The final WG recommendations on terminology that have been adopted by the NFA WG are the
- Using existing terms (e.g. right ascension) in extended ways for the terminology associated
with the new paradigm with a clear specification, rather than introducing new names.
- Using “equinox based” and “CIO based” for referring to the classical and new
Comment: the “Celestial/Terrestrial Intermediate Origin” with the acronym CIO/TIO is proposed here
as the updated terminology to replace the IAU 2000 “Celestial/Terrestrial Ephemeris Origin” with the
acronym CEO/TEO (see below items 3 and 4 and the proposed resolution).
- Using “intermediate” to describe (i) the moving geocentric celestial reference system
defined in the IAU 2000 resolutions (i.e. containing the CIP and the CIO), and (ii) the
moving terrestrial system containing the CIP and the TIO.
Comment: the term “intermediate” has been chosen to specify that these systems are intermediary
systems between the geocentric celestial system and the terrestrial system, which are realized by using
the models, constants and procedures that are conventionally accepted; it conventionally separates the
instantaneous celestial orientation of the Earth into components we label polar motion (in the
terrestrial system) and precession-nutation (in the celestial system).
- Harmonizing the name of the pole and the origin to “intermediate” and therefore
changing CEO/TEO to CIO/TIO.
- Using “system” in a broad sense rather than “frame” in this context of the intermediary
- Using special designations for particular realizations of the intermediate celestial system.
Comment: this applies for example to “the IAU 2000A system” to designate the system which is
realized by transforming the geocentric celestial system GCRS to the intermediate system using the
IAU 2000A precession-nutation and associated frame biases at J2000 (the GCRS being transformed
from the BCRS by using the coordinate transformation specified in the IAU 2000 Resolution B1.3).
- Keeping the classical terminology for “true equator and equinox” (or “true equinox
based”) for the classical equatorial system.
- Choosing “equinox right ascension” (or “RA with respect to the equinox”) and
“intermediate right ascension” (or “CIO right ascension”, or “RA with respect to the
CIO”), for the azimuthal coordinate along the equator in the classical and new paradigms,
respectively. (Note that right ascensions and declinations with respect to the ICRS are
usually designated by αICRS, δICRS).
Comment: this is to be specified only once in the presentation of a paper if there is some risk of
misunderstanding. Afterwards, “right ascension” alone is sufficient.
- Giving the name “equation of the origins” to the distance between the CIO and the
equinox along the intermediate equator, the sign of this quantity being such that it represents
the CIO right ascension of the equinox, or equivalently, the difference between the Earth
Rotation Angle and Greenwich apparent sidereal time.
- Retaining “apparent places” and “mean places” in the equinox based system.
- Not introducing “apparent intermediate places” in the CIO based system, but
introducing instead “intermediate places”.
- Using “ITRF zero-meridian” to designate the plane passing through the geocenter, ITRF
pole and ITRF x-origin and using, if necessary, “TIO meridian” to designate the moving
plane passing through the geocenter, the CIP and the TIO.
- Fixing the default orientation of the BCRS so that for all practical applications, unless
otherwise stated, the BCRS is assumed to be oriented according to the ICRS axes.
Comment: Once the BCRS is spatially oriented according to the ICRS, the spatial GCRS
coordinates get an “ICRS-induced” orientation.
- Re-defining Barycentric Dynamical Time (TDB) as a fixed linear function of TCB:
TDB = TCB - LB × (JDTCB − T0) × 86400 + TDB0,
where T0 = 2443144.5003725,
and LB =1.550519768 × 10-8 and TDB0 = − 6.55 × 10-5 s
are defining constants.
– Considering a terminology associated with other types of apparent places, although it may be
required for specific use, has not been considered as being essential for common astronomical
use and is therefore not part of the NFA WG terminology recommendations.
– No WG consensus having been reached for having strict rules for using or not using capitals
for names for origins, poles and systems, no recommendation on this issue is proposed by the
WG. The policy adopted throughout the NFA document is to capitalize those terms that are
defined in IAU or IUGG resolutions.