Fig. 1.
Convention for the definition of the orientation of the arbitrary frame 
with respect to the ICRF (C = [X Y Z]). The drawing illustrates the configuration when the orientation difference is a pure rotation about the Z axes by the positive angle εZ, i.e. ε = [0, 0, εZ]′. The right ascension of the source at S is α in frame C and 
in frame 
.
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