Date: 1555
Owner: iStock Photo
Source Type: Images


This bronze mariner's astrolabe was one of many similar instruments used to determine a ship's latitude at sea. A navigator would position the astrolabe parallel with the horizon at noon, when the sun reaches its peak in the sky, and measure how many degrees of arc were between the horizon and the sun (a known star could also be used at night). To determine latitude, navigators needed to know an astral body's declination, the variation in its position over time (which would be found by consulting an astronomical almanac), and do some basic arithmetic (90 degrees - the sun or star's altitude + the declination = latitude).

The degree markers running along the ring of this astrolabe would be lined up with the rotating pointer in the middle to achieve a relative measure of accuracy. Several other navigational tools, such as the quadrant and cross staff, also measured the sun's altitude at noon to determine a ship's latitude. Although it was virtually impossible to calculate longitude while at sea, navigators could reach a location of known latitude by sailing to a given parallel and traveling along it until reaching land. Astrolabes were hardly exact and errors of several degrees could put a ship too far north or south of its desired port. Nevertheless, simple, durable, and practical instruments like the astrolabe were essential for Iberian navigators of the 14th to 17th centuries and facilitated European dominance of the New World.

CITATION: Norbert Speicher. Small Astrolabe. Courtesy of iStockphoto. ID: 2874694.