Latitude and Longitude
Wilmari Claasen (University of Zurich)
The determination of longitude posed a much more significant problem than that of latitude. To calculate longitude, sailors needed to compare their local time and the time at their point of origin, the latter of which required complicated estimates. This posed such a problem that the government of the Netherlands promised a reward of five thousand guilders to whoever could provide reliable methods to calculate it, with similar rewards offered by the English and French governments.
Moreover, there was often a disconnect between the theoretical methods of navigation proposed by armchair cosmographers and the realities of calculating latitude and longitude at sea. Apart from the fact that nautical instruments required practice to use effectively, the accuracy of the calculations sailors made with them could easily be compromised by a wide multitude of factors, such as the rocking of the ship on the waves or the refraction of light, which could disrupt attempts at measuring the altitude of the Sun when it was close to the horizon at sunset and sunrise.
Mathematicians and astronomers had to recalculate tables of solar declination every few years, and their calculations often contained errors due to their reliance on imprecise astronomical observations. These tables were also difficult for those without a mathematical background to understand. Methods for navigating by the Sun saw constant innovation in the period, with astronomers and writers of nautical almanacs frequently correcting each other’s work and engaging in debates over the most practical and accurate methods. These debates gave rise to the navigation manual, a new literary genre that provided a primarily theoretical framework to guide aspiring voyagers.