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 tool helps sailors determine their latitude and longitude at sea, allowing for accurate navigation. It consists of a graduated arc, a movable arm called an index arm, and mirrors to reflect images. To use a sextant, the navigator sights the celestial body through the instrument and adjusts the index arm until the image of the body aligns with the horizon. This measurement can then be used in conjunction with nautical charts and tables to calculate the vessel's position.