Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. The octant, also called reflecting quadrant, is a measuring instrument used primarily in navigation. It is a type of reflecting instrument. The name octant derives from the Latin octans meaning eighth part of a circle, because the instrument''s arc is one eighth of a circle. Reflecting quadrant derives from the instrument using mirrors to reflect the path of light to the observer and, in doing so, doubles the angle measured. This allows the instrument to use a one-eighth circle arc to measure a quarter circle or quadrant.