Astrolabe

Accession Number1986.008.0083a

VesselNuestra Señora de Atocha (1622)

Datecirca 1560

DescriptionIn the early seventeenth century, the mariner’s astrolabe was perhaps the most significant of all the ship’s navigational instruments. With this beautifully crafted device, the pilots of the 1622 galleon Nuestra Señora de Atocha, Martin Jimenez and Francisco de Cárdenas Garay, measured the angle of the sun above the horizon; a measurement that could then be used to determine the ship’s latitude and its position in the vast expanse of the sea. This reading was taken by suspending the heavy, wheel-like bronze body side-on to the sun. The pointer (alidade) was pivoted until the sun’s rays shone through the small holes (pinnules) that pierced the sighting vanes. The face of the astrolabe was engraved with a degree scale, and at the point where the sunlight passed through both pinnules, the alidade indicated the angle of the sun. This figure was then adjusted for the time of year to determine latitude. This astrolabe is 19.3 centimeters in diameter and weighs 2.59 kilograms.