An Astrolabe. See the website www.astrolabe.org,

or Wikipedia, or use Google books for a public domain encyclopedia. I own an old encyclopedia britannica, the 1903 edition and love it for answering questions such as this, or those related to woodworking, carpentry, masonry, or industrial machinery; e.g. what does an automatic pattern knitting machine or a jacquard loom look like?.

historically several instruments were used to measure this angle, they include the quadrant, cross staff, the backstaff and the mariner's astrolabe.