A sextant is a navigational instrument used to measure the angle between two visible objects, typically the horizon and a celestial body like the Sun or a star. This angle, known as the altitude, helps sailors determine their latitude and longitude at sea. By using a sextant, navigators can accurately chart their course and ensure safe travel across oceans. The sextant consists of a graduated arc, a movable arm called an index arm, and mirrors that allow the user to align the celestial body with the horizon. It has been an essential tool for maritime navigation since the 18th century, significantly improving the accuracy of sea travel.