Observations Beyond Compare

Abstract The final decade of Bradley’s working life was centred on the survey of the stars, observed by Bradley and his assistants down to magnitude 8, visible from the Royal Observatory. These observations were reduced by Bessel and published in 1818; the reduction involved developing new methods that formed the foundations of nineteenth century positional astronomy. Bradley’s discovery of both the aberration of light and the nutation of the Earth’s axis had earlier laid the foundations of eighteenth century astrometry, while his Greenwich observations laid the bed on which Bessel’s great achievement was made. Both Bradley and Bessel were concerned with the problems of observational error and those of perceptual bias, that is, with seeing what the observer wants to see.