Under the foregone conclusion that light itself has a definite velocity, and that this velocity is the same throughout the universe, Bradley’s observation of the aberration of the stars seemed indeed to make it possible to calculate this velocity from the knowledge of the earth’s own speed and the angle of aberration. This angle could be established by comparing the different directions into which a telescope has to be turned at different times of the year in order to focus a particular star. But what does Bradley’s observation tell us, once we exclude all foregone conclusions?