Vintage Freiberger Prazisions Mechanik Trommel Sextant instrument manufactured in the GDR in original wooden box. A sextant is a navigational instrument used to measure the angle between two visible objects, typically the horizon and a celestial body such as the Sun, Moon, or stars. It is an essential tool for celestial navigation, which is the practice of determining one's position on the Earth's surface by observing celestial bodies.