Sextant is an optical instrument that measures the angular distance between any two points, such as the sun and the horizon. It is used for navigation and surveying. The instrument is named for its shape, which is roughly a sixth part of a circle. A sextant's frame supports a graduated arc, a movable index arm that represents the radius of the circle, two mirrors, and a small telescope. One of the mirrors--called the horizon glass--is fixed. The second mirror--the index glass--is screwed to the index arm. The telescope sharpens the line of the horizon.