This instrument is a reproduction of the so-called "Valencia" nautical astrolabe, after the island of Ireland where it was found, and belonged to the Armada of Philip II (16th century). It is in the National Maritime Museum of Greenwich and the replica used is in the Naval Museum of Madrid. 

The nautical astrolabe is the simplification of the planispheric or astronomical astrolabe, and its main purpose was to calculate the Latitude, observing the height of the sun or other stars such as the Pole Star. (H 23).

Measurements:

Maximum Height: 15cm / Diameter: 10cm / Thickness: 10mm