The mariner's astrolabe, also called sea astrolabe, was an inclinometer used to determine the latitude of a ship at sea by measuring the sun's noon altitude (declination) or the meridian altitude of a star of known declination. Not an astrolabe proper, the mariner's astrolabe was rather a graduated circle with an alidade used to measure vertical angles. They were designed to allow for their use on boats in rough water and/or in heavy winds, which astrolabes are ill-equipped to handle. It was invented by the Portuguese people, a nation known for its maritime prowess that dominated the sea for multiple centuries. In the sixteenth century, the instrument was also called a ring.
Three Mariner's Astrolabes in the Museum of the Forte da Ponta da Bandeira; Lagos, Portugal
Mariner's astrolabe from circa 1600.
Mariner's astrolabe made in 1645 by Nicolao Ruffo at The Mariners' Museum
Oldest mariner's astrolabe in National Museum of Oman, Muscat
An astrolabe is an astronomical instrument dating to ancient times. It serves as a star chart and physical model of visible heavenly bodies. Its various functions also make it an elaborate inclinometer and an analog calculation device capable of working out several kinds of problems in astronomy. In its simplest form it is a metal disc with a pattern of wires, cutouts, and perforations that allows a user to calculate astronomical positions precisely. It is able to measure the altitude above the horizon of a celestial body, day or night; it can be used to identify stars or planets, to determine local latitude given local time, to survey, or to triangulate. It was used in classical antiquity, the Islamic Golden Age, the European Middle Ages and the Age of Discovery for all these purposes.
North African, 9th century AD, Planispheric Astrolabe. Khalili Collection.
A modern astrolabe made in Tabriz, Iran in 2013.
16th-century woodcut of measurement of a building's height with an astrolabe
Armillary Sphere mosaic from the Casa di Leada at Soluntum in Sicily.