This mariner's astrolabe, made by Francisco de Goes, is fitted with a suspension ring, a degree scale (only half of which has been engraved), and an alidade. On the alidade are mounted two large square plates, one of which is perforated. The instrument served to measure the altitude of the Sun, which was indicated by the alidade on the degree scale when a ray passed through the front hole and struck the center of the back plate. Provenance: Robert Dudley bequest to the Medici collections.