Mounted quadrant, historical illustration. 17th-century astronomer using a large mounted quadrant. This instrument is used to measure angles up to 90 degrees. Here, it is being used to measure the position of a star in the sky, determined by the angle it makes with the horizon. Artwork from 'Les terres du ciel: voyage astronomique sur les autres mondes' (1884, 'The Lands of Heaven: An Astronomical Journey to the Other Worlds') by French astronomer Camille Flammarion (1842-1925).