Abstract

In 1823, F.W. Bessel published a startling conclusion: ‘involuntary constant differences’ mark out the recorded astronomical transit times of distinct observers. Bessel’s discovery eventually led to the institutionalization of the ‘personal equation’ for astrometry and spurred psychological investigations into the processes of visual perception. Bessel’s discovery revealed that the epistemic terrain of astrometry was more unpredictable than had been previously thought. Yet the solution to these worries was not a rigorous theory of the observer, but rather a set of cautionary practices in data recording. This paper investigates the reasoning process Bessel used to discover involuntary constant differences and asks whether the apparent perceptual relativity he had discovered constituted a threat to data integrity. Bessel’s solution was epistemically responsible because it treated individual observational differences as a type of potential error that was avoided by preventing mixed observations. Additionally, error was not primarily viewed as a mismatch with reality, but rather as endemic to observational data, which kept the focus on practical matters rather than ontological ones.