Down here in Rio for the XXXI Symposium of the Scientific Instrument Commission, there's quite a bit to interest those with a longitude bent.
Yesterday, we heard an excellent keynote paper from Maria Portuondo about attempts in the 1570s and 1580s to establish the exact geographical positions of places around the Spanish Empire to improve the confidential maps and charts held at the Council of the Indies. The scheme, under the guidance of Juan Lopez de Vasco, first Royal Cosmographer to the Council of the Indies, relied on the use of local Spanish officials, who were generally not trained in mathematics or observation. It was necessary therefore to devise a simple, standardised procedure for operatives to follow.
What Velasco came up with was a set of instructions for making and using a device, called the 'instrument of the Indies', with which to record the beginning and end of lunar eclipses, from which longitude could be determined. This is a one-third scale model that Maria has made:
Essentially it's a moon dial that can be made very easily on the spot. Once it was correctly aligned, the observer simply marked on the semicircular line the place of the moon's shadow when the eclipse began and again when it ended. They were then to copy the marks onto paper and send these results (along with information about the length of the Sun's shadow at noon) back to Spain to be analysed and the longitude determined.
As far as keeping strategically important cartographic information secret was concerned, this was ideal, since it prevented useful knowledge being produced, and possibly leaked, locally. The downside was that the calculations needed to deduce longitudes from the marked papers were extremely complex. There were also, of course, many sources of error, but, as Maria pointed out, there had to be some compromise between precision and simplicity in this ambitious attempt at co-ordinated mapping on a worldwide scale.
For the bold historical explorer, there's more detail in Maria's paper, 'Lunar eclipses, longitude and the New World', Journal of the History of Astronomy, 40 (2009), pp. 249-276.