Sensational Differences: the Case of the Transit of Venus

Abstract

In the nineteenth century, astronomers, physiologists and experimental psychologists noticed that different individuals timed simultaneous phenomena differently. These small “sensational differences”, sometimes referred to as personal equations, augmented considerably in astronomical measurements, in perceptions of movement, and in measurements of rapid phenomena, such as the speed of light. Alarmingly, these differences affected science beyond determinations of time and simultaneity. They appeared in elementary perceptions of lengths, angles, color, and sound. Unlike well-known random errors, they persisted even when observations were repeated and averaged. This paper examines various attempts by scientists to overcome these differences in the determination of the most important constant of celestial mechanics: the solar parallax. It takes us from the famous transit of Venus observations of 1874 to later speed-of-light measurements, tracking fundamental changes in the establishment of measurement-based science.