NavList:

Frank,

you may not be aware of this but longitude (which had already been tabulated by Ptolemäus) became a hot topic between Portugal and Spain in the early 16th century when Portuguese navigators reached the fabled Spice Island in 1512. Whereas before cartographers and cosmographers had dealt with this question it then became crucial to know the exact longitude of the Spice Ilands due to the division of the world agreed between the two nations in the treaty of Tordesillas in 1494. Depending on where the line of demarcation was in the Atlantic, the Anti-Meridian was situated in the East dividing the spheres of influence there. If the islands were west of the antimeridian they would belong to Portugal, if east to Spain (nobody bothered to ask the people living there). The Magellan voyage therefore not only had the aim of finding a west route to the Indies - thereby avoiding the Portuguese realm - but also to find out the position of the Spice Islands. In the preparations for the voyage Francisco and Ruy Faleiro proposed the method that Andres de San Marin apparently used in his observations and calculations (it was also explained in Faleiro's "Tratado del Esphera y delarte del marear" published in 1535 and in appendix to Pigafetta's narrative of the voyage). A similar method was proposed by Anthony Linton in 1609 and probably used by William Baffin in 1612.

Apart from the methodical shortcomings of San Martin's work pointed out by Gaspar in his article, all attempts to discover the longitude by the moon were - as you rightly pointed out - due to fail because reliable lunar tables were only produced by Tobias Mayer in the 18th century. Nonetheless Andres de San Martin's measurements were not simply guesswork but an attempt to use astronomical knowledge in a stringent way.

And, by the way, as so often Wikipedia is wrong in historical detail (at least regarding the history of navigation): there is no evidance that Elcano owned books on astronomy. Andres de San Martin is quoted by Herrera as having used an almanach by Regiomontanus (which is disputed by Gaspar in his article).

Best
Wolfgang