The instrument is of cast bronze with a shackle and suspension ring. There are three spokes and a lower ballast piece with a heart-shaped void. At the base of the ballast is an unidentified mark: an A surmounted by a star.
Around the circumference of the limb are engraved circles. There is a scale in the upper left quadrant, divided to 10, 5 and 1 and numbered 0 to 90 by 10 in both directions, for altitude and zenith distance. There is a '5' between each of the pairs of 10? numbers.
The reverse is blank and the alidade and fixing pin are modern replacements.
The instrument was found during dredging work in 1903 at Vera Cruz harbour in the Gulf of Mexico. This location and the diameter of approximately 8 Spanish inches have suggested a Spanish origin.
The instrument was purchased from its finder Donald Courie by R. T. Gunther.
See A. Stimson, The Mariner's Astrolabe (Utrecht, 1988), p. 68.