Accession Number

1986.008.0083a

Alternate object names

Portuguese Mariner's Astrolabe

Creation Date

circa 1560

Description

In 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.

Exhibition Label

Case Caption (2023): 

The Pilot

Aboard a Spanish ship, the pilot was third in seniority and would have had over sixteen years’ experience. His training included mathematics and celestial navigation. He also had to be familiar with charts of the overall voyage and those of the fleet’s destination in detail. He needed to understand changes in cloud patterns, shifts in ocean currents, and the quality of the ocean floor along different coastlines.

Aboard the Nuestra Señora de Atocha, the pilot, Martin Jiminez, secured his chest carefully. It was still intact when it was discovered by Mel Fisher’s divers almost 400 years later. It held plotting dividers, a small sundial, a cross staff, a jar, gold and silver coins, and gold chains. Most importantly, it held the astrolabe shown here as well as four others. The astrolabe was used to determine latitude. Certainty about the ship’s latitude combined with the pilot’s other knowledge, meant that the ship would reach its destination safely and on time.
Object Caption (2023):

Astrolabe
Bronze (c.1610)
Gift of Jamestown Inc.
1986.008.0083a-c