NONROTATING ASTRONOMICAL RELATIVISTIC REFERENCE FRAMES

The target of this paper is a discussion of the meaning of the Newtoni an concept of a reference frame showing no rotation with respect to a set of distant celestial objects in the framework of general relativit y. Because of the properties of Newtonian absolute space and time and the existence of global inertial reference systems in Newton's theory the theoretical construction of such a globally nonrotating reference frame is obvious. This definitively no longer is the case in a relativ istic framework. There exist no global inertial reference systems and one faces the necessity to give the notion of nonrotating frames a rig orous meaning. Considering possible definitions of nonrotating referen ce frames in both Newtonian and relativistic physics, we conclude that the concept of relative spatial rotation between reference systems pl ays a fundamental role in defining nonrotating astronomical reference frames. It turns out that the property of two relativistic reference s ystems to show no spatial rotation relative to each other, being a coo rdinate-dependent concept, has some properties which cannot be interpr eted within our ''Newtonian common sense''. As an example, we discuss two versions of a geocentric reference system, both of which can be co nsidered to show no rotation relative to distant celestial objects at the present level of observational accuracy, but differing by a time-d ependent rotation of considerable amplitude. Applying the obtained res ults to the recently elaborated formalisms for constructing relativist ic astronomical reference systems, we describe relative spatial rotati ons between the galactic, barycentric, geocentric reference systems an d the reference system of an observer. We find a certain parallel betw een the concept of simultaneity (synchronization of clocks) and the co ncept of reference system showing no rotation relative to distant cele stial objects. Both notions are absolute in Newtonian physics and beco me coordinate-dependent in the relativistic framework, representing, t herefore, a mathematical convention rather than a physically meaningfu l phenomena.