Astrolabe (perhaps) by Gualterus Arsenius

This is one of the two astrolabes given to Pope Leo XIII by Father Vincenzo Gisoldi in 1889. This is the larger astrolabe (with a diameter of 345 mm and a thickness of 8 mm) is of the Flemish school, made perhaps by Gualterus Arsenius (d. 1580); the back shows a stereographic equinoctial projection of the celestial sphere, used to convert between ecliptic and equatorial coordinates.