Sea astrolabe

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. In the sixteenth century, the instrument was also called a ring.

The Corpus Christi Museum of Science and History in Texas, United States, contains the world's oldest mariner's astrolabe with a confirmed date of 1554, salvaged from the wreck of the San Esteban.

Many dates can be found for the appearance of the first mariner's astrolabes. The earliest date, 1295, is offered by the Majorcan astronomer Ramon Llull. Later dates center around the late 15th century, with Samuel Purchas claiming that it was adapted for marine navigation by Martin Behaim, a mariner not considered a trustworthy source by some historians. In any event, the instrument was certainly known by the end of the 15th century. Nevertheless, the creation and perfecting of the mariner's astrolabe is attributed to Portuguese navigators during the beginning of Portuguese discoveries. The earliest known description of how to make and use a sea astrolabe comes from Martín Cortés de Albacar’sArte de Navegar published in 1551, although the basic principle is the same as that of the archipendulum used in constructing the Egyptian pyramids.

There is strong evidence that the mariner's astrolabe was derived directly from the planispheric astrolabe, as the earliest examples retain some of the markings (e.g. umbra recta and umbra versa) of the prior device without having the same components.

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