A Middle Age Qibla Finder and the secret code of Portolan maps

Abstract A hidden cache of medieval weaponry was discovered in Verona in 1915. A strange device found among the weapons has since been regarded as a Middle-Age hanging lantern. Here we suggest a different use as mathematical tool to find the Qibla. The new interpretation has opened up unsuspected and original perspectives in the history of science: the tool embodies the application of al-Biruni's (973–1048) method of “azimuthal equidistant projection”, a geodetic theory for map representation that preserves angles and distances. We demonstrate that the tool embeds the rules of spherical trigonometry that provide the orthodromic and lossodromic distances and the azimuth of two points of given latitude and longitude, to within 1% of the difference between the Spherical Earth model of al-Biruni (radius = 6339.6 km) and the WGS84 Ellipsoid. The geometric relationships that allow conversion of geographic coordinates to planar coordinates are equivalent to the practice of fixing an origin for a wind rose and drawing a distance along the azimuth. It represents the missing piece of the puzzle that enables us to interpret middle age Portolan maps as based on the azimuthal equidistant projection scheme of al-Biruni.