REFERENCE SYSTEMS

A reference system is a relation connecting observables and their math ematical represententions. The principle of general relativity assures that any sort of coordinate system can be used to describe physical p henomena. Thus, any reference system is only a convention. There is no absolutely true reference system. Instead, people seek for a best ref erence system, whose meaning may differ thus need to clarify. Taking a n example from Earth rotation, we discuss how to find such a best refe rence system. The definition of the best system will change as scienti fic understandings deepen and computational environments develop. Ther efore, we can not stop improving reference systems. However, when repl acing an existing widely-spread system, one must take great care to mi nimize the inconvenience caused by its transition, especially the inco nvenience which users might endure. The Standards Of Fundamental Astro nomy (SOFA) project being conducted by the IAU WG on Astronomical Stan dards has the opportunity to ease this troublesome task. The World Wid e Web (WWW) will be a main device to realize the project, namely to pr ovide working standards including reference systems to the world.