The mariner's astrolabe was a simplified version of an instrument originally developed by Arab astronomers for measuring the height of heavenly bodies above the horizon and came into use in navigation by about 1470. In order to keep it steady when used on board ship, the mariner's version was heavier and had parts of the disc cut away to reduce wind resistance.

The instrument was used to help determine the ship's latitude from the height of the Pole Star or of the sun. At night, the Pole Star was sighted directly through small pinholes in the two vanes mounted on the pivoting alidade or rule. The altitude in degrees was then read off from the scale on the outer edge of the instrument. To measure the Sun's position during the day, the astrolabe was held below the waist and the alidade was adjusted so that a beam of sunlight passed through the top pinhole onto the bottom one.

This example was found in 1845 under a rock on Valentia Island, close to the point off southern Ireland where three ships of the Spanish Armada were wrecked in 1588. The throne is low and moulded. The mater has been made from a single casting, cut out in a wheel shape with a greater weight left in the lower half to help the instrument hang vertically. The face of the mater is engraved with circles and decorative lines but there are no numbers on the scale, suggesting that the instrument was never completed - perhaps it was one of a number of pieces of unfinished equipment hurried aboard a Spanish vessel in 1588. An alidade is fitted to the face of the instrument and held in place with a pin through its centre, about which it can rotate.