What is a sextant?

A sextant is an instrument used to measure the angle between two visible objects. It is primarily used as a navigational instrument to determine the elevation of a celestial body above the horizon thereby allowing the observer to calculate a line of position (LOP) on the surface of the earth. With only one LOP, a navigator can be sure that his position is on that line somewhere, but he cannot know for sure where on that line he is. By combining additional LOPs (triangulation), navigators are able to determine their exact latitude and longitude.