I am working through "Spacetime Physics" and encountered exercise 3-9, which concerns aberration of starlight. They ask the following question: "Since the background of stars also shifts due to aberration, how can the effect be measured at all?" I got part of the answer. You measure the angle between the north celestial pole and the position of the star. It shifts depending on what time of year it is. That takes care of one component (I think). But I am puzzled as to how you quantify the aberration in the direction perpendicular to this. Well in the pdf https://www.colorado.edu/physics/phys2170/phys2170_fa06/downloads/stellar_aberration.pdf it says that Bradley did not record the East-West aberration due to practical difficulties. So that perhaps answers my question.