Mariner's astrolabe

They were designed to allow for their use on boats in rough water and/or in heavy winds,[1] 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.

By the late 18th century, mariners began using the sextant and then LORAN C, SatNav/Transit, and then global positioning systems (GPS) starting in the 1980s.

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

[12] A disk-type astrolabe dated between 1496 and 1501, now the world's oldest, was discovered in 2014 by the marine scientist David L. Mearns on the wreck site of a Portuguese nau Esmeralda, which sank off the coast of Oman in 1503.

Heavier cast brass frames began to be made in the mid-sixteenth century and were considerably better.

In order to lower the centre of gravity of the device and thus increase its period of motion as a means of stabilizing it, extra brass was usually added to the bottom of the instrument inside the ring.

The mariner's astrolabe needed to be suspended vertically in order to measure the altitude of the celestial object.

It could not easily be used to measure the angle between two objects, which was necessary for longitude calculations by the lunar distance method (though that technique was not used when the instrument was developed).

Another limitation was that the instrument's angular accuracy was directly proportional to the length of the alidade, which was not very long.