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Astron. Astrophys. 331, L33-L36 (1998)

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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 directions.
  • Changes of stellar positions between two epochs are derived by allowing for proper motion. The new positions are still referred to the ICRS.
  • 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 [FORMULA]) 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, precession-nutation).

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 François Mignard.

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© European Southern Observatory (ESO) 1998

Online publication: March 3, 1998