The so-called planispheric astrolabe has nothing to do with a real, i.e. planispheric astrolabe, but is rather a navigational instrument used by sailors. However, the misnomer «Astrolabium» has persisted over the centuries and, although incorrect, is now an integral part of the nomenclature; this is also the case here.
The nautical astrolabe was developed by Portuguese sailors in the late 15th and early 16th centuries. The Spanish, English, French and Dutch also produced sea astrolabes, but the number of these instruments was never very large. Only 106 copies and a further five fragments from the years 1533 to 1760 have survived, some of which are damaged or heavily corroded (as of 2020; cf. Castro et al.). Sea astrolabes fell into disuse in the 17th century; only two examples from the 18th century have survived.
They have gone up against the more accurate Davis quadrant (Backstaff) and the Jacob's staff (also: degree stick or crossbar) could not prevail.
In contrast to the real astrolabe, nautical astrolabes are used exclusively for altitude measurement of the sun and stars.
The complete description of the instrument as PDF for download
