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Lunar Eclipses, Longitude and the New World

Throughout the first hundred years after the discovery of the New World late in the fifteenth century, determining the location and extent of their overseas domains was a principal preoccupation of the Spanish empire. Not only were terrestrial coordinates vital for navigation, but following the 1494 Treaty of Tordesillas, knowing with certainty the location of newly discovered territories carried significant geo-political implications as well. Further complicating the matter were the technical difficulties of calculating longitude. The Spanish monarchy turned to experts in navigation and cosmography to address the problem, and by the mid-sixteenth century had institutionalized cosmographical practice at the House of Trade (Casa de la Contratación) in Seville, the Council of Indies (the King’s council in Madrid responsible for the administration of the colonies), and at the court of Philip II (1527–98). This paper examines the instruments and techniques used in one of the projects that resulted from their efforts to produce a more accurate description of the world: the project (1577–88) to determine the longitude of all of Spain’s overseas territories using lunar eclipses. The first part of this article discusses the project’s conception, scope, and historical precedents. The second part develops mathematical models for the methods used and instruments employed, and also assesses how these were used to yield longitudinal coordinates. The final section studies six surviving observations made using the project’s protocol and compares these observations to positions computed using modern computer algorithms.