CONCEPT
The Personal Equation
The systematic, measurable difference in reaction times between different human observers—discovered through the 1795 Kinnebrook affair at the Greenwich Observatory—which Schaffer uses to demonstrate that every knowledge system incorporates a human component whose contribution is typically rendered invisible by the conventions of scientific publication.
In 1795, the Astronomer Royal Nevil Maskelyne dismissed his assistant David Kinnebrook from the Royal Observatory at Greenwich. The dismissal was not for incompetence or misconduct but for a discrepancy: Kinnebrook’s recorded observations of stellar transit times differed from Maskelyne’s by approximately half a second. Maskelyne attributed the difference to Kinnebrook’s personal failing. Two decades later, the German astronomer Friedrich Bessel realized something Maskelyne had been unable to see: the discrepancy was not a personal error but a systematic difference in the reaction times of different observers—what Bessel termed the “personal equation.” Every human observer introduced a characteristic delay between the moment a star crossed the telescope’s wire and the moment the observer registered the crossing. Maskelyne’s observations were not more accurate than Kinnebrook’s; they were
differently biased. In
Simon Schaffer’s analysis, the episode establishes a principle with consequences far beyond astronomy: the observer’s body is part of the