In this paper I follow the Swedish astronomer Bengt Ferrner around the spaces of longitude research in mid 18th-century London. While Ferrner is not a central historical actor in a conventional sense, his extensive travel journal from 1759-60 provides an important insight into the intersections between London’s scientific and maritime worlds. There, one can read of Ferrner’s daily interactions with many key figures, such as John Harrison, Nevil Maskelyne and Christopher Irwin, as well as descriptions of English industrial and naval installations. It also shows the social practices governing entry into the spaces of longitude research in London, as well as the work of circulating useful knowledge in early modern Europe. As a stranger and foreigner, Ferrner needed to learn how to navigate the urban spaces of the English capital, as well as how to interact with the scientific networks of the city. Thus, for the Swedish astronomer, participating in London’s science required constant effort and recalibration.

English and Swedish efforts to develop methods for finding longitude at sea were linked to state interests, but were simultaneously researched by scientific communities with strong transnational ties. From Ferrner’s journal, we can study first how these two seemingly contradictory aspects were part of Euopean science, and secondly what tactics Ferrner used to capitalise on these two features of longitude research. Ferrner himself was a professor at the naval academy of Karlskrona in eastern Sweden and thus—like many astronomers of the time— his work was aligned with state and naval interests. The objective of Ferrner’s visit was to establish transnational scientific contacts, as well as to communicate findings of useful English science and industry back to an interested Swedish audience. By following Ferrner around we get a glimpse of how state and navy interests on the one hand and transnational circulation of knowledge on the other drove this important 18th-century field of research.