As sociologists learn more about how scientific knowledge is created, they give historians the opportunity to rework their accounts from a more contextual perspective. It is relatively easy to do so in areas with large theoretical, cosmological or overtly ideological components. It is more difficult, but equally necessary, to open up very empirical accomplishments, and recent sociological analysis of the process of science gives us some interesting insights. This paper employs some of these on the apparently unpromising subject of the (...) ‘discovery of secular magnetic variation’ in 1634 by the Gresham professor Henry Gellibrand. (shrink)