12 Conclusion

The new system, based on the ICRS and related IAU resolutions, is defined and described in this paper and is a logical progression from the old system based on the solar system dynamics and nearby stars. The new system introduces greater rigor in the definitions and some simplifications. Among the simplifications are:

• A fixed, epoch-independent, reference frame based on distant non-moving sources.

• A reference frame independent of the dynamics of the solar system.

• An origin, or departure point, not precessing along the moving equator.

• Proportionality between the new Stellar Angle, or Earth Rotation Angle, and UT1.

• Microsecond accuracy predictability of the departure point for many decades, so it is not theory-dependent.

• Lack of interaction between expressions for the motions of reference frames.

The definitions and applications of the ICRS to astrometry should be formally adopted by the IAU in 2003.

Acknowledgements

It is with appreciation that we acknowledge the suggestions and comments of Patrick Wallace, Bernard Guinot, Michael Soffel, and Nicole Capitaine. We also thank Dr. Heiner Schwan for his judicious recommendations as referee.