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\centerline{\tit Relativistic Effects in Motion and Observations}
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\centerline{\tit of Celestial Bodies}

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\centerline{\dh S.A.Klioner}

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\centerline{\it Institute of Applied Astronomy, 8, Zhdanovskaya st.,
197042, St.Petersburg, Russia}

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  Accuracy of modern astronomical observations is getting so high that
in order to understand what we observe and how to get physically and
astronomically meaningful information from the observations, a
general-relativistic consideration is inevitable. The influence of
general relativity is not confined to some small corrections to
Newtonian models. On the contrary, the whole concept of astronomical
reference frames and astronomical observations should be re-considered
from the relativistic point of view.

  We begin with discussing the problem of relativistic astronomical
reference frames. Although a substantial progress has been made here
during last several years, some questions are still to be answered. A
set of general questions is how to generalize some Newtonian concepts
which are of great importance for astronomy onto the relativistic
framework: terrestrial rotating reference frame, precession, nutation,
rotation with respect to distant celestial bodies, etc. The concepts,
being related to a reference frame as a whole rather than to an
infinitely small test observer, are of Newtonian (Euclidean) origin
and, generally speaking, lose their clear [intuitive] meaning in curved
(Riemannian) space-times. More specific question is related to
rotational state of motion of the relativistic reference systems. It
turns out that the property of two reference systems to show no
rotation with respect to each other is not transitive (and even not
symmetric, in a sense) in the framework of general relativity.  We
consider possible ways to overcome these difficulties.

  We review also the relativistic effects in astronomical observations
which are to be accounted for at the modern level of accuracy. We
discuss relativistic effects in both translational and rotational
motions of celestial bodies as well as those in modelling various kinds
of astronomical data.

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