The value of 20.5 arc-s. is called the constant of stellar aberration. Apparent Lack of Symmetry. A serious problem is revealed when one reads the description of that phenomenon. It has been clearly shown experimentally that it is not so. Observations show clearly that, in contradiction with special relativity, stellar aberration does not depend on the relative motion between the source and the detector but exists only when the detector is moving.
Textbooks explain the results solely when the observer is moving.
There is no known explanation for the case when the source is moving. One can reliably determine the motion of individual stars of a binary system from their spectrum from the periodic Doppler shifting of the spectral lines of the star components. Therefore, stars of a binary system acquire sufficient relative transverse velocity to produce an important aberration. Such an aberration is not observed. Some papers report non realistic hypotheses to explain aberration when the star is moving.