Astron. Astrophys. 331, L33-L36 (1998)
3. Practical consequences of the adoption of the ICRS/ICRF
Kovalevsky & McCarthy (1997) have considered the various
consequences of the change from the J2000 Reference System to ICRS,
with particular emphasis on the conceptual aspects. We summarize
hereafter the consequences in every-day life of the astronomers.
- For all uses with accuracy requirement less stringent than 50
mas, the adoption of ICRS has no significant effect.
- There is no epoch attached to the system, i.e., future updates of
the ICRF will not change the ICRS origin and that of its defining
- Changes of stellar positions between two epochs are derived by
allowing for proper motion. The new positions are still referred to
- For precise applications, the IERS 1996 precession-nutation model
(McCarthy 1996) or the celestial pole offsets published by the IERS
should be used, rather than the less accurate IAU conventional models.
The IERS 1996 model being referred to the mean pole at J2000.0 that is
offset from the ICRS one, two additional constants must be used in
order to refer the motion to the ICRS pole. Estimates of these
constants corresponding to Fig. 1 are - 43.1 mas in longitude
(-17.2 mas/sin ) and - 5.1 mas in
obliquity (IERS 1997).
- The direction of celestial objects in the ICRS are consistent with
the terrestrial coordinates in the ITRS by the use of the IERS Earth
orientation parameters (universal time, polar motion,
The IAU Division 1 (Fundamental Astronomy) has set up an ICRS
Working Group, in charge of studying all consequences of the adoption
of the ICRS, and proposing solutions. This Working Group is chaired by
© European Southern Observatory (ESO) 1998
Online publication: March 3, 1998