression used to describe the equatorial offset at J2000 of the GCRS
right ascension of the inertial dynamical mean equinox (see frame bias).
the difference in apparent direction of an object as seen from two different locations;
annual parallax refers to the difference in directions as seen from the barycenter and the geocenter,
while diurnal parallax refers to the component of parallax due to the observer’s separation from the
the motion of the Earth’s pole with respect to the ITRS. The main components are
the Chandlerian free motion with a period of approximately 430 days, and an annual motion. It
also includes sub-daily variations caused by ocean tides and periodic motions driven by
gravitational torques with periods less than two days. Sub-daily variations are not included in the
values distributed by the IERS, and are therefore to be added, after interpolation to the date of
interest, using a model provided by the IERS Conventions.
angular coordinates of the pole with respect to the terrestrial system (see ITRS
precession of the ecliptic:
the secular part of the motion of the ecliptic with respect to the fixed
precession of the equator (and CIP):
the uniformly progressing motion of the pole of rotation of
a freely rotating body, undergoing torque from external gravitational forces. In the case of the
Earth, the precession of the equator is caused by solar system objects acting on the Earth’s
equatorial bulge making the pole of rotation describe a 26000-year orbit around the ecliptic pole.
precession of the equinox:
results from both the precession of the equator and the precession of
the ensemble of effects of external torques on the motion in space of the
rotation axis of a freely rotating body (see the separate entries for precession and nutation below),
or alternatively, the forced motion of the pole of rotation due to those external torques. In the case
of the Earth, a practical definition consistent with the IAU 2000 resolutions is that precessionnutation
is the motion of the CIP in the GCRS, including FCN and other corrections to the standard
models: precession is the secular part of this motion plus the term of 26000-year period and
nutation is that part of the CIP motion not classed as precession.
direction of an object in the GCRS (e.g. right ascension and declination); geocentric
place that is corrected for light-time, light deflection, annual parallax and annual aberration.
practical realization of a reference system, usually as a catalog of positions and
motions of a certain number of fiducial points. For instance, the ICRF is the realization of the
ICRS, where the ICRF points have no proper motions.
theoretical concept of a system of coordinates, including time and standards
necessary to specify the bases used to define the position and motion of objects in time and space.
the bending of a ray of light as it passed through the Earth’s atmosphere. Most
commonly calculated using pressure, temperature, humidity and wavelength.
angular distance measured eastward along the celestial equator from the CIO, or
equinox, to the hour circle passing through the celestial object. Right ascension is given either in
arc or time units. It is essential that the origin, CIO or equinox, of the right ascension be specified.
stellar angle: the original term used for the Earth Rotation Angle (ERA) in the first definition of the non-rotating
the measure of the angle defined by the apparent diurnal motion of the equinox;
hence, a measure of the rotation of the Earth with respect to the celestial reference frame rather
than the Sun. It is often expressed in hours, minutes, and seconds, one hour being equal to 15°.
see International Atomic Time.
see Barycentric Coordinate Time.
see Geocentric Coordinate Time.
see Barycentric Dynamical Time.
TDT: see Terrestrial Dynamical Time.
TEO: see Terrestrial Ephemeris Origin.
the independent time argument of the JPL and MIT/CfA solar-system ephemerides
(Standish, 1998). The linear drift between Teph and TCB is such that the rates of Teph and TT are
as close as possible for the time span covered by the particular ephemeris. Each ephemeris defines
its own version of Teph; the Teph of the JPL ephemeris DE405 is for practical purposes the same
as TDB defined above.
Terrestrial Dynamical Time (TDT): time scale for apparent geocentric ephemerides defined by a 1979 IAU
resolution and in 1991 was replaced by Terrestrial Time (TT).
Terrestrial Ephemeris Origin (TEO): the original name for the Terrestrial Intermediate Origin (TIO) given
in the IAU 2000 resolutions.
Terrestrial Intermediate Origin (TIO)
: origin of longitude in the Intermediate Terrestrial
Reference System. It is the non-rotating origin in the ITRS that is recommended by the IAU 2000
Resolution B1.8, where it was designated Terrestrial Ephemeris Origin. The name Terrestrial
Intermediate Origin was adopted by IAU 2006 Resolution B2. The TIO was originally set at the
ITRF origin of longitude and throughout 1900-2100 stays within 0.1 mas of the ITRF zeromeridian.
Terrestrial Intermediate Reference System (TIRS)
: a geocentric reference system defined by
the intermediate equator of the CIP and the TIO (IAU 2006 Resolution B2). It is related to the
ITRS by polar motion and s
′ (see TIO locator). It is related to the Celestial Intermediate Reference
System by a rotation of ERA around the CIP, which defines the common z-axis of the two systems.
Since the acronym for this system is close to another acronym (namely ITRS), it is suggested that
wherever possible the complete name be used.
Terrestrial Time (TT):
a coordinate time whose mean rate is close to the mean rate of the proper
time of an observer located on the rotating geoid. At 1977 January 1.0 TAI exactly, the value of
TT was 1977 January 1.0003725 exactly. It is related to the Geocentric Coordinate Time (TCG)
by a conventional linear transformation provided by IAU 2000 Resolution B1.9. TT may be used
as the independent time argument for geocentric ephemerides. An accurate realization of TT is TT
(TAI) = TAI + 32s.184. In the past TT was called Terrestrial Dynamical Time (TDT).
see Terrestrial Intermediate Origin.
TIO locator (denoted s′)
: the difference between the ITRS longitude and the instantaneous
longitude of the intersection of the ITRS and intermediate equators. The TIO was originally set at
the ITRF origin of longitude. As a consequence of polar motion the TIO moves according to the
kinematical property of the non-rotating origin. The TIO is currently located using the quantity
′, the rate of which is of the order of 50 μas/cy and is due to the current polar motion.
moving plane passing through the geocenter, the CIP and the TIO.
see Terrestrial Intermediate Reference System.
a place that is centered at the surface of the Earth and dependent on the geographic
true equator of date:
see celestial intermediate equator.
true equinox of date:
intersection of the ecliptic with the intermediate (true) equator and
designated by the ephemeris of the Earth from which it is obtained (e.g. true equinox of DE 405).
see Terrestrial Time.
Universal Time (UT):
a measure of time that conforms, within a close approximation, to the
mean diurnal motion of the Sun and serves as the basis of all civil timekeeping. The term “UT” is
used to designate a member of the family of Universal Time scales (e.g. UTC, UT1).
Universal Time (UT1):
angle of the Earth’s rotation about the CIP axis defined by its
conventional linear relation to the Earth Rotation Angle (ERA). It is related to Greenwich apparent
sidereal time through the ERA (see equation of the origins). It is determined by observations
(currently from VLBI observations of the diurnal motions of distant radio sources). UT1 can be
regarded as a time determined by the rotation of the Earth. It can be obtained from the uniform
time scale UTC by using the quantity UT1 − UTC
, which is provided by the IERS.
UT1 – UTC:
difference between the UT1 parameter derived from observation and the uniform
time scale UTC, the latter being currently defined as: UTC = TAI + n
, where n
is an integer
number of seconds, such that |UT1 − UTC