The sextant is a scientific instrument used to measure and construct angles. Precise, light, and easy to transport, it is mainly used at sea to measure the angular distances and altitudes of stars. This allows the observer to determine their location by making a calculation using an astronomical "ephemeris" (tables published in nautical almanacs), at the times provided by chronometers. These measurements were required to create nautical charts.
This type of instrument has a metal frame in a circular sector shape starting at 60° amplitude. On the front is an embedded silver limb on which the alidade slides to rotate around the center of the limb's circumference. There are mirrors perpendicular to the limb: one is part of the alidade and the other is part of one of the frame arms. The other frame arm has an astronomical eyepiece. The movement of the eyepiece in parallel with the instrument was devised in 1790 by Spanish Navy General José de Mazarredo, who suggested it to British manufacturer John Bird (1709–76), who then became the first to introduce this improvement to the sextant.
This piece was made by Spanish technician and instrument maker José María Baleato, who worked for Ferrol Naval Base during the last quarter of the 18th century.