Quadrant

This is a navigational instrument of the kind that explorers like Ross would have used. The light of a star or the sun is reflected by the small mirror, which is in a fixed position, onto an adjustable mirror and through the eye piece. Looking at the horizon at the same time, the mirror is adjusted until the reflected star can be seen in the fixed mirror. The angle at which the fixed mirror is when this occurs is the angle needed for navigation.   A quadrant has a 90 degree angle, a sextant a 60 degree angle and an octant a 45 degree angle.